Dynamic Stark Effect in Two-Dimensional Spectroscopy Revealing Modulation of Ultrafast Charge Separation in Bacterial Reaction Centers by an Inherent Electric Field

Despite extensive study, mysteries remain regarding the highly efficient ultrafast charge separation processes in photosynthetic reaction centers (RCs). In this work, transient Stark signals were found to be present in ultrafast two-dimensional electronic spectra recorded for purple bacterial RCs at 77 K. These arose from the electric field that is inherent to the intradimer charge-transfer intermediate of the bacteriochlorophyll pair (P), P_A⁺P_B^-. By comparing three mutated RCs, a correlation was found between the efficient formation of P_A⁺P_B^- and a fast charge separation rate. Importantly, the energy level of P* was changed due to the Stark shift, influencing the driving force for P* → P⁺B_A^- electron transfer and hence its rate. Furthermore, the orientation and amplitude of the inherent electric field varied in different ways upon different mutation, leading to contrasting changes in the rates. This mechanism of modulation provides a solution to a long-lasting inconsistency between experimental observations and activation energy theory.

High-Efficiency Excitation Energy Transfer in Biohybrid Quantum Dot-Bacterial Reaction Center Nanoconjugates

Reaction centers (RCs) are the pivotal component of natural photosystems, converting solar energy into the potential difference between separated electrons and holes that is used to power much of biology. RCs from anoxygenic purple photosynthetic bacteria such as Rhodobacter sphaeroides only weakly absorb much of the visible region of the solar spectrum, which limits their overall light-harvesting capacity. For in vitro applications such as biohybrid photodevices, this deficiency can be addressed by effectively coupling RCs with synthetic light-harvesting materials. Here, we studied the time scale and efficiency of Förster resonance energy transfer (FRET) in a nanoconjugate assembled from a synthetic quantum dot (QD) antenna and a tailored RC engineered to be fluorescent. Time-correlated single-photon counting spectroscopy of biohybrid conjugates enabled the direct determination of FRET from QDs to attached RCs on a time scale of 26.6 ± 0.1 ns and with a high efficiency of 0.75 ± 0.01.

1H, 13C and 15N assignment of the paramagnetic high potential iron-sulfur protein (HiPIP) PioC from Rhodopseudomonas palustris TIE-1

High potential iron-sulfur proteins (HiPIPs) are a class of small proteins (50-100 aa residues), containing a 4Fe-4S iron-sulfur cluster. The 4Fe-4S cluster shuttles between the oxidation states [Fe₄S₄]^3+/2+, with a positive redox potential in the range (500-50 mV) throughout the different known HiPIPs. Both oxidation states are paramagnetic at room temperature. HiPIPs are electron transfer proteins, isolated from photosynthetic bacteria and usually provide electrons to the photosynthetic reaction-center. PioC, the HIPIP isolated from Rhodopseudomonas palustris TIE-1, is the smallest among all known HiPIPs. Despite their small dimensions, an extensive NMR assignment is only available for two of them, because paramagnetism prevents the straightforward assignment of all resonances. We report here the complete NMR assignment of ¹H, ¹³C and ¹⁵N signals for the reduced [Fe₄S₄]²⁺ state of the protein. A set of double and triple resonance experiments performed with standardized parameters/datasets provided the assignment of about 72% of the residues. The almost complete resonance assignment (99.5% of backbone and ca. 90% of side chain resonances) was achieved by combining the above information with those obtained using a second set of NMR experiments, in which acquisition and processing parameters, as well as pulse sequences design, were optimized to account for the peculiar features of this paramagnetic protein.

A new procedure for fast soft staining of BN-PAGEs on photosynthetic complexes

We report a fast and sensitive procedure for blue native PAGE staining, in which the conventional staining step with CBB is avoided. After running, a short exposure to a mix of polar protic solvents (ethanol and acetic acid) leads to a fast and selective removal of the dye from the migration front and a specific binding to the protein bands, while the rest undergo a selective and complete background removal, leading to an intense contrast. This single-step staining-destaining technique is useful in protein samples that bind colored cofactors such as photosystems, which can be selectively discerned by their characteristic green color. After the staining of such samples, the green color persists, while the other unpigmented protein complexes and the molecular standard remain CBB stained, creating a useful reference system for the assignment of the bands. The advantages and chemical basis of this staining procedure are discussed.

Arabidopsis plants grown in the field and climate chambers significantly differ in leaf morphology and photosystem components

BACKGROUND

Plants exhibit phenotypic plasticity and respond to differences in environmental conditions by acclimation. We have systematically compared leaves of Arabidopsis thaliana plants grown in the field and under controlled low, normal and high light conditions in the laboratory to determine their most prominent phenotypic differences.

RESULTS

Compared to plants grown under field conditions, the "indoor plants" had larger leaves, modified leaf shapes and longer petioles. Their pigment composition also significantly differed; indoor plants had reduced levels of xanthophyll pigments. In addition, Lhcb1 and Lhcb2 levels were up to three times higher in the indoor plants, but differences in the PSI antenna were much smaller, with only the low-abundance Lhca5 protein showing altered levels. Both isoforms of early-light-induced protein (ELIP) were absent in the indoor plants, and they had less non-photochemical quenching (NPQ). The field-grown plants had a high capacity to perform state transitions. Plants lacking ELIPs did not have reduced growth or seed set rates, but their mortality rates were sometimes higher. NPQ levels between natural accessions grown under different conditions were not correlated.

CONCLUSION

Our results indicate that comparative analysis of field-grown plants with those grown under artificial conditions is important for a full understanding of plant plasticity and adaptation.

[Ionization energies and infrared spectra studies of histidine using density functional theory]

Histidines provide axial ligands to the primary electron donors in photosynthetic reaction centers (RCs) and play an important role in the protein environments of these donors. In this paper the authors present a systematic study of ionization energies and vibrational properties of histidine using hybrid density functional theory (DFT). All calculations were undertaken by using B3LYP method in combination with four basis sets: 6-31G(d), 6-31G(df, p), 6-31+G(d) and 6-311+G(2d, 2p) with the aim to investigate how the basis sets influence the calculation results. To investigate solvent effects and gain a detailed understanding of marker bands of histidine, the ionization energies of histidine and the vibrational frequencies of histidine which are unlabeled and 13C, 15N, and 2H labeled in the gas phase, CCl4, protein environment, THF and water solution, which span a wide range of dielectric constant, were also calculated. Our results showed that: (1) The main geometry parameters of histidine were impacted by basis sets and mediums, and C2-N3 and N3-C4 bond of imidazole ring of histidine side chain display the maximum bond lengths in the gas phase; (2) single point energies and frequencies calculated were decreased while ionization energies increased with the increasing level of basis sets and diffuse function applied in the same solvent; (3) with the same computational method, the higher the dielectric constant of the solvent used, the lower the ionization energy and vibrational frequency and the higher the intensity obtained. In addition, calculated ionization energy in the gas phase and marker bands of histidine as well as frequency shift upon 13C and 15N labeling at the computationally more expensive 6-311+G(2d, 2p) level are in good agreement with experimental observations available in literatures. All calculations indicated that the results calculated by using higher level basis set with diffuse function were more accurate and closer to the experimental value. In conclusion, the results provide useful information for the further studies of the functional and vibrational properties of chlorophyll-a ligated to histidine residue in photosynthetic reaction center.

Conserved role of proton gradient regulation 5 in the regulation of PSI cyclic electron transport

There are at least two photosynthetic cyclic electron transport (CET) pathways in most C(3) plants: the NAD(P)H dehydrogenase (NDH)-dependent pathway and a pathway dependent upon putative ferredoxin:plastoquinone oxidoreductase (FQR) activity. While the NDH complex has been identified, and shown to play a role in photosynthesis, especially under stress conditions, less is known about the machinery of FQR-dependent CET. Recent studies indicate that FQR-dependent CET is dependent upon PGR5, a small protein of unknown function. In a previous study we found that overexpression of PGR5 causes alterations in growth and development associated with decreased chloroplast development and a transient increase in nonphotochemical quenching (NPQ) after the shift from dark to light. In the current study we examine the spatiotemporal expression pattern of PGR5, and the effects of overexpression of PGR5 in Arabidopsis under a host of light and stress conditions. To investigate the conserved function of PGR5, we cloned PGR5 from a species which apparently lacks NDH, loblolly pine, and overexpressed it in Arabidopsis. Although greening of cotyledons was severely delayed in overexpressing lines under low light, mature plants survived exposure to high light and drought stress better than wild-type. In addition, PSI was more resistant to high light in the PGR5 overexpressors than in wild-type plants, while PSII was more sensitive to this stress. These complex responses corresponded to alterations in linear and cyclic electron transfer, suggesting that over-accumulation of PGR5 induces pleiotropic effects, probably via elevated CET. We conclude that PGR5 has a developmentally-regulated, conserved role in mediating CET.

Computational analysis of photosynthetic systems

The use of various computational techniques for the study of photosynthetic systems is described ranging from genome analysis to density functional simulations of the oxygen evolving complex of PSII. The use of simulations for analyzing protein structures can aid in clarifying ambiguous and incomplete experimental results to identifying underlying rules to create efficient light-initiated charge separation at high efficiency.

Two stereoisomers of spheroidene in the Rhodobacter sphaeroides R26 reaction center: a DFT analysis of resonance Raman spectra

From a theoretical analysis of the resonance Raman spectra of 19 isotopomers of spheroidene reconstituted into the reaction center (RC) of Rhodobacter sphaeroides R26, we conclude that the carotenoid in the RC occurs in two configurations. The normal mode underlying the resonance Raman transition at 1239 cm(-1), characteristic for spheroidene in the RC, has been identified and found to uniquely refer to the cis nature of the 15,15' carbon-carbon double bond. Detailed analysis of the isotope-induced shifts of transitions in the 1500-1550 cm(-1) region proves that, besides the 15,15'-cis configuration, spheroidene in the RC adopts another cis-configuration, most likely the 13,14-cis configuration.

Structure and dynamics of the N-terminal loop of PsbQ from photosystem II of Spinacia oleracea

Infrared and Raman spectroscopy were applied to identify restraints for the structure determination of the 20 amino acid loop between two beta-sheets of the N-terminal region of the PsbQ protein of the oxygen evolving complex of photosystem II from Spinacia oleracea by restraint-based homology modeling. One of the initial models has shown a stable fold of the loop in a 20 ns molecular dynamics simulation that is in accordance with spectroscopic data. Cleavage of the first 12 amino acids leads to a permanent drift in the root means square deviation of the protein backbone and induces major structural changes.

A proteomic approach for investigation of photosynthetic apparatus in plants

The proteome of the photosynthetic apparatus of barley (Hordeum vulgare), obtained by analysis of thylakoids without any previous fractionation, was mapped by native electrophoresis followed by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) as the second dimension two-dimensional-blue native (2-D/BN)/SDS-PAGE). This protocol provided an excellent alternative to the 2-D-isoelectric focusing/sodium dodecyl sulfate-polyacrylamide gel electrophoresis for 2-D separation of the most hydrophobic thylakoid proteins. Monocots and dicots showed significant differences in the first dimension while in the second dimension patterns appeared similar. Identification of each spot was performed by internal peptide primary sequence determination using both nano-electrospray ionization tandem mass spectrometry and, to a lesser extent, peptide mass fingerprinting matrix-assisted laser desorption/ionization-time of flight using MALDI-TOF. This is due in particular to the fact that a limited number of peptides was obtained after trypsin digestion of these highly hydrophobic proteins. A larger number of peptides from hydrophilic intermembrane domains of transmembrane proteins were detected. Despite this, about 70% of the expected proteins were identified, including proteins with grand average of hydropathicity scores higher than 0.5. It is therefore reasonable to assert that protein hydrophobicity is not the limiting factor. Small proteins were not well identified with trypsin digestion. Instead some of these could be identified using acid hydrolysis. The method presented here does not require prefractionation of different thylakoid complexes and consequently gives confidence in comparing the proteome of the photosynthetic apparatus before and after treatment. It thus allows us to understand the molecular mechanisms underlying physiological adaptations of higher plants and to perform screening of photosynthetic mutants.

Large reallocations of carbon, nitrogen, and photosynthetic reductant among phycobilisomes, photosystems, and Rubisco during light acclimation in Synechococcus elongatus strain PCC7942 are constrained in cells under low environmental inorganic carbon

Synechococcus elongatus strain PCC7942 cells were grown in high or low environmental concentrations of inorganic C (high-C(i), low-C(i)) and subjected to a light shift from 50 micromol m(-2) s(-1) to 500 micromol m(-2) s(-1). We quantified photosynthetic reductant (O(2) evolution) and molar cellular contents of phycobilisomes, PSII, PSI, and ribulose-1,5-bisphosphate carboxylase oxygenase (Rubisco) through the light shift. Upon the increase in light, small initial relative decreases in phycobilisomes per cell resulted from near cessation of phycobilisome synthesis and their dilution into daughter cells. Thus, allocation of reductant to phycobilisome synthesis dropped fivefold from pre- to post-light shift. The decrease in phycobilisome synthesis liberated enough material and reductant to allow a doubling of Rubisco and up to a sixfold increase in PSII complexes per cell. Low-C(i) cells had smaller initial phycobilisome pools and upon increased light; their reallocation of reductant from phycobilisome synthesis may have limited the rate and extent of light acclimation, compared to high-C(i) cells. Acclimation to increased light involved large reallocations of C, N, and reductant among different components of the photosynthetic apparatus, but total allocation to the apparatus was fairly stable at ca. 50% of cellular N, and drew 25-50% of reductant from photosynthesis.

Multiple scattering x-ray absorption studies of Zn2+ binding sites in bacterial photosynthetic reaction centers

Binding of transition metal ions to the reaction center (RC) protein of the photosynthetic bacterium Rhodobacter sphaeroides has been previously shown to slow light-induced electron and proton transfer to the secondary quinone acceptor molecule, Q(B). On the basis of x-ray diffraction at 2.5 angstroms resolution a site, formed by AspH124, HisH126, and HisH128, has been identified at the protein surface which binds Cd(2+) or Zn(2+). Using Zn K-edge x-ray absorption fine structure spectroscopy we report here on the local structure of Zn(2+) ions bound to purified RC complexes embedded into polyvinyl alcohol films. X-ray absorption fine structure data were analyzed by combining ab initio simulations and multiparameter fitting; structural contributions up to the fourth coordination shell and multiple scattering paths (involving three atoms) have been included. Results for complexes characterized by a Zn to RC stoichiometry close to one indicate that Zn(2+) binds two O and two N atoms in the first coordination shell. Higher shell contributions are consistent with a binding cluster formed by two His, one Asp residue, and a water molecule. Analysis of complexes characterized by approximately 2 Zn ions per RC reveals a second structurally distinct binding site, involving one O and three N atoms, not belonging to a His residue. The local structure obtained for the higher affinity site nicely fits the coordination geometry proposed on the basis of x-ray diffraction data, but detects a significant contraction of the first shell. Two possible locations of the second new binding site at the cytoplasmic surface of the RC are proposed.

The reaction center H subunit is not required for high levels of light-harvesting complex 1 in Rhodospirillum rubrum mutants

The gene (puhA) encoding the H subunit of the reaction center (RC) was deleted by site-directed interposon mutagenesis by using a kanamycin resistance cassette lacking transcriptional terminators to eliminate polar effects in both the wild-type strain Rhodospirillum rubrum S1 and the carotenoid-less strain R. rubrum G9. The puhA interposon mutants were incapable of photoheterotrophic growth but grew normally under aerobic chemoheterotrophic conditions. Absorption spectroscopy and sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed that the RCs were absent. In minimal medium and also in modified medium containing succinate and fructose, the light-harvesting 1 complex (LH1) levels of the S1-derived mutants were about 70 to 100% of the wild-type levels in the same media. The correct assembly of LH1 in the membrane and the pigment-pigment interaction were confirmed by near-infrared circular dichroism spectroscopy. LH1 formation was almost absent when the carotenoid-less G9-derived puhA mutants were grown in standard minimal medium, suggesting that carotenoids may stabilize LH1. In the fructose-containing medium, however, the LH1 levels of the G9 mutants were 70 to 100% of the parental strain levels. Electron micrographs of thin sections of R. rubrum revealed photosynthetic membranes in all mutants grown in succinate-fructose medium. These studies indicate that the H subunit of the RC is necessary neither for maximal formation of LH1 nor for photosynthetic membrane formation but is essential for functional RC assembly.

Extraction of extracellular polymeric substances from the photosynthetic bacterium Rhodopseudomonas acidophila

Among the four methods for extracting extracellular polymeric substances (EPS) from Rhodopseudomonas acidophila (EDTA, NaOH, H(2)SO(4), heating/centrifugation), EDTA extraction was found to be the most effective. The contents of the major components of EPS from R. acidophila, i.e., carbohydrate, protein and nucleic acid, were 6.5, 58.4 and 5.4 mg g(-1) dry cells, respectively. The optimum extraction time was 1-3 h and the EDTA dosage was approximately 2.8 g g(-1) dry cells. Under these conditions, no cell lysis was observed. The EPS content and the percentage of the three main components were greatly dependent on the extraction method. The intensity of absorption peaks for photosynthetic pigments in the UV-visible spectrum of bacteria remained unchanged prior to and after EDTA extraction; and no pigment peaks appeared in the EPS spectrum. This suggests that few cells were destroyed and lysis did not occur. UV-visible spectrum analysis, an easy and rapid technique, could be used to monitor cell lysis during EPS extraction from R. acidophila.

pH-sensitive fluorescent dye as probe for proton uptake in photosynthetic reaction centers

Isolated and purified reaction centers (RC) from Rhodobacter sphaeroides R-26.1 were solubilised in detergent with excess quinone and external electron donors and illuminated in the presence of pyranine. The pH change accompanying the reaction center photocycle was monitored by recording the variation of the pyranine fluorescence intensity. Using Q(B)-depleted reaction centers or blocking the photocycle with terbutryne strongly reduced the pH change. The usefulness and limits of this technique in monitoring the pH changes during the RC photocycle are also discussed.

Intact mass measurements for unequivocal identification of hydrophobic photosynthetic photosystems I and II antenna proteins

The purpose of this study was to determine the optimal experimental conditions necessary to allow rapid and accurate identification of highly hydrophobic proteins, such as the antenna proteins from photosystems I and II. The antenna proteins were derived from two different species, tomato and Arabidopsis, whose photosynthetic genome is well known. The separation techniques included sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) followed by immunoblotting, microamino acid sequencing, reversed-phase high-performance liquid chromatography (RP-HPLC), intact mass measurements, and peptide mass fingerprinting by mass spectrometry. Immunoblotting was time-consuming, and success was limited due to cross-reactivity between these highly conserved sequences. The best results by far were achieved by separating intact proteins through hyphenation of reversed-phase liquid chromatography on-line to an electrospray ionization mass spectrometer (ESI-MS) and consequently identifying individual proteins from their intact mass measurements (IMMs), whereas peptide mass fingerprinting was hampered by the highly hydrophobic nature of these proteins. RP-HPLC-ESI-MS was superior in the quality of separation. Moreover, the high quality of mass spectra recorded during the RP-HPLC-ESI-MS analysis meant that the relative deviations of the molecular masses determined with a quadrupole ion trap mass analyzer ranged between 100 and 300 ppm. Thus, the correspondence between the intact mass values measured with those deduced from the DNA sequences allowed the different types of antenna proteins to be identified and assigned to their corresponding gene families. By utilizing this correlation, it was possible to spot gene products of previously cloned genes.

The menD and menE homologs code for 2-succinyl-6-hydroxyl-2,4-cyclohexadiene-1-carboxylate synthase and O-succinylbenzoic acid-CoA synthase in the phylloquinone biosynthetic pathway of Synechocystis sp. PCC 6803

The genome of the cyanobacterium Synechocystis sp. PCC 6803 contains genes identified as menD and menE, homologs of Escherichia coli genes that code for 2-succinyl-6-hydroxyl-2,4-cyclohexadiene-1-carboxylate (SHCHC) synthase and O-succinylbenzoic acid-CoA ligase in the menaquinone biosynthetic pathway. In cyanobacteria, the product of this pathway is 2-methyl-3-phytyl-1,4-naphthoquinone (phylloquinone), a molecule used exclusively as an electron transfer cofactor in Photosystem (PS) I. The menD(-) and menE(-) strains were generated, and both were found to lack phylloquinone. Hence, no alternative pathways exist in cyanobacteria to produce O-succinylbenzoyl-CoA. Q-band EPR studies of photoaccumulated quinone anion radical and optical kinetic studies of the P700(+) [F(A)/F(B)](-) backreaction indicate that in the mutant strains, plastoquinone-9 functions as the electron transfer cofactor in the A(1) site of PS I. At a light intensity of 40 microE m(-2) s(-1), the menD(-) and menE(-) mutant strains grew photoautotrophically and photoheterotrophically, but with doubling times slower than the wild type. Both of which are sensitive to high light intensities. Low-temperature fluorescence studies show that in the menD(-) and menE(-) mutants, the ratio of PS I to PS II is reduced relative to the wild type. Whole-chain electron transfer rates in the menD(-) and menE(-) mutant cells are correspondingly higher on a chlorophyll basis. The slower growth rate and high-light sensitivity of the menD(-) and menE(-) mutants are therefore attributed to a lower content of PS I per cell.

Elimination of high-light-inducible polypeptides related to eukaryotic chlorophyll a/b-binding proteins results in aberrant photoacclimation in Synechocystis PCC6803

The hli genes, present in cyanobacteria, algae and vascular plants, encode small proteins [high-light-inducible polypeptides (HLIPs)] with a single membrane-spanning alpha-helix related to the first and third helices of eukaryotic chlorophyll a/b-binding proteins. The HLIPs are present in low amounts in low light and they accumulate transiently at high light intensities. We are investigating the function of those polypeptides in a Synechocystis PCC6803 mutant lacking four of the five hli genes. Growth of the quadruple hli mutant was adversely affected by high light intensities. The most striking effect of the quadruple hli mutation was an alteration of cell pigmentation. Pigment changes associated with cell acclimation to increasing light intensity [i.e. decrease in light-harvesting pigments, accumulation of the carotenoid myxoxanthophyll and decrease in photosystem I (PSI)-associated chlorophylls] were strongly exacerbated in the quadruple hli mutant, resulting in yellowish cultures that bleached in high light and died as light intensities exceeded (>500 micromol photon m(-2) s(-1)). However, these pigment changes were not associated with an inhibition of photosynthesis, as probed by in vivo chlorophyll fluorescence, photoacoustic and O(2)-evolution measurements. On the contrary, the HLIP deficiency was accompanied by a stimulation of the photochemical activity, especially in high-light-grown cells. Western blot analyses revealed that the PSI reaction center level (PsaA/B) was noticeably reduced in the quadruple hli mutant relative to the wild type, whereas the abundance of the PSII reaction center protein D1 was comparatively little affected. The hli mutations did not enhance photoinhibition and photooxidation when cells were exposed over a short term to a very high light intensity. Together, the results of this study indicate that HLIPs are critical in the adaptation of the cyanobacterium to variations in light intensity. The data are consistent with the idea that HLIPs are involved, through a direct or indirect means, in nonphotochemical dissipation of absorbed light energy.

Proteomics of light-harvesting proteins in different plant species. Analysis and comparison by liquid chromatography-electrospray ionization mass spectrometry. Photosystem II

An overview of the intact molecular masses and the hydrophobic properties of the photosystem II (PSII) light-harvesting proteins in 14 different plant species is presented. The protein separation and identification was achieved by means of reversed-phase high-performance liquid chromatography-electrospray ionization-mass spectrometry. The good correspondence of the molecular masses measured by reversed-phase high-performance liquid chromatography-electrospray ionization-mass spectrometry with those deduced from the DNA sequence (0.008%-0.016% relative deviation in Arabidopsis) enabled the identification of the different protein types. Utilizing this correlation, it was possible in several cases to spot a gene product for the previously cloned genes. In PSII, all antenna proteins show hydrophobic properties considerably different within the same as well as among various species, in contrast to observations made previously with PSI. These differences might reflect a tuning of protein-protein interactions that play a role in inducing different supramolecular organizations of PSII: within the same species as a consequence of short-term adaptations, and among species for seasonal species adaptation. The relative antenna stoichiometry was readily established on the basis of relative peak areas of the separated proteins in the ultraviolet chromatograms. The correspondence found between the high copy number of genes with the gene products reveals that the genes are not silent in their protein expression. Moreover, the high copy number of gene products as well as protein heterogeneity observed in PSII suggest a possible plant strategy to realize the high degree of organization and interconnection of the light-harvesting systems under any environmental conditions.

The abundance of minor chlorophyll a/b-binding proteins CP29 and LHCI of barley (Hordeum vulgare L.) during leaf senescence is controlled by light

The abundance of the minor light-harvesting complexes CP29 and LHCI generally declines during the senescence of barley leaves. When light intensity declined due to clouding during the senescence of flag leaves from barley plants grown under field conditions, the levels of both light-harvesting complexes temporarily increased in parallel with photosystem II-efficiency [F(v)/F(m)]. A sudden shift from high light conditions to low light conditions during the growth of barley plants in a growth chamber also resulted in an increase in the abundance of minor light-harvesting complexes and a parallel increase in F(v)/F(m) as well as in the chlorophyll a+b-content of senescing primary foliage leaves. Northern blot analyses with a cDNA probe specific for the barley Lhcb4 gene encoding CP29 showed that the light-dependent changes in the abundance of CP29 during senescence are paralleled by corresponding changes in the transcript level. The results indicate that adjustments of the levels of minor light-harvesting complexes during senescence under high light conditions may serve in the prevention of photo-oxidative damage to the photosynthetic reaction centres and under low light in ensuring efficient photosynthesis of the residual photosynthetic reaction centres.

Light-dependent induction of strongly increased microalgal growth by methanol

Low methanol concentrations (about 0.5% v/v) induce biomass production in cultures of the unicellular green alga Scenedesmus obliquus by more than 300%, compared to controls without this solvent. This effect on the microalgal growth was found to be dependent on the solvent concentration, the packed cell volume (PCV), light intensity and light quality. It could be shown that methanol addition leads to a decrease in size of the light harvesting complex (LHC) on the basis of chlorophylls and proteins, and thus to changes in structure and functioning of the photosynthetic apparatus. These alterations lead to enhanced photosynthesis and respiration rates. The action of methanol on the photosynthetic apparatus is thus comparable to the effect of enhanced CO(2) concentrations. These findings support the previously proposed pathway for methanol metabolization with CO(2) as the final product. We conclude that the subsequent assimilation of the increased CO(2) amounts by the Calvin-Benson cycle is a possible explanation for the methanol-mediated increase in biomass production in terms of PCV. The methanol effect is observed only in the light and in the presence of a functioning photosynthetic apparatus. Preliminary action spectra suggest that the primary photoreceptor is a chlorophyll-protein complex with two absorption maxima at 680 and 430 nm, which may possibly be attributed to the reaction center of photosystem II (PSII).

Proteomic analysis of a highly active photosystem II preparation from the cyanobacterium Synechocystis sp. PCC 6803 reveals the presence of novel polypeptides

A highly active oxygen-evolving photosystem II (PSII) complex was purified from the HT-3 strain of the widely used cyanobacterium Synechocystis sp. PCC 6803, in which the CP47 polypeptide has been genetically engineered to contain a polyhistidine tag at its carboxyl terminus [Bricker, T. M., Morvant, J., Masri, N., Sutton, H. M., and Frankel, L. K. (1998) Biochim. Biophys. Acta 1409, 50-57]. These purified PSII centers had four manganese atoms, one calcium atom, and two cytochrome b(559) hemes each. Optical absorption and fluorescence emission spectroscopy as well as western immunoblot analysis demonstrated that the purified PSII preparation was devoid of any contamination with photosystem I and phycobiliproteins. A comprehensive proteomic analysis using a system designed to enhance resolution of low-molecular-weight polypeptides, followed by MALDI mass spectrometry and N-terminal amino acid sequencing, identified 31 distinct polypeptides in this PSII preparation. We propose a new nomenclature for the polypeptide components of PSII identified after PsbZ, which proceeds sequentially from Psb27. During this study, the polypeptides PsbJ, PsbM, PsbX, PsbY, PsbZ, Psb27, and Psb28 proteins were detected for the first time in a purified PSII complex from Synechocystis 6803. Five novel polypeptides were also identified in this preparation. They included the Sll1638 protein, which shares significant sequence similarity to PsbQ, a peripheral protein of PSII that was previously thought to be present only in chloroplasts. This work describes newly identified proteins in a highly purified cyanobacterial PSII preparation that is being widely used to investigate the structure, function, and biogenesis of this photosystem.

Advantages of quantitative affinity chromatography for the analysis of solute interaction with membrane proteins

The use of membrane proteins as chromatographic stationary phases for the quantitation of biospecific interaction between the proteins and solutes is reviewed. This method is one among the few where a membrane protein is immobilized for repeated analyses of solute binding. To our knowledge, five transmembrane proteins have been immobilized in chromatographic matrices: the glucose and nucleoside transporters from human red blood cells, the photosynthetic reaction center from Rhodobacter sphaeroides, the nicotinic acetylcholine receptor from rat brain and a recombinant P-glycoprotein. Proteoliposomes and membrane vesicles have thereby been entrapped in size-exclusion beads, such as Superdex 200, and membrane proteins have been adsorbed on 'immobilized artificial membrane' monolayers of lipid analogs grafted to silica beads. Encouragingly, immobilized glucose transporter and P-glycoprotein showed constant interactant affinities for months. Analysis is done in the frontal mode at equilibrium because there is no separation between bound and free ligand. Both the affinity constant, which generally coincides with the corresponding constant determined by use of nonchromatographic methods, and the amount of active binding sites are obtained. The method has been successfully applied to functional analysis of membrane proteins in cells or reconstituted in lipid mono- or bilayers, screening of low-molecular interactants, investigation of protein-protein interaction and studies of effects of physico-chemical parameters on solute-protein interaction. The analyses require sensitive detection of the analyte and matching between amount of binding sites and affinity.

[Specific density of leaf as a characteristic of the photosynthetic apparatus]

At early stages of ontogeny (up to 50-60% of the maximum leaf area) of wheat (Triticum aestivum L.), meadow fescue (Festuca pratensis Huds.), reed fescue (F. arindinacea Schreb.), and sugar beet (Beta vulgaris L. var. saccharifera (Alef) Krass), there is correlation between changes in the specific leaf density (SLD), rate of photosynthetic CO2 assimilation; activity of the key photosynthetic enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco, EC 4.1.1.39); and concentration of chlorophyll (Chl) a, Chl b, carotenoids, and soluble leaf proteins. However, there is no correlation of SLD with the activity of phospho(enol)pyruvate carboxylase (EC 4.1.1.31). Senescence of leaves was accompanied by a decrease in the SLD value. Treatment with cytokininomimetics (6-benzylaminopurine and Metribuzin) caused an increase in the SLD value. The specific leaf density is suggested to be a structural and functional characteristic of the photosynthetic apparatus of agricultural plants.

Overexpression of chlorophyllide a oxygenase (CAO) enlarges the antenna size of photosystem II in Arabidopsis thaliana

The light-harvesting efficiency of a photosystem is thought to be largely dependent on its photosynthetic antenna size. It has been suggested that antenna size is controlled by the biosynthesis of chlorophyll b. To verify this hypothesis, we overexpressed the enzyme for chlorophyll b biosynthesis, chlorophyllide a oxygenase (CAO), in Arabidopsis thaliana by transforming the plant with cDNA for CAO under the control of the 35S cauliflower mosaic virus promoter. In the early de-etiolation phase, when the intrinsic CAO expression is very low, the chlorophyll a: b ratio was drastically decreased from 28 to 7.3, indicating that enhancement of chlorophyll b biosynthesis had been successfully achieved. We made the following observations in full-green rosette leaves of transgenic plants. (1) The chlorophyll a : b ratio was reduced from 2.85 to 2.65. (2) The ratio of the peripheral light-harvesting complexes (LHCII) to the core antenna complex (CPa) resolved with the green-gel system increased by 20%. (3) The ratio of the light-harvesting complex II apoproteins (LHCP) to 47-kDa chlorophyll a protein (CP47), which was estimated by the results of immunoblotting, increased by 40%. These results indicated that the antenna size increased by at least 10-20% in transgenic plants, suggesting that chlorophyll b biosynthesis controls antenna size. To the best of our knowledge, this is the first report on enlargement of the antenna size by genetic manipulations.

Mode of action of new diethylamines in lycopene cyclase inhibition and in photosystem II turnover

The new bleaching herbicidal compound N,N-diethyl-N-(2-undecynyl)amine (NDUA) is identified here as an inhibitor of lycopene cyclase and is compared with the known cyclase inhibitors N,N-diethyl-N-[2-(4-chlorophenylthio)ethyl]amine (CPTA) and N,N-diethyl-N-[2-(4-methylphenoxy)ethyl]amine (MTPA). HPLC separation of chloroplast pigments shows lycopene accumulation in NDUA treated tissue. Variation in chain length of the undecynylamine moeity of NDUA from 7 to 21 C atoms reveals an optimum of 11 to 14 C atoms for herbicidal activity. A series of seven further analogues of NDUA and CPTA reveals the structural elements necessary for inhibition of lycopene cyclase. The effect of NDUA derivatives on photosynthesis has been studied in Chlamydomonas reinhardtii. Photosynthesis is highly sensitive, particularly towards the C14 and longer chain length analogues at nanomolar concentrations. It is shown that the breakdown of photosynthesis by NDUA is due to interference with the turnover of the D1 protein of the photosystem II reaction centre that requires the continuous biosynthesis of the two reaction-centre beta-carotene moieties in the reassembly phase. The D1 protein disappearance is most marked under strong light conditions. The depletion of photosystem II occurs before total pigment bleaching. This newly recognized mechanism in herbicidal activity is also the basis for the mode of action of other lycopene cyclase inhibitors as well as phytoene desaturase inhibitors.

Identification and localization of a thylakoid-bound carbonic anhydrase from the green algae Tetraedron minimum (Chlorophyta) and Chlamydomonas noctigama (Chlorophyta)

In order to broaden our understanding of the eukaryotic CO2-concentrating mechanism the occurrence and localization of a thylakoid-associated carbonic anhydrase (EC 4.2.1.1) were studied in the green algae Tetraedron minimum and Chlamydomonas noctigama. Both algae induce a CO2-concentrating mechanism when grown under limiting CO2 conditions. Using mass-spectrometric measurements of 18O exchange from doubly labelled CO2, the presence of a thylakoid-associated carbonic anhydrase was confirmed for both species. From purified thylakoid membranes, photosystem I (PSI), photosystem II (PSII) and the light-harvesting complex of the photosynthetic apparatus were isolated by mild detergent gel. The protein fractions were identified by 77 K fluorescence spectroscopy and immunological studies. A polypeptide was found to immunoreact with an antibody raised against thylakoid carbonic anhydrase (CAH3) from Chlamydomonas reinhardtii. It was found that this polypeptide was mainly associated with PSII, although a certain proportion was also connected to light harvesting complex II. This was confirmed by activity measurements of carbonic anhydrase in isolated bands extracted from the mild detergent gel. The thylakoid carbonic anhydrase isolated from T. minimum had an isoelectric point between 5.4 and 4.8. Together the results are consistent with the hypothesis that thylakoid carbonic anhydrase resides within the lumen where it is associated with the PSII complex.

Oxygen yield from single turnover flashes in leaves: non-photochemical excitation quenching and the number of active PSII

O(2) evolution from single turnover flashes of up to 96 micromol absorbed quanta m(-2) and from multiple turnover pulses of 8.6 and 38.6 ms duration and 12800 and 850 micromol absorbed quanta m(-2) s(-1) intensity, respectively, was measured in sunflower leaves with the help of zirconium O(2) analyser. O(2) evolution from one flash could be measured with 1% accuracy on the background of 10-50 micromol O(2) mol(-1). Before the measurements leaves were pre-adapted either at 30-60 or 1700 micromol quanta m(-2) s(-1) to induce different non-photochemical excitation quenching (q(N)). Short (1 min) exposures at the high light that created only energy-dependent, q(E) type quenching, caused no changes in the O(2) yield from saturating flashes or pulses that could be related to the q(E) quenching, but the yield from low intensity flashes and pulses decreased considerably. Long 30-60-min exposures at the high light induced a reversible inhibitory, q(I) type quenching that decreased the O(2) yield from both, saturating and limiting flashes and pulses (but more from the limiting ones), which reversed within 15 min under the low light. The results are in agreement with the notion that q(E) is caused by a quenching process in the PSII antenna and no changes occur in the PSII centres, but the reversible (15-30 min) q(I) quenching is accompanied by inactivation of a part of PSII centres.

Kinetics of photoacclimation in response to a shift to high light of the red alga Rhodella violacea adapted to low irradiance

The unicellular rhodophyte Rhodella violacea can adapt to a wide range of irradiances. To create a light stress, cells acclimated to low light were transferred to higher irradiance and the kinetics of various changes produced by the light shift were analyzed. The proton gradient generated by excess light led to a non-photochemical quenching of the chlorophyll fluorescence and some photoinhibition of photosystem II centers was also produced by the light stress. After the shift to higher irradiance, the mRNA levels of three chloroplast genes that encode phycoerythrin and phycocyanin apoproteins and heme oxygenase (the first enzyme specific to the bilin synthesis) were negatively regulated. A change in the amount of thylakoids and in the total pigment content of the cells occurred during light acclimation after a light stress. The change in the size of the phycobilisome was limited to dissapearance of the terminal phycoerythrin hexamers in some of the rods. The ability of R. violacea to photoacclimate depends both on large changes in thylakoid number and pigment content and on smaller changes in the antenna size of photosystem II.

Proteomics of the chloroplast: systematic identification and targeting analysis of lumenal and peripheral thylakoid proteins

The soluble and peripheral proteins in the thylakoids of pea were systematically analyzed by using two-dimensional electrophoresis, mass spectrometry, and N-terminal Edman sequencing, followed by database searching. After correcting to eliminate possible isoforms and post-translational modifications, we estimated that there are at least 200 to 230 different lumenal and peripheral proteins. Sixty-one proteins were identified; for 33 of these proteins, a clear function or functional domain could be identified, whereas for 10 proteins, no function could be assigned. For 18 proteins, no expressed sequence tag or full-length gene could be identified in the databases, despite experimental determination of a significant amount of amino acid sequence. Nine previously unidentified proteins with lumenal transit peptides are presented along with their full-length genes; seven of these proteins possess the twin arginine motif that is characteristic for substrates of the TAT pathway. Logoplots were used to provide a detailed analysis of the lumenal targeting signals, and all nuclear-encoded proteins identified on the two-dimensional gels were used to test predictions for chloroplast localization and transit peptides made by the software programs ChloroP, PSORT, and SignalP. A combination of these three programs was found to provide a useful tool for evaluating chloroplast localization and transit peptides and also could reveal possible alternative processing sites and dual targeting. The potential of proteomics for plant biology and homology-based searching with mass spectrometry data is discussed.

Revealing the structure of the oxygen-evolving core dimer of photosystem II by cryoelectron crystallography

Here we present cryoelectron crystallographic analysis of an isolated dimeric oxygen-evolving complex of photosystem II (at a resolution of approximately 0.9 nm), revealing that the D1-D2 reaction center (RC) proteins are centrally located between the chlorophyll-binding proteins, CP43 and CP47. This conclusion supports the hypothesis that photosystems I and II have similar structural features and share a common evolutionary origin. Additional density connecting the two halves of the dimer, which was not observed in a recently described CP47-RC complex that did not include CP43, may be attributed to the small subunits that are involved in regulating secondary electron transfer, such as PsbH. These subunits are possibly also required for stabilization of the dimeric photosystem II complex. This complex, containing at least 29 transmembrane helices in its asymmetric unit, represents one of the largest membrane protein complexes studied at this resolution.

Inactivation of the clpP1 gene for the proteolytic subunit of the ATP-dependent Clp protease in the cyanobacterium Synechococcus limits growth and light acclimation

ClpP functions as the proteolytic subunit of the ATP-dependent Clp protease in eubacteria, mammals and plant chloroplasts. We have cloned a clpP gene, designated clpP1, from the cyanobacterium Synechococcus sp. PCC 7942. The monocistronic 591 bp gene codes for a protein 80% similar to one of four putative ClpP proteins in another cyanobacterium, Synechocystis sp. PCC 6803. The constitutive ClpP1 content in Synechococcus cultures was not inducible by high temperatures, but it did rise fivefold with increasing growth light from 50 to 175 micromol photons m(-2) s(-1). A clpP1 inactivation strain (delta clpP1) exhibited slower growth rates, especially at the higher irradiances, and changes in the proportion of the photosynthetic pigments, chlorophyll a and phycocyanin. Many mutant cells (ca. 35%) were also severely elongated, up to 20 times longer than the wild type. The stress phenotype of delta clpP1 when grown at high light was confirmed by the induction of known stress proteins, such as the heat shock protein GroEL and the alternate form of PSII reaction center D1 protein, D1 form 2. ClpP1 content also rose significantly during short-term photoinhibition, but its loss in delta clpP1 did not exacerbate the extent of inactivation of photosynthesis, nor affect the inducible D1 exchange mechanism, indicating ClpP1 is not directly involved in D1 protein turnover.

Electrospray-ionization mass spectrometry of intact intrinsic membrane proteins

Membrane proteins drive and mediate many essential cellular processes making them a vital section of the proteome. However, the amphipathic nature of these molecules ensures their detailed structural analysis remains challenging. A versatile procedure for effective electrospray-ionization mass spectrometry (ESI-MS) of intact intrinsic membrane proteins purified using reverse-phase chromatography in aqueous formic acid/isopropanol is presented. The spectra of four examples, bacteriorhodopsin and its apoprotein from Halobacterium and the D1 and D2 reaction-center subunits from spinach thylakoids, achieve mass measurements that are within 0.01% of calculated theoretical values. All of the spectra reveal lesser quantities of other molecular species that can usually be equated with covalently modified subpopulations of these proteins. Our analysis of bovine rhodopsin, the first ESI-MS study of a G-protein coupled receptor, yielded a complex spectrum indicative of extensive molecular heterogeneity. The range of masses measured for the native molecule agrees well with the range calculated based upon variable glycosylation and reveals further heterogeneity arising from other covalent modifications. The technique described represents the most precise way to catalogue membrane proteins and their post-translational modifications. Resolution of the components of protein complexes provides insights into native protein/protein interactions. The apparent retention of structure by bacteriorhodopsin during the analysis raises the potential of obtaining tertiary structure information using more developed ESI-MS experiments.

Sodium alkyl ether sulfate preparative electrophoresis for the preparation of reaction centers without H-subunit from Rhodopseudomonas viridis

Sodium alkyl ether sulfate (AES), an analog of sodium dodecyl sulfate (SDS) was used in polyacrylamide gel electrophoresis (PAGE) for the partial decomposition of the photosynthetic reaction center (RC) of Rhodopseudomonas viridis. Unlike SDS, AES did not completely dissociate RC into its subunits but selectively detached H-subunit from RC to give RC(-H) without losing the spectroscopic nature of RC. For the denaturation of RC(-H), AES was found to be as mild as 3-[3-cholamidopropyl)dimethylammonio]-l-propanesulfonate (CHAPS).

Isolation and properties of photochemically active reaction center complexes from the green sulfur bacterium Prosthecochloris aestuarii

A new and rapid procedure was developed for the isolation of the reaction center core (RCC)-complex from the green sulfur bacterium Prosthecochloris aestuarii. Reaction center preparations containing the Fenna Matthews Olson (FMO) protein were also obtained. The procedure involved incubation of broken cells with the detergents Triton X-100 and SB12, sucrose gradient centrifugation and hydroxyapatite chromatography. Three different pigment protein complexes were obtained: one containing (about) three FMO trimers per RCC, one with one FMO per RCC and one consisting of RCC only. The last one contained polypeptides with apparent molecular masses of 64 kDa (pscA) and 35 kDa (pscB, the FA/FB, FeS subunit), but no cytochrome. Bacteriochlorophyll a and the chlorophyll a isomer functioning as primary electron acceptor were present at a ratio of 4.8:1. The complexes were also characterized spectroscopically and in terms of photochemical activity, at room temperature as well as at cryogenic temperatures. Illumination caused oxidation of the primary donor P840, with the highest activity in the RCC complex (DeltaA840/A810 = 0.06). At room temperature in the RCC complex essentially all of the P840+ produced in a flash was re-reduced slowly in the dark (several seconds). At low temperatures (150-10 K) a triplet was formed in a fraction of the reaction centers, presumably by a reversal of the charge separation, whereas in others P840+ formed in the light was re-reduced in 40-50 ms.

Molecular genetic analysis suggesting interactions between AppA and PpsR in regulation of photosynthesis gene expression in Rhodobacter sphaeroides 2.4.1

The AppA protein plays an essential regulatory role in development of the photosynthetic apparatus in the anoxygenic phototrophic bacterium Rhodobacter sphaeroides 2.4.1 (M. Gomelsky and S. Kaplan, J. Bacteriol. 177:4609-4618, 1995). To gain additional insight into both the role and site of action of AppA in the regulatory network governing photosynthesis gene expression, we investigated the relationships between AppA and other known regulators of photosynthesis gene expression. We determined that AppA is dispensable for development of the photosynthetic apparatus in a ppsR null background, where PpsR is an aerobic repressor of genes involved in photopigment biosynthesis and puc operon expression. Moreover, all suppressors of an appA null mutation thus far isolated, showing improved photosynthetic growth, were found to contain mutations in the ppsR gene. Because ppsR gene expression in R. sphaeroides 2.4.1 appears to be largely independent of growth conditions, we suggest that regulation of repressor activity occurs predominately at the protein level. We have also found that PpsR functions as a repressor not only under aerobic but under anaerobic photosynthetic conditions and thereby is involved in regulating the abundance of the light harvesting complex II, depending on light intensity. It seems likely therefore, that PpsR responds to an integral signal (e.g., changes in redox potential) produced either by changes in oxygen tension or light intensity. The profile of the isolated suppressor mutations in PpsR is in accord with this proposition. We propose that AppA may be involved in a redox-dependent modulation of PpsR repressor activity.

Severity of mutant phenotype in a series of chlorophyll-deficient wheat mutants depends on light intensity and the severity of the block in chlorophyll synthesis

Analyses of a series of allelic chlorina mutants of wheat (Triticum aestivum L.), which have partial blocks in chlorophyll (Chl) synthesis and, therefore, a limited Chl supply, reinforce the principle that Chl is required for the stable accumulation of Chl-binding proteins and that only reaction centers accumulate when the supply of Chl is severely limited. Depending on the rate of Chl accumulation (determined by the severity of the mutation) and on the rate of turnover of Chl and its precursors (determined by the environment in which the plant is grown), the mutants each reach an equilibrium of Chl synthesis and degradation. Together these mutants generate a spectrum of phenotypes. Under the harshest conditions (high illumination), plants with moderate blocks in Chl synthesis have membranes with very little Chl and Chl-proteins and membrane stacks resembling the thylakoids of the lethal xantha mutants of barely grown at low to medium light intensities (which have more severe blocks). In contrast, when grown under low-light conditions the same plants with moderate blocks have thylakoids resembling those of the wild type. The wide range of phenotypes of Chl b-deficient mutants has historically produced more confusion than enlightenment, but incomparable growth conditions can now explain the discrepancies reported in the literature.

Capillary electrophoresis of closely related intrinsic thylakoid membrane proteins of the photosystem II light-harvesting complex (LHC II)

The electrophoretic migration behavior of three closely related hydrophobic intrinsic membrane proteins of the photosystem II light-harvesting complex (LHC II) was investigated in free solution capillary electrophoresis at pH 8.0-10 with running electrolyte solutions containing either anionic, zwitter-ionic or nonionic detergents. The complete and repeatable separation of these proteins was accomplished with a running electrolyte solution of 25 mM Tris/192 mM glycine, pH 8.8, containing either sodium dodecyl sulfate or n-octyl beta-D-glucopyranoside at concentration up to 5.0 and 7.0 mM, respectively. Migration times and resolution of the individual LHC II intrinsic membrane proteins were sensitive to the type of detergent. The effect of detergent concentration on the electrophoretic behavior of the LHC II proteins was also investigated. Electroelution of the LHC II components separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis was used to isolate these intrinsic membrane proteins, which were then injected onto the capillary electrophoresis system for peak identification.

Analysis of the absorption spectrum of photosystem II reaction centers: temperature dependence, pigment assignment, and inhomogeneous broadening

In this study a model for decomposition and pigment assignment of the low-temperature (10 K) absorption spectrum of the photosystem II reaction center (D1-D2-cytochrome b559 complex, PSII-RC) is developed. It is based on theoretical calculations of the line shapes of the inhomogeneously broadened pigment spectra, taking into account electron-phonon coupling. The analysis is performed under the hypothesis that exciton coupling is weak, except for the P680 special pair. In this way a detailed decomposition of the absorption spectrum is obtained. Within the model the temperature dependence of the spectrum can be well explained. It is mainly caused by the temperature-dependent changes of the homogeneous absorption spectra of the individual pigments in the PSII-RC. In addition, slight changes in the inhomogeneous distribution functions have to be taken into account. Two slightly different parameter sets are found. We prefer one of these parameter sets which indicates that an accessory chlorophyll (Chl) is the lowest energy pigment in the RC core and that the two antenna Chls have their spectral maxima at 667.7 and 677.9 nm, respectively. The relationship between the shape of the absorption spectrum and the pigment stoichiometry of the sample (ratio of chlorophyll a:pheophytin a), which was noticed by comparison of a variety of different independently prepared samples, can be explained by the presence of "additional" Chl molecules which are nonstoichiometrically bound to part of the PSII-RCs. These Chls can be grouped into three spectrally distinguishable pools. One of them has its absorption maximum at about 683 nm and is responsible for the prominent shoulder that is present in the 10 K absorption spectra of most PSII-RC preparations. Our results suggest that the Chl content of the samples has been underestimated in many spectroscopic studies on the PSII-RC.

Organization of photosystem I polypeptides examined by chemical cross-linking

Photosystem I from the cyanobacterium Synechocystis sp. PCC 6803 was examined using the chemical cross-linkers glutaraldehyde and N-ethyl-1-3-[3-(dimethylamino)propyl]carbodiimide to investigate the organization of the polypeptide subunits. Thylakoid membranes and photosystem I, which was isolated by Triton X-100 fractionation, were treated with cross-linking reagents and were resolved using a Tricine/urea low-molecular-weight resolution gel system. Subunit-specific antibodies and western blotting analysis were used to identify the components of cross-linked species. These analyses identified glutaraldehyde-dependent cross-linking products composed of small amounts of PsaD and PsaC, PsaC and PsaE, and PsaE and PsaF. The novel cross-link between PsaE and PsaF was also observed following treatment with N-ethyl-1-3-[3-(dimethylamino)propyl]carbodiimide. These cross-linking results suggest a structural interaction between PsaE and PsaF and predict a transmembrane topology for PsaF.

Pigment content of D1-D2-cytochrome b559 reaction center preparations after removal of CP47 contamination: an immunological study

Isolated D1-D2-cytochrome b559 photosystem II reaction center preparations with pigment stoichiometry higher than 4 chlorophylls per 2 pheophytins can be contaminated with CP47 proximal antenna complex. Reaction center prepared by a modification of the Nanba-Satoh procedure and containing about 6 chlorophylls per 2 pheophytins showed immuno-cross-reactivity when probed with a monoclonal antibody raised against the CP47 polypeptide. Furthermore, they could be fractionated successfully by Superose-12 sieve chromatography into two different populations. The first few fractions off the column contained a more definitive 435 nm shoulder corresponding to increased chlorophyll content, and showed strong immuno-cross-reactivity with the CP47 antibody. The peak fractions off the column displayed a less prominent 435 nm shoulder, and did not cross-react with the antibody. Moreover, when a 6-chlorophyll preparation was mixed with Sepharose beads coupled to CP47 antibody, the eluted material corresponded to a preparation of about 4 chlorophylls per 2 pheophytins and did not show any cross-reaction with the antibody against CP47. The amount of CP47 protein in the 6-chlorophyll preparation as quantitated using Coomassie Blue staining or from gel blots was sufficient to account for most of the extra 2 chlorophylls. We conclude that D1-D2-cytochrome b559 preparations containing more than 4 chlorophylls per 2 pheophytins can be contaminated with small amounts of CP47-D1-D2-Cyt b559 complex and that native photosystem II reaction centers contain 4 core chlorophylls per 2 pheophytins.

A nuclear-encoded subunit of the photosystem II reaction center

A nuclear-encoded polypeptide of 6.1 kDa was identified in isolated photosystem II (PSII) reaction center from Spinacia oleracea. The hydrophobic membrane protein easily escapes staining procedures such as Coomassie R-250 or silver staining, but it is clearly detected by immunodecoration with peptide-directed IgG. This additional subunit was found to be present in PSII reaction centers previously known to contain only the D1/D2/cytb559 proteins and the psbI gene product. Furthermore, cross-linking experiments using 1-(3-dimethylaminopropyl-) 3-ethylcarbodiimide showed that the nearest neighbors were the D1 and D2 proteins and the cytb559. The 6.1-kDa protein was purified by immune affinity chromatography. N-terminal sequence analysis of the isolated protein confirmed the identity of the 6.1-kDa protein and enabled finding of strong similarities with a randomly obtained cDNA from Arabidopsis thaliana. Using enzyme-linked immunosorbent assay in combination with thylakoid membrane preparations of different orientation, the N terminus of the protein, predicted to span the membrane once, is suggested to be exposed at the lumen side of the membrane. Consequently the 6.1-kDa protein seems to be the only subunit in the PSII reaction center that is nuclear encoded and has its N terminus on the lumen side of the membrane. These findings open for new interesting suggestions concerning the properties of photosystem II reaction center with respect to the photosynthetic activity, regulation and assembly in higher plants.

Mutational analysis of photosystem I polypeptides in the cyanobacterium Synechocystis sp. PCC 6803. Targeted inactivation of psaI reveals the function of psaI in the structural organization of psaL

We cloned, characterized, and inactivated the psaI gene encoding a 4-kDa hydrophobic subunit of photosystem I from the cyanobacterium Synechocystis sp. PCC 6803. The psaI gene is located 90 base pairs downstream from psaL, and is transcribed on 0.94- and 0.32-kilobase transcripts. To identify the function of PsaI, we generated a cyanobacterial strain in which psaI has been interrupted by a gene for chloramphenicol resistance. The wild-type and the mutant cells showed comparable rates of photoautotrophic growth at 25 degrees C. However, the mutant cells grew slower and contained less chlorophyll than the wild-type cells, when grown at 40 degrees C. The PsaI-less membranes from cells grown at either temperature showed a small decrease in NADP+ photoreduction rate when compared to the wild-type membranes. Inactivation of psaI led to an 80% decrease in the PsaL level in the photosynthetic membranes and to a complete loss of PsaL in the purified photosystem I preparations, but had little effect on the accumulation of other photosystem I subunits. Upon solubilization with nonionic detergents, photosystem I trimers could be obtained from the wild-type, but not from the PsaI-less membranes. The PsaI-less photosystem I monomers did not contain detectable levels of PsaL. Therefore, a structural interaction between PsaL and PsaI may stabilize the association of PsaL with the photosystem I core. PsaL in the wild-type and PsaI-less membranes showed equal resistance to removal by chaotropic agents. However, PsaL in the PsaI-less strain exhibited an increased susceptibility to proteolysis. From these data, we conclude that PsaI has a crucial role in aiding normal structural organization of PsaL within the photosystem I complex and the absence of PsaI alters PsaL organization, leading to a small, but physiologically significant, defect in photosystem I function.

Targeted deletion of psaJ from the cyanobacterium Synechocystis sp. PCC 6803 indicates structural interactions between the PsaJ and PsaF subunits of photosystem I

Photosystem I catalyzes the light-driven oxidation of plastocyanin or cytochrome c6 and the reduction of ferredoxin or flavodoxin. PsaJ is a 4.4 kDa hydrophobic subunit of photosystem I from cyanobacteria and chloroplasts. To investigate the function of PsaJ, we generated a mutant strain of the cyanobacterium Synechocystis sp. PCC 6803 in which the psaJ gene is replaced by a gene for chloramphenicol resistance. Deletion of psaJ led to a reduction in the steady state RNA level from psaF which is located upstream from psaJ. Immunoquantification using an anti-PsaF antibody revealed a significant decrease in the amount of PsaF in membranes of the mutant strain. Trimeric photosystem I complexes isolated from the mutant strain using n-dodecyl beta-D-maltoside lacked PsaJ, contained ca. 80% less PsaF, but maintained wild-type levels of other photosystem I subunits. In contrast, the photosystem I purified using Triton X-100 contained less than 2% PsaF when compared to the wild type, showing the more extractable nature of PsaF in PsaJ-less photosystem I in the presence of Triton X-100. PsaE was more accessible to removal by NaI in a mutant strain lacking PsaF and PsaJ than in the wild type. The presence of PsaF in photosystem I from the PsaJ-less strain did not alter the increased susceptibility of PsaE to removal by NaI. These results indicate an interaction between PsaJ and PsaF in the organization of the complex.

Structural analysis of photosystem I polypeptides using chemical crosslinking

Thylakoid membranes, obtained from leaves of 14 d soybean (Glycine max L. cv. Williams) plants, were treated with the chemical crosslinkers glutaraldehyde or 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide (EDC) to investigate the structural organization of photosystem I. Polypeptides were resolved using lithium dodecyl sulfate polyacrylamide gel electrophoresis, and were identified by western blot analysis using a library of polyclonal antibodies specific for photosystem I subunits. An electrophoretic examination of crosslinked thylakoids revealed numerous crosslinked products, using either glutaraldehyde or EDC. However, only a few of these could be identified by western blot analysis using subunit-specific polyclonal antibodies. Several glutaraldehyde dependent crosslinked species were identified. A single band was identified minimally composed of PsaC and PsaD, documenting the close interaction between these two subunits. The most interesting aspect of these studies was a crosslinked species composed of the PsaB subunit observed following EDC treatment of thylakoids. This is either an internally crosslinked species, which will provide structural information concerning the topology of the complex PsaB protein, a linkage with a polypeptide for which we do not yet have an immunological probe, or a masking of epitopes by the EDC linkage at critical locations in the peptide which is linked to PsaB.

Cellular localization of cytochrome c550. Its specific association with cyanobacterial photosystem II

Cellular localization of cytochrome (cyt) c550, a low potential, c-type monoheme cytochrome, in a thermophilic cyanobacterium Synechococcus vulcanus was investigated by systematic fractionation of the cells followed by its enzymatic and immunological detection. While cyt c-553, a soluble cyt that donates an electron to P700, was detected in the supernatant of osmotic disruption of lysozyme-treated cells, cyt c550 was detected only in the thylakoid membrane fraction, being tightly bound to thylakoids, and its removal required sonication in the presence of 1 M CaCl2. Upon further fractionation of the thylakoids into photosystem (PS) I and PSII by lauryl dimethylamine N-oxide solubilization, cyt c550 was exclusively concentrated in the crude PSII fraction together with cyt f, with no significant amount being detected in any of the soluble and PSI fractions. Upon further fractionation of the crude PSII by n-dodecyl beta-D-maltoside solubilization followed by column chromatography, cyt c550 was detected exclusively in the purified PSII core complex fraction but not in any other fractions. A 12-kDa protein, one of the extrinsic components of cyanobacterial PSII, exhibited completely the same behavior as that of cyt c550 during these fractionation procedures. These results, coupled with our previous results that cyt c550 binds stoichiometrically to the cyanobacterial PSII core complex and enhances O2 evolution (Shen, J.-R., and Inoue, Y. (1993) Biochemistry 32, 1825-1832), indicate that there is only one species of cyt c550 in cyanobacterial cells and that this cyt is exclusively associated with PSII as a functional component for O2 evolution.

Mapping the lateral distribution of photosystem II and the cytochrome b6/f complex by direct immune labeling of the thylakoid membrane

By direct immunolabeling we have mapped the distribution of photosystem II (PS II) and cytochrome b6/f on the surfaces of photosynthetic membranes isolated from spinach. Photosynthetic membranes were attached to a support and gently disrupted to expose the occluded outer stacked surface, prior to labeling. Polyclonal antibodies against PS II intensely labeled the outer stacked surfaces while the outer nonstacked surface had minimal labeling. This confirms previous fractionation and immunolocalization studies which demonstrated that PS II is largely restricted to the stacked regions of the membrane. Inside-out membranes were also heavily labeled with PS II antibodies. Antibodies against cytochrome f were evenly distributed between the stacked and nonstacked outer surfaces and were found clumped together on the membrane outer surface. Previous fractionation and immunolocalization studies have indicated that cytochrome b6/f is located in both the stacked and nonstacked regions, but this is the first report to provide direct evidence that the complex may be clustered in the membrane. The clustering of antibodies to cytochrome b6/f supports the idea that this electron transport component exists as a multimeric complex within the membranes and that such complexes are found in both stacked and nonstacked regions of the photosynthetic membrane. No evidence was seen of any special differentiation of the marginal regions of the membrane, which link stacked and nonstacked regions.

Carotenoid-binding proteins of photosystem II

The distribution of the photosynthetic pigments of the chlorophyll-binding proteins or photosystem-II membranes, isolated from dark-adapted maize leaves was determined. Most (80%) of a xanthophyll, violaxanthin, was found in the three minor chlorophyll-a/b proteins CP24, CP26 and CP29 whose function is unknown. Violaxanthin is the precursor of zeaxanthin, which is involved in dissipating excess excitation energy into heat [Demmig-Adams, B. (1991) Biochim. Biophys. Acta 1020, 1-24] under conditions of high transmembrane pH gradient [Gilmore, A. M. & Yamamoto, H. Y. (1992) Proc. Natl Acad. Sci. USA 89, 1899-1903]. We propose that a role for the minor photosystem-II chlorophyll-a/b proteins is the regulation of energy transfer to the reaction centre. It was also confirmed that the photosystem II reaction centre (D1-D2-cytochrome b559) contains beta-carotene as the only carotenoid. However, the two other chlorophyll-a-binding proteins of photosystem II, CP47 and CP43, bind not only beta-carotene, but also the xanthophyll lutein, previously thought to be restricted to chlorophyll-a/b proteins.