Breaking biological symmetry in membrane proteins: the asymmetrical orientation of PsaC on the pseudo-C2 symmetric Photosystem I core

The elucidation of assembly pathways of multi-subunit membrane proteins is of growing interest in structural biology. In this study, we provide an analysis of the assembly of the asymmetrically oriented PsaC subunit on the pseudo C(2)-symmetric Photosystem I core. Based on a comparison of the differences in the NMR solution structure of unbound PsaC with that of the X-ray crystal structure of bound PsaC, and on a detailed analysis of the PsaC binding site surrounding the F(X) iron-sulfur cluster, two models can be envisioned for what are likely the last steps in the assembly of Photosystem I. Here, we dissect both models and attempt to address heretofore unrecognized issues by proposing a mechanism that includes a thermodynamic perspective. Experimental strategies to verify the models are proposed. In closing, the evolutionary aspects of the assembly process will be considered, with special reference to the structural arrangement of the PsaC binding surface.

Iron-sulfur clusters in type I reaction centers

Type I reaction centers (RCs) are multisubunit chlorophyll-protein complexes that function in photosynthetic organisms to convert photons to Gibbs free energy. The unique feature of Type I RCs is the presence of iron-sulfur clusters as electron transfer cofactors. Photosystem I (PS I) of oxygenic phototrophs is the best-studied Type I RC. It is comprised of an interpolypeptide [4Fe-4S] cluster, F(X), that bridges the PsaA and PsaB subunits, and two terminal [4Fe-4S] clusters, F(A) and F(B), that are bound to the PsaC subunit. In this review, we provide an update on the structure and function of the bound iron-sulfur clusters in Type I RCs. The first new development in this area is the identification of F(A) as the cluster proximal to F(X) and the resolution of the electron transfer sequence as F(X)-->F(A)-->F(B)-->soluble ferredoxin. The second new development is the determination of the three-dimensional NMR solution structure of unbound PsaC and localization of the equal- and mixed-valence pairs in F(A)(-) and F(B)(-). We provide a survey of the EPR properties and spectra of the iron-sulfur clusters in Type I RCs of cyanobacteria, green sulfur bacteria, and heliobacteria, and we summarize new information about the kinetics of back-reactions involving the iron-sulfur clusters.

Recruitment of a foreign quinone into the A1 site of photosystem I. In vivo replacement of plastoquinone-9 by media-supplemented naphthoquinones in phylloquinone biosynthetic pathway mutants of Synechocystis sp. PCC 6803

Interruption of the phylloquinone (PhQ) biosynthetic pathway by interposon mutagenesis of the menA and menB genes in Synechocystis sp. PCC 6803 results in plastoquinone-9 (PQ-9) occupying the A(1) site and functioning in electron transfer from A(0) to the FeS clusters in photosystem (PS) I (Johnson, T. W., Shen, G., Zybailov, B., Kolling, D., Reategui, R., Beauparlant, S., Vassiliev, I. R., Bryant, D. A., Jones, A. D., Golbeck, J. H., and Chitnis, P. R. (2000) J. Biol. Chem. 275, 8523-8530. We report here the isolation of menB26, a strain of the menB mutant that grows in high light by virtue of a higher PS I to PS II ratio. PhQ can be reincorporated into the A(1) site of the menB26 mutant strain by supplementing the growth medium with authentic PhQ. The reincorporation of PhQ also occurs in cells that have been treated with protein synthesis inhibitors, consistent with a displacement of PQ-9 from the A(1) site by mass action. The doubling time of the menB26 mutant cells, but not the menA mutant cells, approaches the wild type when the growth medium is supplemented with naphthoquinone (NQ) derivatives such as 2-CO(2)H-1,4-NQ and 2-CH(3)-1,4-NQ. Since PhQ replaces PQ-9 in the supplemented menB26 mutant cells, but not in the menA mutant cells, the phytyl tail accompanies the incorporation of these quinones into the A(1) site. Studies with menB26 mutant cells and perdeuterated 2-CH(3)-1,4-NQ shows that phytylation occurs at position 3 of the NQ ring because the deuterated 2-methyl group remains intact. Therefore, the specificity of the phytyltransferase enzyme is selective with respect to the group present at ring positions 2 and 3. Supplementing the growth medium of menB26 mutant cells with 1,4-NQ also leads to its incorporation into the A(1) site, but typically without either the phytyl tail or the methyl group. These findings open the possibility of biologically incorporating novel quinones into the A(1) site by supplementing the growth medium of menB26 mutant cells.

Photoinduced transient absorbance spectra of P840/P840(+) and the FMO protein in reaction centers of Chlorobium vibrioforme

The kinetics of photoinduced absorbance changes in the 400-ns to 100-ms time range were studied between 770 and 1025 nm in reaction center core (RCC) complexes isolated from the green sulfur bacterium Chlorobium vibrioforme. A global, multiple stretched-exponential analysis shows the presence of two distinct but strongly overlapping spectra. The spectrum of the 70-micros component consists of a broad bleaching with two minima at 810 and 825 nm and a broad positive band at wavelengths greater than 865 nm and is assigned to the decay of (3)Bchl a of the Fenna-Matthews-Olson (FMO) protein. The contribution of the 70-micros component correlates with the amount of FMO protein in the isolated RCC complex. The spectrum of the 1.6-micros component has a sharp bleaching at 835 nm, a maximum at 805 nm, a broad positive band at wavelengths higher than 865 nm, and a broad negative band at wavelengths higher than 960 nm. When the RCC is incubated with inorganic iron and sulfur, the 1.6-micros component is replaced by a component with a lifetime of approximately 40 micros, consistent with the reconstruction of the F(X) cluster. We propose that the 1.6-micros component results from charge recombination between P840(+) and an intermediate electron acceptor operating between A(0) and F(X). Our studies in Chlorobium RCCs show that approaches that employ a single wavelength in the measurement of absorption changes have inherent limitations and that a global kinetic analysis at multiple wavelengths in the near-infrared is required to reliably separate absorption changes due to P840/P840(+) from the decay of (3)Bchl a in the FMO protein.

Electron transfer may occur in the chlorosome envelope: the CsmI and CsmJ proteins of chlorosomes are 2Fe-2S ferredoxins

Chlorosomes of the green sulfur bacterium Chlorobium tepidum have previously been shown to contain at least 10 polypeptides [Chung, S., Frank, G., Zuber, H., and Bryant, D. A. (1994) Photosynth. Res. 41, 261-275]. Based upon the N-terminal amino acid sequences determined for two of these proteins, the corresponding genes were isolated using degenerate oligonucleotide hybridization probes. The csmI and csmJ genes encode proteins of 244 and 225 amino acids, respectively. A third gene, denoted csmX, that predicts a protein of 221 amino acids with strong sequence similarity to CsmI and CsmJ, was found to be encoded immediately upstream from the csmJ gene. All three proteins have strong sequence similarity in their amino-terminal domains to [2Fe-2S] ferredoxins of the adrenodoxin/putidaredoxin subfamily of ferredoxins. CsmI and CsmJ were overproduced in Escherichia coli, and both proteins were shown by EPR spectroscopy to contain iron-sulfur clusters. The g-tensor and relaxation properties are consistent with their assignment as [2Fe-2S] clusters. Isolated chlorosomes were also shown to contain [2Fe-2S] clusters whose properties were similar to those of the recombinant CsmI and CsmJ proteins. Redox titration of isolated chlorosomes showed these clusters to have potentials of about -201 and +92 mV vs SHE. The former potential is similar to that measured by redox titration of the clusters in inclusion bodies of CsmJ. Possible roles for these iron-sulfur proteins in electron transport and light harvesting are discussed.

Recruitment of a foreign quinone into the A1 site of photosystem I. Altered kinetics of electron transfer in phylloquinone biosynthetic pathway mutants studied by time-resolved optical, EPR, and electrometric techniques

Interruption of the menA or menB gene in Synechocystis sp. PCC 6803 results in the incorporation of a foreign quinone, termed Q, into the A(1) site of photosystem I with a number of experimental indicators identifying Q as plastoquinone-9. A global multiexponential analysis of time-resolved optical spectra in the blue region shows the following three kinetic components: 1) a 3-ms lifetime in the absence of methyl viologen that represents charge recombination between P700(+) and an FeS(-) cluster; 2) a 750-microseconds lifetime that represents electron donation from an FeS(-) cluster to methyl viologen; and 3) an approximately 15-microseconds lifetime that represents an electrochromic shift of a carotenoid pigment. Room temperature direct detection transient EPR studies of forward electron transfer show a spectrum of P700(+) Q(-) during the lifetime of the spin polarization and give no evidence of a significant population of P700(+) FeS(-) for t </= 2-3 microseconds. The UV difference spectrum measured 5 microseconds after a flash shows a maximum at 315 nm, a crossover at 280 nm, and a minimum at 255 nm as well as a shoulder at 290-295 nm, all of which are characteristic of the plastoquinone-9 anion radical. Kinetic measurements that monitor Q at 315 nm show a major phase of forward electron transfer to the FeS clusters with a lifetime of approximately 15 microseconds, which matches the electrochromic shift at 485 nm of the carotenoid, as well as an minor phase with a lifetime of approximately 250 microseconds. Electrometric measurements show similar biphasic kinetics. The slower kinetic phase can be detected using time-resolved EPR spectroscopy and has a spectrum characteristic of a semiquinone anion radical. We estimate the redox potential of plastoquinone-9 in the A(1) site to be more oxidizing than phylloquinone so that electron transfer from Q(-) to F(X) is thermodynamically unfavorable in the menA and menB mutants.

Paramagnetic 1H NMR spectroscopy of the reduced, unbound photosystem I subunit PsaC: sequence-specific assignment of contact-shifted resonances and identification of mixed- and equal-valence Fe-Fe pairs in [4Fe-4S] centers FA- and FB-

The PsaC subunit of Photosystem I (PS I) is a 9.3-kDa protein that binds two important cofactors in photosynthetic electron transfer: the [4Fe-4S] clusters FA and FB. The g-tensor orientation of FA- and FB- is believed to be correlated to the preferential localization of the mixed-valence and equal-valence (ferrous) iron pairs in each [4Fe-4S]+ cluster. The preferential position of the mixed-valence and equal-valence pairs, in turn. can be inferred from the study of the temperature dependence of contact-shifted resonances by 1H NMR spectroscopy. For this, a sequence-specific assignment of these signals is required. The 1H NMR spectrum of reduced, unbound PsaC from Synechococcus sp. PCC 7002 at 280.4 K in 99% D2O solution shows 18 hyperfine-shifted resonances. The non-solvent-exchangeable, hyperfine-shifted resonances of reduced PsaC are clearly identified as belonging to the cysteines coordinating the clusters FA- and FB- by their downfield chemical shifts, by their temperature dependencies, and by their short T1 relaxation times. The usual fast method of assigning the 1H NMR spectra of reduced [4Fe-4S] proteins through magnetization transfer from the oxidized to the reduced state was not feasible in the case of reduced PsaC. Therefore, a de novo self-consistent sequence-specific assignment of the hyperfine-shifted resonances was obtained based on dipolar connectivities from 1D NOE difference spectra and on longitudinal relaxation times using the X-ray structure of Clostridium acidi urici 2[4Fe-4S] cluster ferredoxin at 0.94 A resolution as a model. The results clearly show the same sequence-specific distribution of Curie and anti-Curie cysteines for unbound, reduced PsaC as established for other [4Fe-4S]-containing proteins; therefore, the mixed-valence and equal-valence (ferrous) Fe-Fe pairs in FA- and FB- have the same preferential positions relative to the protein. The analysis reveals that the magnetic properties of the two [4Fe-4S] clusters are essentially indistinguishable in unbound PsaC, in contrast to the PsaC that is bound as a component of the PS I complex.

The bound electron acceptors in green sulfur bacteria: resolution of the g-tensor for the F(X) iron-sulfur cluster in Chlorobium tepidum

The photosynthetic reaction center (RC) of green sulfur bacteria contains two [4Fe-4S] clusters named F(A) and F(B), by analogy with photosystem I (PS I). PS I also contains an interpolypeptide [4Fe-4S] cluster named F(X); however, spectroscopic evidence for an analogous iron-sulfur cluster in green sulfur bacteria remains equivocal. To minimize oxidative damage to the iron-sulfur clusters, we studied the sensitivity of F(A) and F(B) to molecular oxygen in whole cells of Chlorobium vibrioforme and Chlorobium tepidum and obtained highly photoactive membranes and RCs from Cb. tepidum by adjusting isolation conditions to maximize the amplitude of the F(A)(-)/F(B)(-) electron paramagnetic resonance signal at g = 1.89 (measured at 126 mW of microwave power and 14 K) relative to the P840(+) signal at g = 2.0028 (measured at 800 microW of microwave power and 14 K). In these optimized preparations we were able to differentiate F(X)(-) from F(A)(-)/F(B)(-) by their different relaxation properties. At temperatures between 4 and 9 K, isolated membranes and RCs of Cb. tepidum show a broad peak at g = 2.12 and a prominent high-field trough at g = 1.76 (measured at 126 mW of microwave power). The complete g-tensor of F(X)(-), extracted by numerical simulation, yields principal values of 2.17, 1.92, and 1. 77 and is similar to F(X) in PS I. An important difference from PS I is that because the bound cytochrome is available as a fast electron donor in Chlorobium, it is not necessary to prereduce F(A) and F(B) to photoaccumulate F(X)(-).

Recruitment of a foreign quinone into the A(1) site of photosystem I. II. Structural and functional characterization of phylloquinone biosynthetic pathway mutants by electron paramagnetic resonance and electron-nuclear double resonance spectroscopy

Electron paramagnetic resonance (EPR) and electron-nuclear double resonance studies of the photosystem (PS) I quinone acceptor, A(1), in phylloquinone biosynthetic pathway mutants are described. Room temperature continuous wave EPR measurements at X-band of whole cells of menA and menB interruption mutants show a transient reduction and oxidation of an organic radical with a g-value and anisotropy characteristic of a quinone. In PS I complexes, the continuous wave EPR spectrum of the photoaccumulated Q(-) radical, measured at Q-band, and the electron spin-polarized transient EPR spectra of the radical pair P700(+) Q(-), measured at X-, Q-, and W-bands, show three prominent features: (i) Q(-) has a larger g-anisotropy than native phylloquinone, (ii) Q(-) does not display the prominent methyl hyperfine couplings attributed to the 2-methyl group of phylloquinone, and (iii) the orientation of Q(-) in the A(1) site as derived from the spin polarization is that of native phylloquinone in the wild type. Electron spin echo modulation experiments on P700(+) Q(-) show that the dipolar coupling in the radical pair is the same as in native PS I, i.e. the distance between P700(+) and Q(-) (25.3 +/- 0.3 A) is the same as between P700(+) and A(1)(-) in the wild type. Pulsed electron-nuclear double resonance studies show two sets of resolved spectral features with nearly axially symmetric hyperfine couplings. They are tentatively assigned to the two methyl groups of the recruited plastoquinone-9, and their difference indicates a strong inequivalence among the two groups when in the A(1) site. These results show that Q (i) functions in accepting an electron from A(0)(-) and in passing the electron forward to the iron-sulfur clusters, (ii) occupies the A(1) site with an orientation similar to that of phylloquinone in the wild type, and (iii) has spectroscopic properties consistent with its identity as plastoquinone-9.

Recruitment of a foreign quinone into the A(1) site of photosystem I. I. Genetic and physiological characterization of phylloquinone biosynthetic pathway mutants in Synechocystis sp. pcc 6803

Genes encoding enzymes of the biosynthetic pathway leading to phylloquinone, the secondary electron acceptor of photosystem (PS) I, were identified in Synechocystis sp. PCC 6803 by comparison with genes encoding enzymes of the menaquinone biosynthetic pathway in Escherichia coli. Targeted inactivation of the menA and menB genes, which code for phytyl transferase and 1,4-dihydroxy-2-naphthoate synthase, respectively, prevented the synthesis of phylloquinone, thereby confirming the participation of these two gene products in the biosynthetic pathway. The menA and menB mutants grow photoautotrophically under low light conditions (20 microE m(-2) s(-1)), with doubling times twice that of the wild type, but they are unable to grow under high light conditions (120 microE m(-2) s(-1)). The menA and menB mutants grow photoheterotrophically on media supplemented with glucose under low light conditions, with doubling times similar to that of the wild type, but they are unable to grow under high light conditions unless atrazine is present to inhibit PS II activity. The level of active PS II per cell in the menA and menB mutant strains is identical to that of the wild type, but the level of active PS I is about 50-60% that of the wild type as assayed by low temperature fluorescence, P700 photoactivity, and electron transfer rates. PS I complexes isolated from the menA and menB mutant strains contain the full complement of polypeptides, show photoreduction of F(A) and F(B) at 15 K, and support 82-84% of the wild type rate of electron transfer from cytochrome c(6) to flavodoxin. HPLC analyses show high levels of plastoquinone-9 in PS I complexes from the menA and menB mutants but not from the wild type. We propose that in the absence of phylloquinone, PS I recruits plastoquinone-9 into the A(1) site, where it functions as an efficient cofactor in electron transfer from A(0) to the iron-sulfur clusters.

A kinetic assessment of the sequence of electron transfer from F(X) to F(A) and further to F(B) in photosystem I: the value of the equilibrium constant between F(X) and F(A)

The x-ray structure analysis of photosystem I (PS I) crystals at 4-A resolution (Schubert et al., 1997, J. Mol. Biol. 272:741-769) has revealed the distances between the three iron-sulfur clusters, labeled F(X), F(1), and F(2), which function on the acceptor side of PS I. There is a general consensus concerning the assignment of the F(X) cluster, which is bound to the PsaA and PsaB polypeptides that constitute the PS I core heterodimer. However, the correspondence between the acceptors labeled F(1) and F(2) on the electron density map and the F(A) and F(B) clusters defined by electron paramagnetic resonance (EPR) spectroscopy remains controversial. Two recent studies (Diaz-Quintana et al., 1998, Biochemistry. 37:3429-3439;, Vassiliev et al., 1998, Biophys. J. 74:2029-2035) provided evidence that F(A) is the cluster proximal to F(X), and F(B) is the cluster that donates electrons to ferredoxin. In this work, we provide a kinetic argument to support this assignment by estimating the rates of electron transfer between the iron-sulfur clusters F(X), F(A), and F(B). The experimentally determined kinetics of P700(+) dark relaxation in PS I complexes (both F(A) and F(B) are present), HgCl(2)-treated PS I complexes (devoid of F(B)), and P700-F(X) cores (devoid of both F(A) and F(B)) from Synechococcus sp. PCC 6301 are compared with the expected dependencies on the rate of electron transfer, based on the x-ray distances between the cofactors. The analysis, which takes into consideration the asymmetrical position of iron-sulfur clusters F(1) and F(2) relative to F(X), supports the F(X) --> F(A) --> F(B) --> Fd sequence of electron transfer on the acceptor side of PS I. Based on this sequence of electron transfer and on the observed kinetics of P700(+) reduction and F(X)(-) oxidation, we estimate the equilibrium constant of electron transfer between F(X) and F(A) at room temperature to be approximately 47. The value of this equilibrium constant is discussed in the context of the midpoint potentials of F(X) and F(A), as determined by low-temperature EPR spectroscopy.

Electrometrical study of electron transfer from the terminal FA/FB iron-sulfur clusters to external acceptors in photosystem I

An electrometrical technique was used to investigate electron transfer between the terminal iron-sulfur centers F(A)/F(B) and external electron acceptors in photosystem I (PS I) complexes from the cyanobacterium Synechococcus sp. PCC 6301 and from spinach. The increase of the relative contribution of the slow components of the membrane potential decay kinetics in the presence of both native (ferredoxin, flavodoxin) and artificial (methyl viologen) electron acceptors indicate the effective interaction between the terminal 14Fe-4S] cluster and acceptors. The finding that FA fails to donate electrons to flavodoxin in F(B)-less (HgCl2-treated) PS I complexes suggests that F(B) is the direct electron donor to flavodoxin. The lack of additional electrogenicity under conditions of effective electron transfer from the F(B) redox center to soluble acceptors indicates that this reaction is electrically silent.

Location of the iron-sulfur clusters FA and FB in photosystem I: an electron paramagnetic resonance study of spin relaxation enhancement of P700+

Photosystem I (PS I) mediates electron-transfer from plastocyanin to ferredoxin via a photochemically active chlorophyll dimer (P700), a monomeric chlorophyll electron acceptor (A0), a phylloquinone (A1), and three [4Fe-4S] clusters (FX/A/B). The sequence of electron-transfer events between the iron-sulfur cluster, FX, and ferredoxin is presently unclear. Owing to the presence of a 2-fold symmetry in the PsaC protein to which the iron-sulfur clusters F(A) and F(B) are bound, the spatial arrangement of these cofactors with respect to the C2-axis of symmetry in PS I is uncertain as well. An unequivocal determination of the spatial arrangement of the iron-sulfur clusters FA and FB within the protein is necessary to unravel the complete electron-transport chain in PS I. In the present study, we generate EPR signals from charge-separated spin pairs (P700+-FredX/A/B) in PS I and characterize them by progressive microwave power saturation measurements to determine the arrangement of the iron-sulfur clusters FX/A/B relative to P700. The microwave power at half saturation (P1/2) of P700+ is greater when both FA and FB are reduced in untreated PS I than when only FA is reduced in mercury-treated PS I. The experimental P1/2 values are compared to values calculated by using P700-FA/B crystallographic distances and assuming that either FA or FB is closer to P700+. On the basis of this comparison of experimental and theoretical values of spin relaxation enhancement effects on P700+ in P700+ [4Fe-4S]- charge-separated pairs, we find that iron-sulfur cluster FA is in closer proximity to P700 than the FB cluster.

Role of D1-His190 in the proton-coupled oxidation of tyrosine YZ in manganese-depleted photosystem II

To further characterize the role of D1-His190 in the oxidation of tyrosine Y(Z) in photosystem II, the pH dependence of P(680)(*)()(+) reduction was measured in H190A and Mn-depleted wild-type PSII particles isolated from the cyanobacterium, Synechocystis sp. PCC 6803. Measurements were conducted in the presence and absence of imidazole and other small organic bases. In H190A PSII particles, rapid reduction of P(680)(*)()(+) attributed to electron transfer from Y(Z) increased dramatically above pH 9, with an apparent pK(A) of approximately 10.3. In the presence of ethanolamine and imidazole, this dramatic increase occurred at lower pH values, with the efficiency of Y(Z) oxidation correlating with the solution pK(A) value of the added base. We conclude that the pK(A) of Y(Z) is approximately 10.3 in D1-H190A PSII particles. In Mn-depleted wild-type PSII particles, P(680)(*)()(+) reduction was accelerated by all exogenous bases examined (substituted imidazoles, histidine, Tris, and 1,4-diazabicyclo[2.2.2]octane). We conclude that Y(Z) is solvent accessible in Mn-depleted wild-type PSII particles and that its pK(A) is near that of tyrosine in solution. In Mn-depleted wild-type PSII particles, over 80% of the kinetics of P(680)(*)()(+) reduction after a flash could be described by three kinetic components. The individual rate constants of these components varied slightly with pH, but their relative proportions varied dramatically with pH, showing apparent pK(A) values of 7.5 and 6.25 (6.9 and 5.8 in the presence of Ca(2+) and Mg(2+) ions). An additional pK(A) value (pK(A) < 4.5) may also be present. To describe these data, we propose (1) the pK(A) of His190 is 6.9-7.5, depending on buffer ions, (2) the deprotonation of Y(Z) is facilitated by the transient formation of a either a hydrogen bond or a hydrogen-bonded water bridge between Y(Z) and D1-His190, and (3) when protonated, D1-His190 interacts with nearby residues having pK(A) values near 6 and 4. Because Y(Z) and D1-His190 are located near the Mn cluster, these residues may interact with the Mn cluster in the intact system.

Photosystem I is indispensable for photoautotrophic growth, CO2 fixation, and H2 photoproduction in Chlamydomonas reinhardtii

Certain Chlamydomonas reinhardtii mutants deficient in photosystem I due to defects in psaA mRNA maturation have been reported to be capable of CO2 fixation, H2 photoevolution, and photoautotrophic growth (Greenbaum, E., Lee, J. W., Tevault, C. V., Blankinship, S. L. , and Mets, L. J. (1995) Nature 376, 438-441 and Lee, J. W., Tevault, C. V., Owens, T. G.; Greenbaum, E. (1996) Science 273, 364-367). We have generated deletions of photosystem I core subunits in both wild type and these mutant strains and have analyzed their abilities to grow photoautotrophically, to fix CO2, and to photoevolve O2 or H2 (using mass spectrometry) as well as their photosystem I content (using immunological and spectroscopic analyses). We find no instance of a strain that can perform photosynthesis in the absence of photosystem I. The F8 strain harbored a small amount of photosystem I, and it could fix CO2 and grow slowly, but it lost these abilities after deletion of either psaA or psaC; these activities could be restored to the F8-psaADelta mutant by reintroduction of psaA. We observed limited O2 photoevolution in mutants lacking photosystem I; use of 18O2 indicated that this O2 evolution is coupled to O2 uptake (i.e. respiration) rather than CO2 fixation or H2 evolution. We conclude that the reported instances of CO2 fixation, H2 photoevolution, and photoautotrophic growth of photosystem I-deficient mutants result from the presence of unrecognized photosystem I.

The cysteine-proximal aspartates in the Fx-binding niche of photosystem I. Effect of alanine and lysine replacements on photoautotrophic growth, electron transfer rates, single-turnover flash efficiency, and EPR spectral properties

The FX electron acceptor in Photosystem I (PS I) is a highly electronegative (Em = -705 mV) interpolypeptide [4Fe-4S] cluster ligated by cysteines 556 and 565 on PsaB and cysteines 574 and 583 on PsaA in Synechocystis sp. PCC 6803. An aspartic acid is adjacent to each of these cysteines on PsaB and adjacent to the proline-proximal cysteine on PsaA. We investigated the effect of D566PsaB and D557PsaB on electron transfer through FX by changing each aspartate to the neutral alanine or to the positively charged lysine either singly (D566APsaB, D557APsaB, D566KPsaB, and D557KPsaB) or in pairs (D557APsaB/D566APsaB and D557KPsaB/D566APsaB). All mutants except for D557KPsaB/D566APsaB grew photoautotrophically, but the growth of D557KPsaB and D557APsaB/D566APsaB was impaired under low light. The doubling time was increased, and the chlorophyll content per cell was lower in D557KPsaB and D557APsaB/D566APsaB relative to the wild type and the other mutants. Nevertheless, the rates of NADP+ photoreduction in PS I complexes from all mutants were no less than 75% of that of the wild type. The kinetics of back-reaction of the electron acceptors on a single-turnover flash showed efficient electron transfer to the terminal acceptors FA and FB in PS I complexes from all mutants. The EPR spectrum of FX was identical to that in the wild type in all but the single and double D566APsaB mutants, where the high-field resonance was shifted downfield. We conclude that the impaired growth of some of the mutants is related to a reduced accumulation of PS I rather than to photosynthetic efficiency. The chemical nature and the charge of the amino acids adjacent to the cysteine ligands on PsaB do not appear to be significant factors in the efficiency of electron transfer through FX.

Role of D1-His190 in proton-coupled electron transfer reactions in photosystem II: a chemical complementation study

Recent models for water oxidation in photosystem II propose that His190 of the D1 polypeptide facilitates electron transfer from tyrosine YZ to P680+ by accepting the hydroxyl proton from YZ. To test these models, and to further define the role of D1-His190 in the proton-coupled electron transfer reactions of PSII, the rates of P680+ reduction, YZ oxidation, QA- oxidation, and YZ* reduction were measured in PSII particles isolated from several D1-His190 mutants constructed in the cyanobacterium Synechocystis sp. PCC 6803. These measurements were conducted in the absence and presence of imidazole and other small organic bases. In all mutants examined, the rates of P680+ reduction, YZ oxidation, and YZ* reduction after a single flash were slowed dramatically and the rate of QA- oxidation was accelerated to values consistent with the reduction of P680+ by QA- rather than by YZ. There appeared to be little correlation between these rates and the nature of the residue substituted for D1-His190. However, in nearly all mutants examined, the rates of P680+ reduction, YZ oxidation, and YZ* reduction were accelerated dramatically in the presence of imidazole and other small organic bases (e.g., methyl-substituted imidazoles, histidine, methylamine, ethanolamine, and TRIS). In addition, the rate of QA- oxidation was decelerated substantially. For example, in the presence of 100 mM imidazole, the rate of electron transfer from YZ to P680+ in most D1-His190 mutants increased 26-87-fold. Furthermore, in the presence of 5 mM imidazole, the rate of YZ* reduction in the D1-His190 mutants increased to values comparable to that of Mn-depleted wild-type PSII particles in the absence of imidazole. On the basis of these results, we conclude that D1-His190 is the immediate proton acceptor for YZ and that the hydroxyl proton of YZ remains bound to D1-His190 during the lifetime of YZ*, thereby facilitating the reduction of YZ*.

The eight-amino acid internal loop of PSI-C mediates association of low molecular mass iron-sulfur proteins with the P700-FX core in photosystem I

The PSI-C subunit of photosystem I (PS I) shows similarity to soluble 2[4Fe-4S] ferredoxins. PSI-C contains an eight residue internal loop and a 15 residue C-terminal extension which are absent in the ferredoxins. The eight-residue loop has been shown to interact with PSI-A/PSI-B (Naver, H., Scott, M. P., Golbeck, J. H., Moller, B. L., and Scheller, H. V. (1996) J. Biol. Chem. 271, 8996-9001). Four mutant proteins were constructed. Two were modified barley PSI-C proteins, one lacking the loop and the C terminus (PSI-Ccore) and one where the loop replace the C-terminal extension (PSI-CcoreLc-term). Two were modified Clostridium pasteurianum ferredoxins, one with the loop of barley PSI-C and one with both the loop and the C terminus of PSI-C. Wild-type proteins and the mutants were used to reconstitute barley P700-FX cores lacking PSI-C, -D, and-E. Western blotting showed that PSI-CcoreLc-term binds to PS I, whereas PSI-Ccore does not. Without PSI-D the PSI-CcoreLc-term mutant accepts electrons from FX in contrast to PSI-C mutants without the loop. Flash photolysis of P700-FX cores reconstituted with C. pasteurianum ferredoxin showed that only the ferredoxin mutants with the loop accepted electrons from FX. From this, it is concluded that the loop of PSI-C is necessary and sufficient for the association between PS I and PSI-C, and that the loop is functional as an interaction domain even when positioned at the C terminus of PSI-C or on a low molecular mass, soluble ferredoxin.

Electrogenicity accompanies photoreduction of the iron-sulfur clusters F(A) and F(B) in photosystem I

Photovoltage responses accompanying electron transfer on the acceptor side of photosystem I (PS I) were investigated in proteoliposomes containing PS I complexes from the cyanobacterium Synechococcus sp. PCC 6301 using a direct electrometrical technique. The relative contributions of the F(X) --> F(B) and the F(X) --> F(A) electron transfer reactions to the overall electrogenicity were elucidated by comparing the sodium dithionite-induced decrease in the magnitude of the total photoelectric responses in control and in F(B)-less (HgCl2-treated) PS I complexes. The results obtained suggest that the electrogenesis on the acceptor side of PS I is related to electron transfers between both F(X) and F(A) and F(A) and F(B). Based on the electrogenic nature of the latter reaction in PS I complexes, we conclude that F(A) rather than F(B) is the acceptor proximal to F(X).

Structure and properties in solution of PsaD, an extrinsic polypeptide of photosystem I

PsaD is a small, extrinsic polypeptide located on the stromal side (cytoplasmic side in cyanobacteria) of the photosystem I reaction centre complex. The gene from the cyanobacterium Nostoc sp. PCC 8009 was expressed in Escherichia coli and the structure of the recovered protein in solution investigated. Size-exclusion chromatography, dynamic light scattering and measurement of 15N transverse relaxation times showed that the protein is a stable dimer in solution, whereas in the reaction centre complex it is a monomer. NMR experiments showed that the dimer is symmetrical and that there are at least two domains, one structured and the remainder unstructured. The structured domain contains a small amount of beta-sheet. Three-dimensional heteronuclear NMR spectra of [13C, 15N]PsaD showed that the structured domain is associated with the central part of the sequence while the N- and C-terminal regions are mobile. Evidence was obtained for a shift in equilibrium between two slightly different conformational states at about pH 6, and the protein was shown to bind to PsaE preferentially at neutral pH. Addition of trifluoroethanol was shown to induce the formation of a small amount of alpha-helix, and the form present in 30% trifluoroethanol appears to be more closely related to the in situ structure, which has been reported to contain one short helix in crystals [Schubert, W.-D., Klukas, O., Krauss, N., Saenger, W., Fromme, P. & Witt, H. T. (1997) J. Mol. Biol. 272, 741-769]. The significance of these findings for the assembly of the complex is discussed.

PsaC subunit of photosystem I is oriented with iron-sulfur cluster F(B) as the immediate electron donor to ferredoxin and flavodoxin

The PsaC subunit of photosystem I (PS I) binds two [4Fe-4S] clusters, F(A) and F(B), functioning as electron carriers between F(X) and soluble ferredoxin. To resolve the issue whether F(A) or F(B) is proximal to F(X), we used single-turnover flashes to promote step-by-step electron transfer between electron carriers in control (both F(A) and F(B) present) and HgCl2-treated (F(B)-less) PS I complexes from Synechococcus sp. PCC 6301 and analyzed the kinetics of P700+ reduction by monitoring the absorbance changes at 832 nm in the presence of a fast electron donor (phenazine methosulfate (PMS)). In control PS I complexes exogenously added ferredoxin, or flavodoxin could be photoreduced on each flash, thus allowing P700+ to be reduced from PMS. In F(B)-less complexes, both in the presence and in the absence of ferredoxin or flavodoxin, P700+ was reduced from PMS only on the first flash and was reduced from F(X)- on the following flashes, indicating lack of electron transfer to ferredoxin or flavodoxin. In the F(B)-less complexes, a normal level of P700 photooxidation was detected accompanied by a high yield of charge recombination between P700+ and F(A)- in the presence of a slow donor, 2,6-dichlorophenol-indophenol. This recombination remained the only pathway of F(A)- reoxidation in the presence of added ferredoxin, consistent with the lack of forward electron transfer. F(A)- could be reoxidized by methyl viologen in F(B)-less PS I complexes, although at a concentration two orders of magnitude higher than is required in wild-type PS I complexes, thus implying the presence of a diffusion barrier. The inhibition of electron transfer to ferredoxin and flavodoxin was completely reversed after reconstituting the F(B) cluster. Using rate versus distance estimates for electron transfer rates from F(X) to ferredoxin for two possible orientations of PsaC, we conclude that the kinetic data are best compatible with PsaC being oriented with F(A) as the cluster proximal to F(X) and F(B) as the distal cluster that donates electrons to ferredoxin.

Menaquinone-7 in the reaction center complex of the green sulfur bacterium Chlorobium vibrioforme functions as the electron acceptor A1

Photosynthetically active reaction center complexes were prepared from the green sulfur bacterium Chlorobium vibrioforme NCIMB 8327, and the content of quinones was determined by extraction and high-performance liquid chromatography. The analysis showed a stoichiometry of 1.7 molecules of menaquinone-7/reaction center. No other quinones were detected in the isolated reaction centers, whereas membrane preparations also contained chlorobiumquinone. The possible involvement of quinones in electron transport was investigated by electron paramagnetic resonance (EPR) spectroscopy. A highly anisotropic radical was detected by Q-band EPR spectroscopy in both membranes and isolated reaction centers following dark reduction with sodium dithionite and photoaccumulation at 205 K. At 34 GHz, the EPR spectrum is characterized by a g tensor with gxx = 2.0063, gyy = 2.0052, gzz = 2.0020 and delta B of 0.7 mT, consistent with its identification as a quinone. This spectrum is highly similar in terms of g values and line widths to photoaccumulated A1- in photosystem I of Synechococcus sp. PCC 7002. The results indicate that menaquinone-7 in the green sulfur bacterial reaction center is analogous to phylloquinone in photosystem I.

Electrogenic reduction of the primary electron donor P700+ in photosystem I by redox dyes

The kinetics of reduction of the photo-oxidized primary electron donor P700+ by redox dyes N,N,N',N'-tetramethyl-p-phenylendiamine, 2,6-dichlorophenol-indophenol and phenazine methosulfate was studied in proteoliposomes containing Photosystem I complexes from cyanobacteria Synechocystis sp. PCC 6803 using direct electrometrical technique. In the presence of high concentrations of redox dyes, the fast generation of a membrane potential related to electron transfer between P700 and the terminal iron-sulfur clusters F(A)/F(B) was followed by a new electrogenic phase in the millisecond time domain, which contributes approximately 20% to the overall photoelectric response. This phase is ascribed to the vectorial transfer of an electron from the redox dye to the protein-embedded chlorophyll of P700+. Since the contribution of this electrogenic phase in the presence of artificial redox dyes is approximately equal to that of the phase observed earlier in the presence of cytochrome c6, it is likely that electrogenic reduction of P700+ in vivo occurs due to vectorial electron transfer within RC molecule rather than within the cytochrome c6-P700 complex.

Choline monooxygenase, an unusual iron-sulfur enzyme catalyzing the first step of glycine betaine synthesis in plants: prosthetic group characterization and cDNA cloning

Plants synthesize the osmoprotectant glycine betaine via the route choline --> betaine aldehyde --> glycine betaine. In spinach, the first step is catalyzed by choline monooxygenase (CMO), a ferredoxin-dependent stromal enzyme that has been hypothesized to be an oligomer of identical subunits and to be an Fe-S protein. Analysis by HPLC and matrix-assisted laser desorption ionization MS confirmed that native CMO contains only one type of subunit (Mr 42,864). Determination of acid-labile sulfur and nonheme iron demonstrated that there is one [2Fe-2S] cluster per subunit, and EPR spectral data indicated that this cluster is of the Rieske type--i.e., coordinated by two Cys and two His ligands. A full-length CMO cDNA (1,622 bp) was cloned from spinach using a probe generated by PCR amplification for which the primers were based on internal peptide sequences. The ORF encoded a 440-amino acid polypeptide that included a 60-residue transit peptide. The deduced amino acid sequence included two Cys-His pairs spaced 16 residues apart, a motif characteristic of Rieske-type Fe-S proteins. Larger regions that included this motif also showed some sequence similarity (approximately 40%) to Rieske-type proteins, particularly bacterial oxygenases. Otherwise there was very little similarity between CMO and proteins from plants or other organisms. RNA and immunoblot analyses showed that the expression of CMO in leaves increased several-fold during salinization. We conclude that CMO is a stress-inducible representative of a new class of plant oxygenases.

Strains of synechocystis sp. PCC 6803 with altered PsaC. I. Mutations incorporated in the cysteine ligands of the two [4Fe-4S] clusters FA and FB of photosystem I

Two [4Fe-4S] clusters, FA and FB, function as terminal electron carriers in Photosystem I (PS I), a thylakoid membrane-bound protein-pigment complex. To probe the function of these two clusters in photosynthetic electron transport, site-directed mutants were created in the transformable cyanobacterium Synechocystis sp. PCC 6803. Cysteine ligands in positions 14 or 51 to FB and FA, respectively, were replaced with aspartate, serine, or alanine, and the effect on the genetic, physiological, and biochemical characteristics of PS I complexes from the mutant strains were studied. All mutant strains were unable to grow photoautotrophically, and compared with wild type, mixotrophic growth was inhibited under normal light intensity. The mutant cells supported lower rates of whole-chain photosynthetic electron transport. Thylakoids isolated from the aspartate and serine mutants have lower levels of PS I subunits PsaC, PsaD, and PsaE and lower rates of PS I-mediated substrate photoreduction compared with the wild type. The alanine and double aspartate mutants have no detectable levels PsaC, PsaD, and PsaE. Electron transfer rates, measured by cytochrome c6-mediated NADP+ photoreduction, were lower in purified PS I complexes from the aspartate and serine mutants. By measuring the P700(+) kinetics after a single turnover flash, a large percentage of the backreaction in the aspartate and serine mutants was found to be derived from A1 and FX, indicating an inefficiency at the FX --> FA/FB electron transfer step. The alanine and double aspartate mutants failed to show any backreaction from [FA/FB]-. These results indicate that the various mutations of the cysteine 14 and 51 ligands to FB and FA affect biogenesis and electron transfer differently depending on the type of substitution, and that the effects of mutations on biogenesis and function can be biochemically separated and analyzed.

Strains of Synechocystis sp. PCC 6803 with altered PsaC. II. EPR and optical spectroscopic properties of FA and FB in aspartate, serine, and alanine replacements of cysteines 14 and 51

A psaC deletion mutant of the unicellular cyanobacterium Synechocystis sp. PCC 6803 was utilized to incorporate site-specific amino acid substitutions in the cysteine residues that ligate the FA and FB iron-sulfur clusters in Photosystem I (PS I). Cysteines 14 and 51 of PsaC were changed to aspartic acid (C14DPsaC, C51DPsaC, C14D/C51DPsaC), serine (C14SPsaC, C51SPsaC), and alanine (C14APsaC, C51APsaC), and the properties of FA and FB were characterized by electron paramagnetic resonance spectroscopy and time-resolved optical spectroscopy. The C14DPsaC-PS I and C14SPsaC-PS I complexes showed high levels of photoreduction of FA with g values of 2.045, 1. 944, and 1.852 after illumination at 15 K, but there was no evidence of reduced FB in the g = 2 region. The C51DPsaC-PS I and C51SPsaC-PS I complexes showed low levels of photoreduction of FB with g values of 2.067, 1.931, and 1.881 after illumination at 15 K, but there was no evidence of reduced FA in the g = 2 region. The presence of FB was inferred in C14DPsaC-PS I and C14SPsaC-PS I, and the presence of FA was inferred in C51DPsaC-PS I and C51SPsaC-PS I by magnetic interaction in the photoaccumulated spectra and by the equal spin concentration of the irreversible P700(+) cation generated by illumination at 77 K. Flash-induced optical absorbance changes at 298 K in the presence of a fast electron donor indicate that two electron acceptors function after FX in the four mutant PS I complexes at room temperature. These data suggest that a mixed-ligand [4Fe-4S] cluster is present in the mutant sites of C14X-PS I and C51X-PS I (where X = D or S), but that the proposed spin state of S = 3/2 renders the resonances undetectable in the g = 2 region. The C14APsaC-PS I, C51APsaC-PS I and C14D/C51DPsaC-PS I complexes show only the photoreduction of FX, consistent with the absence of PsaC. These results show that only those PsaC proteins that contain two [4Fe-4S] clusters are capable of assembling onto PS I cores in vivo.

Redox titration of two [4Fe-4S] clusters in the photosynthetic reaction center from the anaerobic green sulfur bacterium Chlorobium vibrioforme

Anaerobic green sulfur bacteria contain photosynthetic reaction centers analogous to photosystem I (PS I) of plants and cyanobacteria. These reaction centers, termed type I, are characterized by the presence of bound iron-sulfur clusters as the terminal electron acceptors. In this work, the iron-sulfur clusters in Chlorobium vibrioforme were studied using selective light-induced reduction protocols, spin quantifications, and chemical redox titrations coupled with EPR detection. Illumination of a dark-frozen sample at 12 K results in the appearance of a spectrum termed signal I. Chemical reduction in darkness at solution potentials between -414 mV and -492 mV results in the appearance of a different spectrum termed signal II. Illumination of these chemically poised samples at 12 K results in the appearance of signal I such that the sum of the intensity of signal I + signal II is nearly constant for every ratio of signal I/signal II. As the solution potential is lowered to -545 mV, the spectrum shifts to yet a third set of resonances, termed signal III. Concomitant with this shift is a loss of low temperature light-induced reduction of signal I. Photoaccumulation of a sample poised at a solution potential of -50 mV results also in the appearance of signal III at nearly the same spin concentration as the chemically reduced sample. Spin quantifications imply that signals I and II are both derived from the reduction of one iron-sulfur cluster, termed center I; signal III is derived from simultaneous reduction of two iron-sulfur clusters, centers I and II. By measuring the EPR signal intensities over a range of solution potentials, centers I and II were shown to have Em (pH 10.0) values of -446 mV and -501 mV, respectively. The observations are consistent with a structural and functional analogy of centers I and II with F(A) and F(B) of PS I.

Near-IR absorbance changes and electrogenic reactions in the microsecond-to-second time domain in Photosystem I

The back-reaction kinetics in Photosystem I (PS I) were studied on the microsecond-to-s time scale in cyanobacterial preparations, which differed in the number of iron-sulfur clusters to assess the contributions of particular components to the reduction of P700+. In membrane fragments and in trimeric P700-FA/FB complexes, the major contribution to the absorbance change at 820 nm (delta A820) was the back-reaction of FA- and/or FB- with lifetimes of approximately 10 and 80 ms (approximately 10% and 40% relative amplitude). The decay of photoinduced electric potential (delta psi) across a membrane with directionally incorporated P700-FA/FB complexes had similar kinetics. HgCl2-treated PS I complexes, which contain FA but no FB, retain both of these kinetic components, indicating that neither can be assigned uniquely to a specific acceptor. These results suggest that FA- reduces P700+ directly and argue for a rapid electron equilibration between FA and FB, which would eliminate their kinetic distinction in a back-reaction. In PsaC-depleted P700-Fx cores, as well as in P700-FA/FB complexes with chemically reduced FA and FB, the major contribution to the delta A820 and the delta psi decay is a biphasic back-reaction of F-X (approximately 400 microseconds and 1.5 ms) with some contribution from A-1 (approximately 10 microseconds and 100 microseconds), the latter of which is variable depending on experimental conditions. The delta A820 decay in a P700-A1 core devoid of all iron-sulfur clusters comprises two phases with lifetimes of 10 microseconds and 130 microseconds (2.7:1 ratio). The biexponential back-reaction kinetics found for each of the electron acceptors may be related to existence of different conformational states of the PS I complex. In all preparations studied, excitation at 532 nm with flash energies exceeding 10 mJ gives rise to formation of antenna 3Chl, which also contributes to delta A820 decay on the tens-of-microsecond time scale. A distinction between delta A820 components related to back-reactions and to 3Chl decay can be made by analysis of flash saturation dependencies and by measurements of kinetics with preoxidized P700.

Modified ligands to FA and FB in photosystem I. Proposed chemical rescue of a [4Fe-4S] cluster with an external thiolate in alanine, glycine, and serine mutants of PsaC

The FB and FA electron acceptors in Photosystem I (PS I) are [4Fe-4S] clusters ligated by cysteines provided by PsaC. In a previous study (Mehari, T., Qiao, F., Scott, M. P., Nellis, D., Zhao, J., Bryant, D., and Golbeck, J. H. (1995) J. Biol. Chem. 270, 28108-28117), we showed that when cysteines 14 and 51 were replaced with serine or alanine, the free proteins contained a S = 1/2, [4Fe-4S] cluster at the unmodified site and a mixed population of S = 1/2, [3Fe-4S] and S = 3/2, [4Fe-4S] clusters at the modified site. We show here that these mutant PsaC proteins can be rebound to P700-FX cores, resulting in fully functional PS I complexes. The low temperature EPR spectra of the C14XPsaC.PS I complexes (where X = S, A, or G) show the photoreduction of a wild-type FA cluster and a modified FB' cluster, the latter with g values of 2.115, 1.899, and 1.852 and linewidths of 110, 70, and 85 MHz. Since neither alanine nor glycine contains a suitable side group, an external thiolate provided by beta-mercaptoethanol has likely been recruited to supply the requisite ligand to the [4Fe-4S] cluster. The EPR spectrum of the C51SPsaC.PS I complex differs from that of the C51APsaC.PS I or C51GPsaC.PS I complexes by the presence of an additional set of resonances, which may be derived from the serine oxygen-ligated cluster. In all other mutant PS I complexes, a wild-type spin-coupled interaction spectrum appears when FA and FB are simultaneously reduced. Single turnover flash studies indicate approximately 50% efficient electron transfer to FA/FB in the C14SPsaC.PS I, C51SPsaC.PS I, C14GPsaC.PS I, and C51GPsaC.PS I mutants and less than 40% in the C14APsaC.PS I and C51APsaC.PS I mutants, compared with approximately 76% in the PS I core reconstructed with wild-type PsaC. These data are consistent with the measurements of the rates of cytochrome c6-NADP+ reductase activity, indicating lower rates in the alanine mutants. It is proposed that the chemical rescue of a [4Fe-4S] cluster with a recruited external thiolate at the modified site allows the mutant PsaC proteins to rebind to PS I and to function in forward electron transfer.

Mutational analysis of photosystem I polypeptides. Role of PsaD and the lysyl 106 residue in the reductase activity of the photosystem I

The ADC4 mutant of the cyanobacterium Synechocystis sp. PCC 6803 was studied to determine the structural and functional consequences of the absence of PsaD in photosystem I. Isolated ADC4 membranes were shown to be deficient in ferredoxin-mediated NADP(+) reduction, even though charge separation between P700 and FA/FB occurred with high efficiency. Unlike the wild type, FB became preferentially photoreduced when ADC4 membranes were illuminated at 15 K, and the EPR line shapes were relatively broad. Membrane fragments oriented in two dimensions on thin mylar films showed that the g tensor axes of FA- and FB- were identical in the ADC4 and wild type strains, implying that PsaC is oriented similarly on the reaction center. PsaC and the FA/FB iron-sulfur clusters are lost more readily from the ADC4 membranes after treatment with Triton X-100 or chaotropic agents, implying a stabilizing role for PsaD. The specific role of Lys106 of PsaD, which can be crosslinked to Glu93 of ferredoxin (Lelong et al. (1994) J. Biol. Chem. 269, 10034-10039), was probed by site-directed mutagenesis. Chemical cross-linking and protease treatment experiments did not reveal any drastic alterations in the conformation of the mutant PsaD proteins. The EPR spectra of FA and FB in membranes of the Lys106 mutants were similar to those of the wild type. Membranes of all Lys106 mutants showed wild type rates of flavodoxin reduction and flavodoxin-mediated NADP+ reduction, but had 10-54% decrease in the ferredoxin-mediated NADP+ reduction rates. This implies that Lys106 is a dispensable component of the docking site on the reducing side of photosystem I and an ionic interaction between Lys106 of PsaD and Glu93 of ferredoxin is not essential for electron transfer to ferredoxin. These results demonstrate that PsaD serves distinct roles in modulating the EPR spectral characteristics of FA and FB, in stabilizing PsaC on the reaction center, and in facilitating ferredoxin-mediated NADP+ photoreduction on the reducing side of photosystem I.

Reconstitution of barley photosystem I with modified PSI-C allows identification of domains interacting with PSI-D and PSI-A/B

The PSI-C subunit of photosystem I shows similarity to soluble 2[4Fe-4S] ferredoxins. Alignment analysis clearly shows that PSI-C contains an 8-residue internal loop and a 15-residue C-terminal extension that are absent in the ferredoxins. The remaining residues in PSI-C are likely to be folded in a way similar to the soluble 2[4Fe-4S] ferredoxins. Two modified PSI-C subunits lacking either the 8-residue loop or 10 residues of the C terminus were expressed in Escherichia coli and used to reconstitute a barley P700-FX core prepared to specifically lack PSI-C, PSI-D, and PSI-E. As shown by EPR spectroscopy, the modified proteins carry two [4Fe-4S] clusters with characteristics similar to those of native PSI-C. Western blot analysis of the reconstituted photosystem I complexes showed that the modified PSI-C proteins bind to the P700-FX core. Flash photolysis revealed that in photosystem I complexes reconstituted in the presence of PSI-D with the C-terminally deleted PSI-C, the FA/FB back-reaction was less efficiently restored than with wild-type PSI-C. The loop-deleted PSI-C was even less efficient. We attribute these differences to altered binding properties of the modified proteins. Comparison of reconstitutions performed in the presence and absence of PSI-D shows that the loop-deleted PSI-C is unable to bind without PSI-D, whereas the C-terminally deleted PSI-C binds only weakly with PSI-D. These results imply that the internal loop of PSI-C interacts with the PSI-A/B heterodimer and that the C terminus of PSI-C interacts with PSI-D.

Modified ligands to FA and FB in photosystem I. I. Structural constraints for the formation of iron-sulfur clusters in free and rebound PsaC

Cysteines 14, 21, 34, 51, or 58 in PsaC of photosystem I (PS I) were replaced with aspartic acid (C21D and C58D), serine (C14S, C34S, and C51S), and alanine (C14A, C34A, and C51A). When free in solution, the C34S and C34A holoproteins contained two S = 1/2 ground state [4Fe-4S] clusters; all other mutant proteins contained [3Fe-4S] clusters and [4Fe-4S] clusters; in addition, there was evidence in C14S, C51S, C14A, and C51A for high spin (S = 3/2) [4Fe-4S] clusters, presumably in the modified site. These findings are consistent with the assignment of C14, C21, C51, and C58, but not C34, as ligands to FA and FB. The [4Fe-4S] clusters in the unmodified sites in C14S, C51S, C14A, and C51A remained highly electronegative, with Em values ranging from -495 to -575 mV. The [3Fe-4S] clusters in the modified sites were driven 400 to 450 mV more oxidizing than the native [4Fe-4S] clusters, with Em values ranging from -98 mV to -171 mV. A C14D/C51D double mutant contains [3Fe-4S] and S = 1/2 [4Fe-4S] clusters, showing that the 3Cys.1Asp motif is also able to accommodate a low spin cubane. When C34S, C34A, C14S, C51S, C14A, and C51A were rebound to P700-FX cores, electron transfer to FA/FB was regained, but functional reconstitution has not yet been achieved for C21D, C58D, or C14D/C51D. These data imply that PsaC requires two iron-sulfur clusters to refold, one of which must be a cubane. Since two [4Fe-4S] clusters are found in all reconstituted PS I complexes, the presence of two cubanes in free PsaC may be a necessary precondition for binding to P700-FX cores.

Modified ligands to FA and FB in photosystem I. II. Characterization of a mixed ligand [4Fe-4S] cluster in the C51D mutant of PsaC upon rebinding to P700-Fx cores

A Photosystem I (PS I) complex reconstituted with PsaC-C51D (aspartate in lieu of cysteine in position 51) shows light-induced EPR signals with g values, line widths, and photoreduction behavior characteristic of FB. Contrary to an earlier report, a [3Fe-4S] cluster was not located in the reconstituted PS I complex. Instead, a second set of resonances with g values of 2.044, 1.942, and 1.853 becomes EPR-visible when the C51D-PS I complex is measured at 4.2 K. This fast relaxing center, termed FA' is likely to represent a [4Fe-4S] cluster in the mixed ligand (3Cys.1Asp) site. Redox studies show that the Em of FA' and FB are -630 mV and -575 mV, respectively. Room temperature optical studies support the presence of two functioning electron acceptors subsequent to Fx, and NADP+ photoreduction rates mediated by ferredoxin and flavodoxin are nearly equivalent to the wild type. In addition to [3Fe-4S] clusters and S = 1/2 ground state [4Fe-4S] clusters, the free PsaC-C51D protein shows resonances near g = 5.5, which may represent a population of high spin (S = 3/2) [4Fe-4S] clusters in the mixed ligand FA' site. Similar to the C14D-PS I mutant complex, it is proposed that the P700-Fx core selectively rebinds those free PsaC-C51D proteins that contain two [4Fe-4S] clusters. These studies show that primary photochemistry and electron transfer rates in PS I are relatively unaffected by the presence of a highly reducing, mixed ligand cluster in the FA' site.

Reconstitution of iron-sulfur center FB results in complete restoration of NADP (+) photoreduction in Hg-treated Photosystem I complexes from Synechococcus sp. PCC 6301

The FB iron-sulfur cluster is destroyed preferentially by treating Photosystem I complexes with HgCl2(Kojima Y, Niinomi Y, Tsuboi S, Hiyama T and Sakurai H (1987) Bot Mag 100: 243-53). When FB is 95% depleted but FAis quantitatively retained in cyanobacterial PS I complexes, the reduction potential of FA remains highly electronegative (Em=-530 mV, n=1), the EPR spectral and spin relaxation properties of FA and FXremain unchanged, but NADP(+) photoreduction rates decline from 552 to 72 μmol mg Chl(-1) h(-1).When FB is reconstituted with FeCl3, Na2S and β-mercaptoethanol, NADP(+)photoreduction rates recover to 528 μmol mg Chl(-1) h(-1). The correlation between the presence of FBand NADP(+) photoreduction provides direct experimental evidence that this iron-sulfur cluster is required for electron throughput from cytochromec 6 to flavodoxin or ferredoxin in Photosystem I.

A mixed-ligand iron-sulfur cluster (C556SPaB or C565SPsaB) in the Fx-binding site leads to a decreased quantum efficiency of electron transfer in photosystem I

The proposed structure of Photosystem I depicts two cysteines on the PsaA polypeptide and two cysteines on the PsaB polypeptide in a symmetrical environment, each providing ligands for the interpolypeptide Fx cluster. We studied the role of Fx in electron transfer by substituting serine for cysteine (C565SPsaB and C556SPsaB), thereby introducing the first example of a genetically engineered, mixed-ligand [4Fe-4S] cluster into a protein. Optical kinetic spectroscopy shows that after a single-turnover flash at 298 K, the contribution of A1- (lifetime of 10 microseconds, 40% of total and lifetime of 100 microseconds, 20% of total) and Fx- (lifetime of 500-800 microseconds, 10-15% of total) to the overall P700+ back reaction have increased in C565SPsaB and C556SPsaB at the expense of the back reaction from [FA/FB]-. The electron paramagnetic resonance spectrum of Fx shows g-values of 2.04, 1.94, and 1.81 in both mutants and a similarly decreased amount of FA and FB reduced at 15 K after a single-turnover flash. These results indicate that the mixed-ligand (3 cysteines, 1 serine) Fx cluster is an inefficient electron carrier, but that a small leak through Fx still permits FA and FB to be reduced quantitatively when the samples are frozen during continuous illumination. The data confirm that Fx is a necessary intermediate in the electron transfer pathway from A1 to FA and FB in Photosystem I.

Spectral and kinetic characterization of electron acceptor A1 in a Photosystem I core devoid of iron-sulfur centers F X, F B and F A

The kinetic and spectroscopic properties of the secondary electron acceptor A1 were determined by flash absorption spectroscopy at room and cryogenic temperatures in a Photosystem I (PS I) core devoid of the iron-sulfur clusters FX, FB and FA. It was shown earlier (Warren, P.V., Golbeck, J.H. and Warden, J.T. (1993) Biochemistry 32: 849-857) that the majority of the flash-induced absorbance increase at 820 nm, reflecting formation of P700(+), decays with a t1/2 of 10 μs due to charge recombination between P700(+) and A1 (-). Following A1 (-) directly around 380 nm, where absorbance changes due to the formation of P700(+) are negligible, two major decay components were resolved in this study with t1/2 of ≈ 10 μs and 110 μs at an amplitude ratio of ≈ 2.5:1. The difference spectra between 340 and 490 nm of the two kinetic phases are highly similar, showing absorbance increases from 340 to 400 nm characteristic of the one-electron reduction of the phylloquinone A1. When measured at 10 K, the flash-induced absorbance changes around 380 nm can be fitted with two decay phases of t1/2 ≈ 15 μs and 150 μs at an amplitude ratio ≈ 1:1. The difference spectra of both kinetic phases from 340 to 400 nm are similar to those determined at 298 K and are therefore attributed to charge recombination in the pair P700(+)A1 (-). These results indicate that the backreaction between P700(+) and A1 (-) is multiphasic when FX, FB and FA are removed, and only slightly temperature dependent in the range of 298 K to 10 K.

Evidence for a mixed-ligand [4Fe-4S] cluster in the C14D mutant of PsaC. Altered reduction potentials and EPR spectral properties of the FA and FB clusters on rebinding to the P700-FX core

PsaC-C14D (cysteine 14 replaced by aspartic acid) contains a [3Fe-4S] and a [4Fe-4S] cluster in the FB and FA sites of the free protein [Yu, L., Zhao, J., Lu, W., Bryant, D. A., & Golbeck, J. H. (1993) Biochemistry 32, 8251-8258]. When PsaC-C14D is rebound to a photosystem I (PS I) core, the g-values of 2.043, 1.939, and 1.853 appear similar to FA in a wild-type PS I complex [Zhao, J. D., Li, N., Warren, P. V., Golbeck, J. H., & Bryant, D. A. (1992) Biochemistry 31, 5093-5099]. The reconstituted PsaC-C14D-PS I complex does not contain a [3Fe-4S] cluster; rather, a set of resonances with a rhombic line shape, a gav of approximately 1.97, and broad line widths indicate the presence of a mixed-ligand [4Fe-4S] cluster, termed FB', in the aspartate site. Both FA and FB' become photoreduced at 15 K, and show an interaction spectrum when reduced within the same reaction center. An electrochemical redox study shows that FA and FB' titrate with midpoint potentials near -600 mV at pH 10.0. Single-turnover flash experiments indicate that FA and FB' function as efficient electron acceptors at room temperature, and NADP+ photoreduction rates are about 70% that of a reconstituted PsaC-PS I complex. A population of S = 3/2, [4Fe-4S] clusters was tentatively identified in the free PsaC-C14D protein by characteristic EPR resonances in the g = 5.3 region.(ABSTRACT TRUNCATED AT 250 WORDS)

Superoxide contributes to the rapid inactivation of specific secondary donors of the photosystem II reaction center during photodamage of manganese-depleted photosystem II membranes

The role of superoxide in the mechanism of photoinactivation of the secondary donors of the reaction center of photosystem II membranes depleted of Mn by extraction with NH2OH plus EDTA (NH2OH/EDTA-PSII) was assessed. EPR analyses (g = 2 region) in continuous light, optical kinetic spectrophotometric analyses of P680+ and Car+, and AT-band emission measurements were made after various durations of weak and strong light treatment of NH2OH/EDTA-PSII in the presence and absence of superoxide dismutase, or of PSII electron acceptors to suppress superoxide formation. Additionally, flash-induced variable fluorescence of chlorophyll a and the capabilities of the membranes of photooxidize Mn2+ (in the presence of H2O2) via a high-affinity site (Km approximately 180 nM) and to carry out the photoactivation of the Mn-cluster were determined. In the absence of any additions to the NH2OH/EDTA-PSII membranes which were highly depleted of Mn, weak light treatment caused rapid (t1/2 approximately 20 s) and parallel losses of (a) the approximately 10 microseconds phase of P680+ reduction, which reflects the TyrZ-->P680+ reaction, (b) the amplitude of chlorophyll a variable fluorescence, (c) the capability to accumulate the TyrZ(+)-radical in continuous light, and (d) the capability to photooxidize Mn2+/H2O2 in continuous light. As reported previously [Blubaugh et al. (1991) Biochemistry 30, 7586-7597], a dark-stable 12-G-wide featureless EPR signal centered at g = 2.004 was formed rapidly during illumination. This signal previously was tentatively identified as a Car+ radical and was suggested to contribute to the quenching of chlorophyll a variable fluorescence and to the slowing of the TyrZ-->P680+ reaction. However, we failed to detect Car+ formation by sensitive optical spectrophotometry and obtained no definable evidence for either a quencher of fluorescence other than P680+ itself or a slowing of the TyrZ-->P680+ reaction. Addition of a saturating concentration (96 units/mL) of superoxide dismutase diminished the rate of photodamage(s) by approximately 30-fold, but did not abolish it completely. Superoxide dismutase similarly suppressed strong light-induced photodamages, causing the loss of capability to photooxidize Mn2+/H2O2, to carry out photoactivation, and to generate the AT-band emission as well as TyrZ+ EPR signal. In contrast to others, we found no evidence that the initial target(s) of photodamage is (are) different in weak versus strong light treatment. The totality of the results suggests that the initial event in either weak light or strong light photodamage of NH2OH/EDTA-PSII is a decoupling of the redox activity of TyrZ from P680.(ABSTRACT TRUNCATED AT 400 WORDS)

Mutational analysis of photosystem I polypeptides in Synechocystis sp. PCC 6803. Subunit requirements for reduction of NADP+ mediated by ferredoxin and flavodoxin

The subunit requirements for NADP+ reduction by photosystem I were assessed in mutants of Synechocystis sp. PCC 6803 created by targeted inactivation of the psaD, psaE, psaF, and psaL genes. The PsaE-less, PsaF-PsaJ-less, and PsaL-less mutants showed normal photoautotrophic growth, while the growth of PsaD-less mutants was slower without glucose. In isolated wild-type membranes, the rate of flavodoxin reduction and flavodoxin-mediated NADP+ reduction were 800 and 480 mumol/mg of chlorophyll/h, respectively. The rate of ferredoxin-mediated NADP+ photoreduction was 460 mumol/mg of chlorophyll/h. There was no diminution in NADP+ photoreduction in membranes isolated from the PsaF-less and PsaL-less mutants. The rates of ferredoxin-mediated NADP+ photoreduction in membranes of the PsaE-less mutants were 25 mumol/mg of chlorophyll/h. However, the rate of flavodoxin reduction was 380 mumol/mg of chlorophyll/h, and that of flavodoxin-mediated NADP+ photoreduction was 170 mumol/mg of chlorophyll/h. PsaD-less membranes showed < 20% of the wild-type rates of flavodoxin-mediated NADP+ photoreduction, but were completely deficient in ferredoxin-mediated NADP+ photoreduction. Therefore, the roles of PsaE and PsaD are more crucial for "docking" of ferredoxin than of flavodoxin. Proteolysis studies showed that while PsaD was susceptible to rapid in vitro degradation by thermolysin, the number and sizes of protease-resistant fragments were not affected by the absence of PsaE. Protease accessibility studies further indicated that the C-terminal domain of PsaD is surface-exposed on the n-side. These results suggest that PsaE and the C-terminal domain of PsaD generate the docking site for the electron acceptors of photosystem I.

Iron-sulfur centers in the photosynthetic reaction center complex fromChlorobium vibrioforme. Differences from and similarities to the iron-sulfur centers in Photosystem I

The photosynthetic reaction center complex from the green sulfur bacteriumChlorobium vibrioforme has been isolated under anaerobic conditions. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis reveals polypeptides with apparent molecular masses of 80, 40, 30, 18, 15, and 9 kDa. The 80- and 18-kDa polypeptides are identified as the reaction center polypeptide and the secondary donor cytochromec 551 encoded by thepscA andpscC genes, respectively. N-terminal amino acid sequences identify the 40-kDa polypeptide as the bacteriochlorophylla-protein of the baseplate (the Fenna-Matthews-Olson protein) and the 30-kDa polypeptide as the putative 2[4Fe-4S] protein encoded bypscB. Electron paramagnetic resonance (EPR) analysis shows the presence of an iron-sulfur cluster which is irreversibly photoreduced at 9K. Photoaccumulation at higher temperature shows the presence of an additional photoreduced cluster. The EPR spectra of the two iron-sulfur clusters resemble those of FA and FB of Photosystem I, but also show significantly differentg-values, lineshapes, and temperature and power dependencies. We suggest that the two centers are designated Center I (with calculatedg-values of 2.085, 1.898, 1.841), and Center II (with calculatedg-values of 2.083, 1.941, 1.878). The data suggest that Centers I and II are bound to thepscB polypeptide.

PsaE Is Required for in Vivo Cyclic Electron Flow around Photosystem I in the Cyanobacterium Synechococcus sp. PCC 7002

Electron transfer rates to P700+ have been determined in wild-type and three interposon mutants (psaE-, ndhF-, and psaE- ndhF-) of Synechococcus sp. PCC 7002. All three mutants grew significantly more slowly than wild type at low light intensities, and each failed to grow photoheterotrophically in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) and a metabolizable carbon source. The kinetics of P700+ reduction were similar in the wild-type and mutant whole cells in the absence of DCMU. In the presence of DCMU, the P700+ reduction rate in the psaE mutant was significantly slower than in the wild type. In the presence of DCMU and potassium cyanide, added to inhibit the outflow of electrons through cytochrome oxidase, P700+ reduction rates increased for both the psaE- and ndhF- strains. The reduction rates for these two mutants were nonetheless slower than that observed for the wild-type strain. The further addition of methyl viologen caused the rate of P700+ reduction in the wild type to become as slow as that for the psaE mutant in the absence of methyl viologen. Given the ability of methyl viologen to intercept electrons from the acceptor side of photosystem I, this response reveals a lesion in cyclic electron flow in the psaE mutant. In the presence of DCMU, the rate of P700+ reduction in the psaE ndhF double mutant was very slow and nearly identical with that for the wild-type strain in the presence of 2,4-dibromo-3-methyl-6-isopropyl-p-benzoquinone, a condition under which physiological electron donation to P700+ should be completely inhibited. These results suggest that NdhF- and PsaE-dependent electron donation to P700+ occurs only via plastoquinone and/or cytochrome b6/f and indicate that there are three major electron sources for P700+ reduction in this cyanobacterium. We conclude that, although PsaE is not required for linear electron flow to NADP+, it is an essential component in the cyclic electron transport pathway around photosystem I.

Characterization of the [3Fe-4S] and [4Fe-4S] clusters in unbound PsaC mutants C14D and C51D. Midpoint potentials of the single [4Fe-4S] clusters are identical to FA and FB in bound PsaC of photosystem I

In a previous paper we showed that the C51D mutant of PsaC contains a [3Fe-4S] cluster in the FA site and a [4Fe-4S] cluster in the FB site and that the C14D mutant contains an uncharacterized cluster in the FB site and a [4Fe-4S] cluster in the FA site [Zhao, J. D., Li, N., Warren, P. V., Golbeck, J. H., & Bryant, D. A. (1992) Biochemistry 31, 5093-5099]. In this paper we describe the electrochemical and electron spin resonance properties of the recombinant C14D and C51D holoproteins after in vitro reinsertion of the iron-sulfur clusters. Unbound PsaC shows no significant resonances in the oxidized state, but the unbound C14D and C51D mutant proteins show an intense set of resonances at g approximately 2.02 and 1.99 characteristic of an oxidized [3Fe-4S]1+/0 cluster. The Em' values for the [3Fe-4S]1+/0 clusters in C14D (FB*) and C51D (FA*) are -98 mV, and both represent one-electron transfers. After reduction with dithionite at pH 10.0, wild-type PsaC shows a broad set of resonances resulting from the superposition of FA- and FB- characterized by a low-field peak at an apparent g value of 2.051 and a high-field trough at an apparent g value of 1.898. The FB resonances in C51D were slightly narrower, with a low-field peak at an apparent g value of 2.039 and high-field trough at an apparent g value of 1.908.(ABSTRACT TRUNCATED AT 250 WORDS)

Cloning and characterization of the psaE gene of the cyanobacterium Synechococcus sp. PCC 7002: characterization of a psaE mutant and overproduction of the protein in Escherichia coli

The psaE gene, encoding a 7.5 kDa peripheral protein of the photosystem I complex, has been cloned and characterized from the cyanobacterium Synechococcus sp. PCC 7002. The gene is transcribed as an abundant monocistronic transcript of approximately 325 nt. The PsaE protein has been overproduced in Escherichia coli, purified to homogeneity, and used to raise polyclonal antibodies. Mutant strains, in which the psaE gene was insertionally inactivated by interposon mutagenesis, were constructed and characterized. Although the PS I complexes of these strains were similar to those of the wild type, the strains grew more slowly under conditions which favour cyclic electron transport and could not grow at all under photoheterotrophic conditions. The results suggest that PsaE plays a role in cyclic electron transport in cyanobacteria.

Targeted inactivation of the gene psaL encoding a subunit of photosystem I of the cyanobacterium Synechocystis sp. PCC 6803

Photosystem I is a multisubunit pigment-protein complex that functions as a light-driven plastocyanin-ferredoxin oxidoreductase in thylakoid membranes of cyanobacteria and higher plants. A 16-kDa protein subunit of photosystem I complex was isolated from the cyanobacterium Synechocystis sp. PCC 6803. The sequence of its NH2-terminal residues was determined and a corresponding oligonucleotide probe was used to isolate the gene encoding this subunit. The gene, designated as psaL, codes for a protein of 16,605 Da. The deduced amino acid sequence is homologous to the subunit PsaL of barley photosystem I. There are two conserved hydrophobic regions in the subunit PsaL that may cross or interact with thylakoid membranes. The gene psaL exists as a single copy in the genome and is expressed as a monocistronic RNA. Stable mutant strains in which the gene psaL was interrupted by a gene conferring resistance to chloramphenicol, were generated by targeted mutagenesis. Growth and photosynthetic characteristics of a selected mutant strain under photoautotrophic conditions were similar to those of the wild type, suggesting that the function of PsaL is dispensable for photosynthesis in Synechocystis sp. PCC 6803. Western analysis and subunit composition of purified photosystem I revealed that the mutant strain contained other subunits of photosystem I in thylakoid membranes and in the assembled complex. When photosystem II activity was inhibited and glucose was supplied in the medium, mutant strains grew faster than the wild type. Under these conditions of growth, re-reduction of P700 in the mutant cells, but not in the wild type cells, showed a component with an uncharacteristically rapid half-time.

Site-directed conversion of cysteine-565 to serine in PsaB of photosystem I results in the assembly of [3Fe-4S] and [4Fe-4S] clusters in Fx. A mixed-ligand [4Fe-4S] cluster is capable of electron transfer to FA and FB

We reported earlier [Smart, L. B., Warren, P. V., Golbeck, J. H., & McIntosh, L. (1993) Proc. Natl. Acad. Sci. U.S.A. 90, 1132-1136] that the site-directed conversion of cysteine-565 to serine (C565S) in PsaB of Synechocystis sp. PCC 6803 leads to an accumulation of photosystem I polypeptides and the low-temperature photoreduction of the terminal electron acceptors FA and FB. In this paper, we report the occurrence of a [3Fe-4S]1 + ,0 cluster in dodecyl maltoside-solubilized photosystem I complexes prepared from the C565S mutant. The [3Fe-4S] cluster is reducible with dithionite at pH 6.5, implying a midpoint potential considerably more oxidizing than either FA or FB. Similar to the behavior of FX, the [3Fe-4S] cluster undergoes partial, reversible photoreduction when the complex is illuminated at 15 K, and complete photoreduction when the sample is illuminated during freezing. Contrary to the result expected in the presence of a relatively high-potential FX, there is significant low-temperature and room temperature photoreduction of FA and FB in the C565S complex. Although the FA and FB resonances are more intense when the complex is frozen during illumination, they still account for < 60% of FA and FB found by chemical reduction. When the FA and FB clusters are prereduced with dithionite at pH 10.0, a new set of resonances appear upon illumination at g = 2.015, 1.941, and 1.811, and disappear on subsequent darkness. The species giving rise to this signal is most likely a mixed-ligand [4Fe-4S]2+,1+ cluster located in the FX site.(ABSTRACT TRUNCATED AT 250 WORDS)

Shared thematic elements in photochemical reaction centers

The structural, functional, and evolutionary relationships between photosystem II and the purple nonsulfur bacterial reaction center have been recognized for several years. These can be classified as "quinone type" (type II) photosystems because the terminal electron acceptor is a mobile quinone molecule. The analogous relationship between photosystem I and the green sulfur bacterial (and helicobacterial) reaction centers has only recently become clear. These can be classified as "iron-sulfur type" (type I) photosystems because the terminal electron acceptor consists of one or more bound iron-sulfur clusters. At a fundamental level, the quinone type and iron-sulfur type reaction centers share a common photochemical motif in the early process of charge separation, leading to the speculation that all photochemical reaction centers have a common evolutionary origin. This review summarizes the current state of knowledge in comparative reaction center biochemistry between prokaryotic bacteria, cyanobacteria, and green plants.