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

Adaptation mechanisms of microalgal-bacterial granular sludge to outdoor light-limited conditions

Microalgal-bacterial granular sludge (MBGS) has attached attention for sustainable wastewater treatment, but it remains elusive whether it can adapt to outdoor light-limited conditions. This paper investigated the biological adaptation mechanisms of MBGS to outdoor light-limited diel conditions using real municipal wastewater. The results indicated that MBGS still had excellent pollutants removal performance, and that both the extracellular polymeric substances and glycogen content of MBGS increased significantly. The main functional microalgae and bacteria were revealed to be Leptolyngbyaceae and Rhodanobacteria, respectively. Further analyses indicated that the abundance of genes encoding PsbA, PsbD, PsbE, PsbJ, PsbP, Psb27, Psb28-2, PsaC, PsaE, PsaL, PsbX, PetB, PetA, and PetE increased in photosystem. Meanwhile, the abundance of gene encoding Rubisco decreased but the gene abundance regarding to crassulacean acid metabolism cycle increased. These suggested that MBGS could adjust the photosynthetic pathway to ensure the completion of photosynthesis. This study is anticipated to add fundamental insights for the MBGS process operated under outdoor light-limited conditions.

Presence of a [3Fe-4S] cluster in a PsaC variant as a functional component of the photosystem I electron transfer chain in Synechococcus sp. PCC 7002

A site-directed C14G mutation was introduced into the stromal PsaC subunit of Synechococcus sp. strain PCC 7002 in vivo in order to introduce an exchangeable coordination site into the terminal F_B [4Fe-4S] cluster of Photosystem I (PSI). Using an engineered PSI-less strain (psaAB deletion), psaC was deleted and replaced with recombinant versions controlled by a strong promoter, and the psaAB deletion was complemented. Modified PSI accumulated at lower levels in this strain and supported slower photoautotrophic growth than wild type. As-isolated PSI complexes containing PsaC_C14G showed resonances with g values of 2.038 and 2.007 characteristic of a [3Fe-4S]¹⁺ cluster. When the PSI complexes were illuminated at 15 K, these resonances partially disappeared and two new sets of resonances appeared. The majority set had g values of 2.05, 1.95, and 1.85, characteristic of F_A^-, and the minority set had g values of 2.11, 1.90, and 1.88 from F_B' in the modified site. The S = 1/2 spin state of the latter implied the presence of a thiolate as the terminal ligand. The [3Fe-4S] clusters could be partially reconstituted with iron, producing a larger population of [4Fe-4S] clusters. Rates of flavodoxin reduction were identical in PSI complexes isolated from wild type and the PsaC_C14G variant strain; this implied equivalent capacity for forward electron transfer in PSI complexes that contained [3Fe-4S] and [4Fe-4S] clusters. The development of this cyanobacterial strain is a first step toward translation of in vitro PSI-based biosolar molecular wire systems in vivo and provides new insights into the formation of Fe/S clusters.

Molecular dynamics simulation and bioinformatics study on chloroplast stromal ridge complex from rice (Oryza sativa L.)

Background

Rice (Oryza sativa L.) is one of the most important cereal crops in the world and its yield is closely related to the photosynthesis efficiency. The chloroplast stromal ridge complex consisting of PsaC-PsaD-PsaE plays an important role in plant photosynthesis, which has been a subject of many studies. Till now, the recognition mechanism between PsaC and PsaD in rice is still not fully understood.

Results

Here, we present the interaction features of OsPsaC and OsPsaD by molecular dynamics simulations and bioinformatics. Firstly, we identified interacting residues in the OsPsaC-OsPsaD complex during simulations. Significantly, important hydrogen bonds were observed in residue pairs R19-E103, D47-K62, R53-E63, Y81-R20, Y81-R61 and L26-V105. Free energy calculations suggested two salt bridges R19-E103 and D47-K62 were essential to maintain the OsPsaC-OsPsaD interaction. Supportively, electrostatic potentials surfaces of OsPsaD exhibited electrostatic attraction helped to stabilize the residue pairs R19-E103 and D47-K62. In particular, the importance of R19 was further verified by two 500 ns CG-MD simulations. Secondly, this study compared the stromal ridge complex in rice with that in other organisms. Notably, alignments of amino acids showed these two salt bridges R19-E103 and D47-K62 also existed in other organisms. Electrostatic potentials surfaces and X-ray structural analysis strongly suggested the stromal ridge complex in other organisms adopted a similar and general recognition mechanism.

Conclusions

These results together provided structure basis and dynamics behavior to understand recognition and assembly of the stromal ridge complex in rice.

Electronic supplementary material

The online version of this article (doi:10.1186/s12859-016-0877-0) contains supplementary material, which is available to authorized users.

Elucidating the design principles of photosynthetic electron-transfer proteins by site-directed spin labeling EPR spectroscopy

Site-directed spin labeling electron paramagnetic resonance (SDSL EPR) spectroscopy is a powerful tool to determine solvent accessibility, side-chain dynamics, and inter-spin distances at specific sites in biological macromolecules. This information provides important insights into the structure and dynamics of both natural and designed proteins and protein complexes. Here, we discuss the application of SDSL EPR spectroscopy in probing the charge-transfer cofactors in photosynthetic reaction centers (RC) such as photosystem I (PSI) and the bacterial reaction center (bRC). Photosynthetic RCs are large multi-subunit proteins (molecular weight≥300 kDa) that perform light-driven charge transfer reactions in photosynthesis. These reactions are carried out by cofactors that are paramagnetic in one of their oxidation states. This renders the RCs unsuitable for conventional nuclear magnetic resonance spectroscopy investigations. However, the presence of native paramagnetic centers and the ability to covalently attach site-directed spin labels in RCs makes them ideally suited for the application of SDSL EPR spectroscopy. The paramagnetic centers serve as probes of conformational changes, dynamics of subunit assembly, and the relative motion of cofactors and peptide subunits. In this review, we describe novel applications of SDSL EPR spectroscopy for elucidating the effects of local structure and dynamics on the electron-transfer cofactors of photosynthetic RCs. Because SDSL EPR Spectroscopy is uniquely suited to provide dynamic information on protein motion, it is a particularly useful method in the engineering and analysis of designed electron transfer proteins and protein networks. This article is part of a Special Issue entitled Biodesign for Bioenergetics--the design and engineering of electronic transfer cofactors, proteins and protein networks, edited by Ronald L. Koder and J.L. Ross Anderson.

Growing green electricity: progress and strategies for use of photosystem I for sustainable photovoltaic energy conversion

Oxygenic photosynthesis is driven via sequential action of Photosystem II (PSII) and (PSI)reaction centers via the Z-scheme. Both of these pigment-membrane protein complexes are found in cyanobacteria, algae, and plants. Unlike PSII, PSI is remarkably stable and does not undergo limiting photo-damage. This stability, as well as other fundamental structural differences, makes PSI the most attractive reaction centers for applied photosynthetic applications. These applied applications exploit the efficient light harvesting and high quantum yield of PSI where the isolated PSI particles are redeployed providing electrons directly as a photocurrent or, via a coupled catalyst to yield H₂. Recent advances in molecular genetics, synthetic biology, and nanotechnology have merged to allow PSI to be integrated into a myriad of biohybrid devices. In photocurrent producing devices, PSI has been immobilized onto various electrode substrates with a continuously evolving toolkit of strategies and novel reagents. However, these innovative yet highly variable designs make it difficult to identify the rate-limiting steps and/or components that function as bottlenecks in PSI-biohybrid devices. In this study we aim to highlight these recent advances with a focus on identifying the similarities and differences in electrode surfaces, immobilization/orientation strategies, and artificial redox mediators. Collectively this work has been able to maintain an annual increase in photocurrent density (Acm⁻²) of ~10-fold over the past decade. The potential drawbacks and attractive features of some of these schemes are also discussed with their feasibility on a large-scale. As an environmentally benign and renewable resource, PSI may provide a new sustainable source of bioenergy. This article is part of a special issue entitled: photosynthesis research for sustainability: keys to produce clean energy.

Differential degradation of photosystem I subunits under iron deficiency in rice

Rice (Oryza sativa) is one of the staple foods of the world. Iron (Fe) deficiency is a major abiotic stress factor that contributes world-wide to losses in crop yield and decline in nutritional quality. As cofactor for many enzymes and proteins, iron is an essential element. It plays a pivotal role in chlorophyll (Chl) biosynthesis, and iron deficiency may result in decreased Chl production and, thus, reduced photosynthetic capacity. Photosystem I (PSI) is a prime target of iron deficiency because of its high iron content (12 Fe per PS). To understand the protein level changes in the light-harvesting complex (LHC) of PSI (LHCI) under iron deficiency, rice seedlings were grown in Hoagland's nutrient medium with and without Fe. Chlorophyll content and photosynthetic efficiency decreased under iron deficiency. Protein gel blots probed with antibodies against the PSI core and Lhca 1-4 proteins revealed that the core subunits PsaA and PsaB remained stable under iron deficiency, whereas PsaC and PsaD decreased by about 50%, and PsaE was completely degraded. Among the LHCI subunits, Lhca1 and Lhca2 decreased by 40 and 50%, respectively, whereas Lhca3 and Lhca4 were completely degraded. We propose that the dissociation of LHCI subunits may be due to increased levels of reactive oxygen species, which is suggested by the increased activity of superoxide dismutase.

Detecting functional groups of Arabidopsis mutants by metabolic profiling and evaluation of pleiotropic responses

Metabolic profiles and fingerprints of Arabidopsis thaliana plants with various defects in plastidic sugar metabolism or photosynthesis were analyzed to elucidate if the genetic mutations can be traced by comparing their metabolic status. Using a platform of chromatographic and spectrometric tools data from untargeted full MS scans as well as from selected metabolites including major carbohydrates, phosphorylated intermediates, carboxylates, free amino acids, major antioxidants, and plastidic pigments were evaluated. Our key observations are that by multivariate statistical analysis each mutant can be separated by a unique metabolic signature. Closely related mutants come close. Thus metabolic profiles of sugar mutants are different but more similar than those of photosynthesis mutants. All mutants show pleiotropic responses mirrored in their metabolic status. These pleiotropic responses are typical and can be used for separating and grouping of the mutants. Our findings show that metabolite fingerprints can be taken to classify mutants and hence may be used to sort genes into functional groups.

Structural and functional changes of PSI-LHCI supercomplexes of Chlamydomonas reinhardtii cells grown under high salt conditions

The eVect of high salt concentration (100 mM NaCl) on the organization of photosystem I-light harvesting complex I supercomplexes (PSI-LHCI) of Chlamydomonas reinhardtii was studied. The electron transfer activity was reduced by 39% in isolated PSI-LHCI supercomplexes. The visible circular dichroism (CD) spectra associated with strongly coupled chlorophyll (Chl) dimers were reduced in intensity, indicating that pigment-pigment interactions were disrupted. This data is consistent with results from Xuorescence streak camera spectroscopy, which suggest that red-shifted pigments in the PSI-LHCI antenna had been lost. Denaturing gel electrophoresis and immunoblot analysis reveals that levels of the PSI reaction center proteins PsaD, PsaE and PsaF were reduced due to salt stress. PsaE is almost completely absent under high salt conditions. It is known that the membrane-extrinsic subunits PsaD and E form the ferredoxin-docking site. Our results indicate that the PSI-LHCI supercomplex is damaged by reactive oxygen species at high salt concentration, with particular impact on the ferredoxin-docking site and the PSILHCI interface.

Understanding of the binding interface between PsaC and the PsaA/PsaB heterodimer in photosystem I

The PsaC subunit of Photosystem I (PS I) is tightly bound to the PsaA/PsaB heterodimer via an extensive network of ionic and hydrogen bonds. To improve our understanding of the design of the PsaC-PsaA/PsaB binding interface, variants of PsaC were generated, each lacking a key binding contact with the PsaA/PsaB heterodimer. The characteristics of the reconstituted, variant PS I complexes were monitored by time-resolved optical spectroscopy, low-temperature EPR spectroscopy, and electron transfer throughput measurements. In the absence of the ionic bond forming contacts R52(C) or R65(C), a markedly slower charge recombination occurs between P(700)(+) and [F(A)/F(B)](-). The addition of PsaD leads to the restoration of native recombination kinetics in a fraction of the PS I complexes reconstituted with R52A(C), but not with R65A(C). Contrary to expectation, the absence of Y80(C), which forms two symmetry-breaking H-bonds with PsaB, does not significantly affect the binding of PsaC as judged by the rate of charge recombination between P(700)(+) and [F(A)/F(B)](-). However, the removal of the entire C-terminus results in a dramatic decrease in the rate of charge recombination. Low-temperature EPR spectra of the variant PS I complexes indicate that the magnetic environments of F(A) and F(B) are altered when compared to that of native PS I. The slowing of the rate of charge recombination in the variant PS I complexes could be due to an increase in the distance between F(X) and F(A)/F(B) as the result of non-native binding or to an altered reduction potential of the iron-sulfur clusters, which would result in a different rate of thermalization up the electron acceptor chain. The most significant finding is that the variant PS I complexes support lower rates of light-induced flavodoxin reduction and that the rates deteriorate rapidly on exposure to dioxygen due to the degradation of F(A) and F(B). We suggest that the extensive set of ionic bonds and H-bonds between PsaC and the PsaA/PsaB heterodimer has evolved to ensure an exceedingly tight binding interface, thereby rendering the [4Fe-4S] clusters in PsaC inaccessible to dioxygen at the onset of oxygenic photosynthesis.

Docking of photosystem I subunit C using a constrained geometric simulation

The elucidation of assembly pathways of multi-subunit protein complexes is a problem of great interest in structural biology and biomolecular modeling. In this study, we use a new computer algorithm for the simulation of large-scale motion in proteins to dock the subunit PsaC onto Photosystem I. We find that a complicated docking pathway involving multiple conformational changes can be quickly simulated by actively targeting only a few residues at a time to their target positions. Simulations for two possible docking scenarios are explored, and experimental approaches to distinguish between them are discussed.

The E subunit of photosystem I is not essential for linear electron flow and photoautotrophic growth in Arabidopsis thaliana

PSI-E is part of the stromal side of photosystem I (PSI). In Arabidopsis thaliana, the two nuclear genes PsaE1 and PsaE2 code for PSI-E, and transcripts of PsaE1 are markedly more abundant than PsaE2 transcripts. Stable null alleles of the two PsaE genes, psae1-3 and psae2-1, were identified and characterised. The psae2-1 mutant exhibited wild-type like PSI-E abundance and photosynthetic performance, whereas in the psae1-3 mutant PSI-E accumulation was decreased by 85%, together with an impaired thylakoid electron flow and plant growth rate. The psae1-3 psae2-1 double mutant totally lacked PSI-E but was still able to grow photoautotrophically, implying that PSI-E is not essential for PSI accumulation and thylakoid electron flow.

The PsbZ subunit of Photosystem II in Synechocystis sp. PCC 6803 modulates electron flow through the photosynthetic electron transfer chain

The psbZ gene of Synechocystis sp. PCC 6803 encodes the approximately 6.6 kDa photosystem II (PSII) subunit. We here report biophysical, biochemical and in vivo characterization of Synechocystis sp. PCC 6803 mutants lacking psbZ. We show that these mutants are able to perform wild-type levels of light-harvesting, energy transfer, PSII oxygen evolution, state transitions and non-photochemical quenching (NPQ) under standard growth conditions. The mutants grow photoautotrophically; however, their growth rate is clearly retarded under low-light conditions and they are not capable of photomixotrophic growth. Further differences exist in the electron transfer properties between the mutants and wild type. In the absence of PsbZ, electron flow potentially increased through photosystem I (PSI) without a change in the maximum electron transfer capacity of PSII. Further, rereduction of P700(+) is much faster, suggesting faster cyclic electron flow around PSI. This implies a role for PsbZ in the regulation of electron transfer, with implication for photoprotection.

Regulation of intracellular free calcium concentration during heterocyst differentiation by HetR and NtcA in Anabaena sp. PCC 7120

Calcium ions are important to some prokaryotic cellular processes, such as heterocyst differentiation of cyanobacteria. Intracellular free Ca(2+)concentration, [Ca(2+)](i), increases several fold in heterocysts and is regulated by CcbP, a Ca(2+)-binding protein found in heterocyst-forming cyanobacteria. We demonstrate here that CcbP is degraded by HetR, a serine-type protease that controls heterocyst differentiation. The degradation depends on Ca(2+) and appears to be specific because HetR did not digest other tested proteins. CcbP was found to bind two Ca(2+) per molecule with K(D) values of 200 nM and 12.8 microM. Degradation of CcbP releases bound Ca(2+) that contributes significantly to the increase of [Ca(2+)](i) during the process of heterocyst differentiation in Anabaena sp. strain PCC 7120. We suggest that degradation of CcbP is a mechanism of positive autoregulation of HetR. The down-regulation of ccbP in differentiating cells and mature heterocysts, which also is critical to the regulation of [Ca(2+)](i), depends on NtcA. Coexpression of ntcA and a ccbP promoter-controlled gfp in Escherichia coli diminished production of GFP, and the decrease is enhanced by alpha-ketoglutarate. It was also found that NtcA could bind a fragment of the ccbP promoter containing an NtcA-binding sequence in a alpha-ketoglutarate-dependent fashion. Therefore, [Ca(2+)](i) is regulated by a collaboration of HetR and NtcA in heterocyst differentiation in Anabaena sp. strain PCC 7120.

FesM, a membrane iron-sulfur protein, is required for cyclic electron flow around photosystem I and photoheterotrophic growth of the cyanobacterium Synechococcus sp. PCC 7002

While it is known that cyclic electron flow around photosystem I is an important pathway of photosynthetic electron transfer for converting light energy to chemical energy, some components of cyclic electron flow remain to be revealed. Here, we show that fesM, encoding a novel membrane iron-sulfur protein, is essential to cyclic electron flow in the cyanobacterium Synechococcus sp. PCC 7002. The FesM protein is predicted to have a cAMP-binding domain, an NtrC-like domain, a redox sensor motif, and an iron-sulfur (4Fe-4S) motif. Deletion of fesM (fesM-D) led to an inability for Synechococcus cells to grow in the presences of glycerol and 3-(3,4-dichlorophenyl)-1,1-dimethylurea. Photoheterotrophic growth was restored by a complete fesM gene present on a replicable plasmid. A mutant fesM gene encoding a truncated FesM protein lacking the cAMP domain failed to restore the phenotype, suggesting this domain is important to the function of FesM. Measurements of reduction of P700(+) and the redox state of interphotosystem electron carriers showed that cells had a slower rate of respiratory electron donation to the interphotosystem electron transport chain, and cyclic electron flow around photosystem I in fesM-D was impaired, suggesting that FesM is a critical protein for respiratory and cyclic electron flow. Phosphatase fusion analysis showed that FesM contains nine membrane-spanning helices, and all functional domains of FesM are located on the cytoplasmic face of the thylakoid membranes.

Structure of cyanobacterial photosystem I

Photosystem I is one of the most fascinating membrane protein complexes for which a structure has been determined. It functions as a bio-solar energy converter, catalyzing one of the first steps of oxygenic photosynthesis. It captures the light of the sun by means of a large antenna system, consisting of chlorophylls and carotenoids, and transfers the energy to the center of the complex, driving the transmembrane electron transfer from plastoquinone to ferredoxin. Cyanobacterial Photosystem I is a trimer consisting of 36 proteins to which 381 cofactors are non-covalently attached. This review discusses the complex function of Photosystem I based on the structure of the complex at 2.5 A resolution as well as spectroscopic and biochemical data.

HetR homodimer is a DNA-binding protein required for heterocyst differentiation, and the DNA-binding activity is inhibited by PatS

HetR plays a key role in regulation of heterocyst differentiation. When the Cys-48 residue of the HetR from Anabaena sp. PCC 7120 was replaced with an Ala residue, the mutant HetR (HetR(C48A)) could not dimerize, indicating that HetR forms a homodimer through a disulfide bond. The Anabaena strain C48, containing the hetRc48a gene, could not produce HetR homodimer and failed to form heterocyst. We show that HetR is a DNA-binding protein and that its homodimerization is required for the DNA binding. HetR binds the promoter regions of hetR, hepA, and patS, suggesting a direct control of the expression of these genes by HetR. We present evidence that shows that the up-regulation of patS and hetR depends on DNA binding by HetR dimer. The pentapeptide RGSGR, which is present at the C terminus of PatS and blocks heterocyst formation, inhibits the DNA binding of HetR and prevents hetR up-regulation.

Mutants for photosystem I subunit D of Arabidopsis thaliana: effects on photosynthesis, photosystem I stability and expression of nuclear genes for chloroplast functions

In Arabidopsis thaliana, the D-subunit of photosystem I (PSI-D) is encoded by two functional genes, PsaD1 and PsaD2, which are highly homologous. Knock-out alleles for each of the loci have been identified by a combination of forward and reverse genetics. The double mutant psad1-1 psad2-1 is seedling-lethal, high-chlorophyll-fluorescent and deficient for all tested PSI subunits, indicating that PSI-D is essential for photosynthesis. In addition, psad1-1 psad2-1 plants show a defect in the accumulation of thylakoid multiprotein complexes other than PSI. Of the single-gene mutations, psad2 plants behave like wild-type (WT) plants, whereas psad1-1 markedly affects the accumulation of PsaD mRNA and protein, and photosynthetic electron flow. Additional effects of the psad1-1 mutation include a decrease in growth rate under greenhouse conditions and downregulation of the mRNA expression of most genes involved in the light phase of photosynthesis. In the same mutant, a marked decrease in the levels of PSI and PSII polypeptides is evident, as well as a light-green leaf coloration and increased photosensitivity. Increased dosage of PsaD2 in the psad1-1 background restores the WT phenotype, indicating that PSI-D1 and PSI-D2 have redundant functions.

Kinetic analyses of state transitions of the cyanobacterium Synechococcus sp. PCC 7002 and its mutant strains impaired in electron transport

The state transitions of the cyanobacterium Synechococcus sp. PCC 7002 and of three mutant strains, which were impaired in PsaE-dependent cyclic electron transport (psaE(-)), respiratory electron transport (ndhF(-)) and both activities (psaE(-)ndhF(-)), were analyzed. Dark incubation of the wild type and psaE(-) cells led to a transition to state 2, while the ndhF(-) strains remained in state 1 after dark incubation. The ndhF(-) cells adapted to state 2 when the cells were incubated under anaerobic conditions or in the presence of potassium cyanide; these results suggest that the ndhF(-) cells were inefficient in performing state 1 to state 2 transitions in the dark unless cytochrome oxidase activity was inhibited. In the state 2 to state 1 transition of wild-type cells induced by light in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), there was still a significant reduction of the interphotosystem electron carriers by both respiration and cyclic electron flow around PSI. Kinetic analysis of the state 2 to state 1 transition shows that, in the absence of PSII activity, the relative contribution to the reduced state of the interphotosystem electron carriers by respiratory and cyclic electron transfer is about 72% and 28%, respectively. The state 2 to state 1 transition was prevented by the cytochrome b(6)f inhibitor 2,5-dibromo-3-methyl-6-isopropylbenzoquinone (DBMIB). On the other hand, the state 1 to state 2 transition was induced by DBMIB with half times of approximately 8 s in all strains. The externally added electron acceptor 2,5-dimethyl-benzoquinone (DMBQ) induced a state 2 to state 1 transition in the dark and this transition could be prevented by DBMIB. The light-induced oxidation of P700 showed that approximately 50% of PSI could be excited by 630-nm light absorbed by phycobilisomes (PBS) under state 2 conditions. P700 oxidation measurements with light absorbed by PBS also showed that the dark-induced state 1 to state 2 transition occurred in wild-type cells but not in the ndhF(-) cells. The possible mechanism for sensing an imbalanced light regime in cyanobacterial state transitions is discussed.

Structure of the cytochrome b6f complex of oxygenic photosynthesis: tuning the cavity

The cytochrome b6f complex provides the electronic connection between the photosystem I and photosystem II reaction centers of oxygenic photosynthesis and generates a transmembrane electrochemical proton gradient for adenosine triphosphate synthesis. A 3.0 angstrom crystal structure of the dimeric b6f complex from the thermophilic cyanobacterium Mastigocladus laminosus reveals a large quinone exchange cavity, stabilized by lipid, in which plastoquinone, a quinone-analog inhibitor, and a novel heme are bound. The core of the b6f complex is similar to the analogous respiratory cytochrome bc1 complex, but the domain arrangement outside the core and the complement of prosthetic groups are strikingly different. The motion of the Rieske iron-sulfur protein extrinsic domain, essential for electron transfer, must also be different in the b6f complex.

Assembly of protein subunits within the stromal ridge of photosystem I. Structural changes between unbound and sequentially PS I-bound polypeptides and correlated changes of the magnetic properties of the terminal iron sulfur clusters

The X-ray structure of Photosystem I (PS I) from Synechococcus elongatus was recently solved at 2.5A resolution (PDB entry 1JB0). It provides a structural model for the stromal subunits PsaC, PsaD and PsaE, which comprise the "stromal ridge" of PS I. In a separate set of studies the three-dimensional solution structures of the unbound, recombinant PsaC (PDB entry 1K0T) and PsaE (PDB entries 1PSF, 1QP2 and 1GXI) subunits were solved by NMR. The PsaC subunit of PS I is a small (9.3 kDa) protein that harbors binding sites for two [4Fe-4S] clusters F(A) and F(B), which are the terminal electron acceptors in PS I. Comparison of the PsaC structure in solution with that in the X-ray structure of PS I reveals significant differences between them which are summarized and evaluated here. Changes in the magnetic properties of [4Fe-4S] centers F(A) and F(B) are related to changes in the protein structure of PsaC, and they are further influenced by the presence of PsaD. Based on experimental evidence, three assembly stages are analyzed: PsaC(free), PsaC(only), PsaC(PS I). Unbound, recombinant PsaD, studied by NMR, has only a few elements of secondary structure and no stable three-dimensional structure in solution. When PsaD is bound in PS I, it has a well-defined three-dimensional structure. For PsaE the three-dimensional structure is very similar in solution and in the PS I-bound form, with the exception of two loop regions. We suggest that the changes in the structures of PsaC and PsaD are caused by the sequential formation of multiple networks of contacts between the polypeptides of the stromal ridge and between those polypeptides and the PsaA/PsaB core polypeptides. The three-dimensional structure of the C(2)-symmetric F(X)-binding loops on PsaA and PsaB were also analyzed and found to be significantly different from the binding sites of other proteins that contain interpolypeptide [4Fe-4S] clusters. The aim of this work is to relate contact information to structural changes in the proteins and to propose a model for the assembly of the stromal ridge of PS I based on this analysis.

Expression of the glutamyl-tRNA synthetase gene from the cyanobacterium Synechococcus sp PCC 7942 depends on nitrogen availability and the global regulator NtcA

We report here transcriptional analyses of a cyanobacterial gene encoding an aminoacyl-tRNA synthetase (aaRS), the gltX gene from Synechoccocus sp. PCC 7942, coding for the glutamyl-tRNA synthetase. We show that the transcript levels of gltX in Synechococcus depend on nitrogen availability and do not increase with the growth rate, which is at odds with observations from other bacteria. We also demonstrate the involvement of the cyanobacterial global regulator NtcA in transcriptional control of gltX according to nitrogen status. Our results support a regulatory model in which the gltX transcript level is finely tuned by a dynamic equilibrium between activation and repression relying upon the cellular concentration of NtcA.

Assembly of photosystem I. II. Rubredoxin is required for the in vivo assembly of F(X) in Synechococcus sp. PCC 7002 as shown by optical and EPR spectroscopy

The rubA gene was insertionally inactivated in Synechococcus sp. PCC 7002, and the properties of photosystem I complexes were characterized spectroscopically. X-band EPR spectroscopy at low temperature shows that the three terminal iron-sulfur clusters, F(X), F(A), and F(B), are missing in whole cells, thylakoids, and photosystem (PS) I complexes of the rubA mutant. The flash-induced decay kinetics of both P700(+) in the visible and A(1)- in the near-UV show that charge recombination occurs between P700(+) and A(1)- in both thylakoids and PS I complexes. The spin-polarized EPR signal at room temperature from PS I complexes also indicates that forward electron transfer does not occur beyond A(1). In agreement, the spin-polarized X-band EPR spectrum of P700(+) A(1)- at low temperature shows that an electron cycle between A(1)- and P700(+) occurs in a much larger fraction of PS I complexes than in the wild-type, wherein a relatively large fraction of the electrons promoted are irreversibly transferred to [F(A)/F(B)]. The electron spin polarization pattern shows that the orientation of phylloquinone in the PS I complexes is identical to that of the wild type, and out-of-phase, spin-echo modulation spectroscopy shows the same P700(+) to A(1)- center-to-center distance in photosystem I complexes of wild type and the rubA mutant. In contrast to the loss of F(X), F(B), and F(A), the Rieske iron-sulfur protein and the non-heme iron in photosystem II are intact. It is proposed that rubredoxin is specifically required for the assembly of the F(X) iron-sulfur cluster but that F(X) is not required for the biosynthesis of trimeric P700-A(1) cores. Since the PsaC protein requires the presence of F(X) for binding, the absence of F(A) and F(B) may be an indirect result of the absence of F(X).

Single and double knockouts of the genes for photosystem I subunits G, K, and H of Arabidopsis. Effects on photosystem I composition, photosynthetic electron flow, and state transitions

Photosystem I (PSI) of higher plants contains 18 subunits. Using Arabidopsis En insertion lines, we have isolated knockout alleles of the genes psaG, psaH2, and psaK, which code for PSI-G, -H, and -K. In the mutants psak-1 and psag-1.4, complete loss of PSI-K and -G, respectively, was confirmed, whereas the residual H level in psah2-1.4 is due to a second gene encoding PSI-H, psaH1. Double mutants, lacking PSI-G, and also -K, or a fraction of -H, together with the three single mutants were characterized for their growth phenotypes and PSI polypeptide composition. In general, the loss of each subunit has secondary, in some cases additive, effects on the abundance of other PSI polypeptides, such as D, E, H, L, N, and the light-harvesting complex I proteins Lhca2 and 3. In the G-less mutant psag-1.4, the variation in PSI composition suggests that PSI-G stabilizes the PSI-core. Levels of light-harvesting complex I proteins in plants, which lack simultaneously PSI-G and -K, indicate that PSI subunits other than G and K can also bind Lhca2 and 3. In the same single and double mutants, psag-1.4, psak-1, psah2-1.4, psag-1.4/psah2-1.4, and psag-1.4/psak-1 photosynthetic electron flow and excitation energy quenching were analyzed to address the roles of the various subunits in P700 reduction (mediated by PSI-F and -N) and oxidation (PSI-E), and state transitions (PSI-H). Based on the results, we also suggest for PSI-K a role in state transitions.

Proteins of the cyanobacterial photosystem I

Cyanobacterial photosystem (PS) I is remarkably similar to its counterpart in the chloroplast of plants and algae. Therefore, it has served as a prototype for the type I reaction centers of photosynthesis. Cyanobacterial PS I contains 11-12 proteins. Some of the cyanobacterial proteins are modified post-translationally. Reverse genetics has been used to generate subunit-deficient cyanobacterial mutants, phenotypes of which have revealed the functions of the missing proteins. The cyanobacterial PS I proteins bind cofactors, provide docking sites for electron transfer proteins, participate in tertiary and quaternary organization of the complex and protect the electron transfer centers. Many of these mutants are now being used in sophisticated structure-function analyses. Yet, the roles of some proteins of the cyanobacterial PS I are unknown. It is necessary to examine functions of these proteins on a global scale of cell physiology, biogenesis and evolution.

Ferredoxin and flavodoxin reduction by photosystem I

Ferredoxin and flavodoxin are soluble proteins which are reduced by the terminal electron acceptors of photosystem I. The kinetics of ferredoxin (flavodoxin) photoreduction are discussed in detail, together with the last steps of intramolecular photosystem I electron transfer which precede ferredoxin (flavodoxin) reduction. The present knowledge concerning the photosystem I docking site for ferredoxin and flavodoxin is described in the second part of the review.

Structure of photosystem I

In plants and cyanobacteria, the primary step in oxygenic photosynthesis, the light induced charge separation, is driven by two large membrane intrinsic protein complexes, the photosystems I and II. Photosystem I catalyses the light driven electron transfer from plastocyanin/cytochrome c(6) on the lumenal side of the membrane to ferredoxin/flavodoxin at the stromal side by a chain of electron carriers. Photosystem I of Synechococcus elongatus consists of 12 protein subunits, 96 chlorophyll a molecules, 22 carotenoids, three [4Fe4S] clusters and two phylloquinones. Furthermore, it has been discovered that four lipids are intrinsic components of photosystem I. Photosystem I exists as a trimer in the native membrane with a molecular mass of 1068 kDa for the whole complex. The X-ray structure of photosystem I at a resolution of 2.5 A shows the location of the individual subunits and cofactors and provides new information on the protein-cofactor interactions. [P. Jordan, P. Fromme, H.T. Witt, O. Klukas, W. Saenger, N. Krauss, Nature 411 (2001) 909-917]. In this review, biochemical data and results of biophysical investigations are discussed with respect to the X-ray crystallographic structure in order to give an overview of the structure and function of this large membrane protein.

Photoinhibition and light-induced cyclic electron transport in ndhB(-) and psaE(-) mutants of Synechocystis sp. PCC 6803

The ndhB(-) and psaE(-) mutants of the cyanobacterium Synechocystis sp. PCC 6803 are partly deficient in PSI-driven cyclic electron transport. We compared photoinhibition in these mutants to the wild type to test the hypothesis that PSI cyclic electron transport protects against photoinhibition. Photoinhibitory treatment greatly accelerated PSI cyclic electron transport in the wild type and also in both the mutants. The psaE(-) mutant showed rates of PSI cyclic electron transport similar to the wild type under all conditions tested. The ndhB(-) mutant showed much lower rates of PSI cyclic electron transport than the wild type following brief dark adaptation but exceeded wild type rates after exposure to photoinhibitory light. The wild type and both mutants showed similar rates of photoinhibition damage and photoinhibition repair at PSII. Photoinhibition at PSI was much slower than at PSII and was also similar between the wild type and both mutants, despite the known instability of PSI in the psaE(-) mutant. We conclude that photoinhibitory light induces sufficient PSI-driven cyclic electron transport in both the ndhB(-) and psaE(-) mutants to fulfill any role that cyclic electron transport plays in protection against photoinhibition.

Abnormal regulation of photosynthetic electron transport in a chloroplast ycf9 inactivation mutant

The ycf9 (orf62) gene of the plastid genome encodes a 6.6-kDa protein (ORF62) of thylakoid membranes. To elucidate the role of the ORF62 protein, the coding region of the gene was disrupted with an aadA cassette, yielding mutant plants that were nearly (more than 95%) homoplasmic for ycf9 inactivation. The ycf9 mutant had no altered phenotype under standard growth conditions, but its growth rate was severely reduced under suboptimal irradiances. On the other hand, it was less susceptible to photodamage than the wild type. ycf9 inactivation resulted in a clear reduction in protein amounts of CP26, the NAD(P)H dehydrogenase complex, and the plastid terminal oxidase. Furthermore, depletion of ORF62 led to a faster flow of electrons to photosystem I without a change in the maximum electron transfer capacity of photosystem II. Despite the reduction of CP26 in the mutant thylakoids, no differences in PSII oxygen evolution rates were evident even at low light intensities. On the other hand, the ycf9 mutant presented deficiencies in the capacity for PSII-independent electron transport (ferredoxin-dependent cyclic electron transport and NAD(P)H dehydrogenase-mediated plastoquinone reduction). Altogether, it is shown that depletion of ORF62 leads to anomalies in the photosynthetic electron transfer chain and in the regulation of electron partitioning among the different routes of electron transport.

Photosystem stoichiometry and state transitions in a mutant of the cyanobacterium Synechococcus sp. PCC 7002 lacking phycocyanin

Phycobilisomes (PBS) function as light-harvesting antenna complexes in cyanobacteria, red algae and cyanelles. They are composed of two substructures: the core and peripheral rods. Interposon mutagenesis of the cpcBA genes of Synechococcus sp. PCC 7002 resulted in a strain (PR6008) lacking phycocyanin and thus the ability to form peripheral rods. Difference absorption spectroscopy of whole cells showed that intact PBS cores were assembled in vivo in the cpcBA mutant strain PR6008. Fluorescence induction measurements demonstrated that the PBS cores are able to deliver absorbed light energy to photosystem (PS) II, and fluorescence induction transients in the presence of DCMU showed that PR6008 cells could perform a state 2 to state 1 transition with similar kinetics to that of the wild-type cells. Thus, PBS core assembly, light-harvesting functions and energy transfer to PS I were not dependent upon the assembly of the peripheral rods. The ratio of PS II:PS I in the PR6008 cells was significantly increased, nearly twice that of the wild-type cells, possibly a result of long-term adaptation to compensate for the reduced antenna size of PS II. However, the ratio of PBS cores:chlorophyll remained unchanged. This result indicates that approximately half of the PS II reaction centers in the PR6008 cells had no closely associated PBS cores.

The FAPY-DNA glycosylase (Fpg) is required for survival of the cyanobacterium Synechococcus elongatus under high light irradiance

The gene for the DNA repair enzyme Fpg from Synechococcus elongatus was detected immediately downstream of the photosystem I gene psaE. fpg is likely expressed together with psaE by transcriptional readover while psaE is mostly expressed independently. Segregated psaE and fpg deletion strains were obtained upon insertional inactivation of both genes in S. elongatus. These mutants are viable under photoautotrophic conditions, but fail to grow under high light regimes that likely cause oxidative stress. These high light sensitive phenotypes suggest that the Fpg protein, which has been shown to repair DNA lesions caused by reactive oxygen species in Escherichia coli, may be involved in the photoprotection of cyanobacteria against oxidative damage caused under high irradiance.

Purification, characterization, and molecular cloning of the gene of a seed-specific antimicrobial protein from pokeweed

A small cysteine-rich protein with antimicrobial activity was isolated from pokeweed (Phytolacca americana) seeds and purified to homogeneity. The protein inhibits the growth of several filamentous fungi and gram-positive bacteria. The protein was highly basic, with a pI higher than 10. The entire amino acid sequence of the protein was determined to be homologous to antimicrobial protein (AMP) from Mirabilis jalapa. The cDNA encoding the P. americana AMP (Pa-AMP-1) and chromosomal DNA containing the gene were cloned and sequenced. The deduced amino acid sequence shows the presence of a signal peptide at the amino terminus, suggesting that the protein is synthesized as a preprotein and secreted outside the cells. The chromosomal gene shows the presence of an intron located within the region encoding the signal peptide. Southern hybridization showed that there was small gene family encoding Pa-AMP. Immunoblotting showed that Pa-AMP-1 was only present in seeds, and was absent in roots, leaves, and stems. The Pa-AMP-1 protein was secreted into the environment of the seeds during germination, and may create an inhibitory zone against soil-borne microorganisms. The disulfide bridges of Pa-AMP-1 were identified. The three-dimensional modeling of Pa-AMP-1 indicates that the protein has a small cystine-knot folding, a positive patch, and a hydrophobic patch.

Essential role of a single arginine of photosystem I in stabilizing the electron transfer complex with ferredoxin

PsaE is one of the photosystem I subunits involved in ferredoxin binding. The central role of arginine 39 of this 8-kDa peripheral polypeptide has been established by a series of mutations. The neutral substitution R39Q leads to a 250-fold increase of the dissociation constant K(d) of the photosystem I-ferredoxin complex, as large as the increase induced by PsaE deletion. At pH 8.0, this K(d) value strongly depends on the charge of the residue substituting Arg-39: 0.22 microM for wild type, 1.5 microM for R39K, 56 microM for R39Q, and more than 100 microM for R39D. The consequences of arginine 39 substitution for the titratable histidine were analyzed as a function of pH. The K(d) value of R39H is increased 140 times at pH 8.0 but only 5 times at pH 5.8, which is assigned to the protonation of histidine at low pH. In the mutant R39Q, the association rate of ferredoxin was decreased 3-fold compared with wild type, whereas an 80-fold increase is calculated for the dissociation rate. We propose that a major contribution of PsaE is to provide a prominent positive charge at position 39 for controlling the electrostatic interaction and lifetime of the complex with ferredoxin.

Identification of the active site of HetR protease and its requirement for heterocyst differentiation in the cyanobacterium Anabaena sp. strain PCC 7120

HetR is a serine-type protease required for heterocyst differentiation in heterocystous cyanobacteria under conditions of nitrogen deprivation. We have identified the active Ser residue of HetR from Anabaena sp. strain PCC 7120 by site-specific mutagenesis. By changing the S152 residue to an Ala residue, the mutant protein cannot be labeled by Dansyl fluoride, a specific serine-type protein inhibitor. The mutant protein showed no autodegradation in vitro. The mutant hetR gene was introduced into Anabaena strain 884a, a hetR mutant. The resultant strain, Anabaena strain S152A, could not form heterocysts under conditions of nitrogen deprivation even though the up-regulation of the mutant hetR gene was induced upon removal of combined nitrogen. The Anabaena strain 216, which carries a mutant hetR gene encoding S179N HetR and could not form heterocysts, also produced HetR protein upon induction. Sequence comparison shows that Ser152 is conserved in all cyanobacterial HetR. Immunoblotting was used to study HetR induction in both the wild-type and mutant strains. The amount of mutant HetR in strain S152A and in strain 216 increased continuously for 24 h after nitrogen step-down, while the amount of HetR in wild-type cells reached a maximum level within 6 h after nitrogen step-down. Our results show the Ser152 is the active site of HetR. The protease activity is required for heterocyst differentiation and might be needed for repression of HetR overproduction under conditions of nitrogen deprivation.

Photosystem I, an improved model of the stromal subunits PsaC, PsaD, and PsaE

An improved electron density map of photosystem I (PSI) calculated at 4-A resolution yields a more detailed structural model of the stromal subunits PsaC, PsaD, and PsaE than previously reported. The NMR structure of the subunit PsaE of PSI from Synechococcus sp. PCC7002 (Falzone, C. J., Kao, Y.-H., Zhao, J., Bryant, D. A., and Lecomte, J. T. J. (1994) Biochemistry 33, 6052-6062) has been used as a model to interpret the region of the electron density map corresponding to this subunit. The spatial orientation with respect to other subunits is described as well as the possible interactions between the stromal subunits. A first model of PsaD consisting of a four-stranded beta-sheet and an alpha-helix is suggested, indicating that this subunit partly shields PsaC from the stromal side. In addition to the improvements on the stromal subunits, the structural model of the membrane-integral region of PSI is also extended. The current electron density map allows the identification of the N and C termini of the subunits PsaA and PsaB. The 11-transmembrane alpha-helices of these subunits can now be assigned uniquely to the hydrophobic segments identified by hydrophobicity analyses.

Measurement of photosystem I activity with photoreduction of recombinant flavodoxin

Flavodoxin can function as an alternative electron acceptor for photosystem I (PSI) in place of ferredoxin under iron-limiting conditions. The isiB gene, encoding the flavodoxin in Synechococcus sp. PCC 7002, was overexpressed in Escherichia coli. Under the conditions employed, most recombinant flavodoxin (rFlvd) was in soluble form with cofactor correctly inserted. The absorption spectrum of rFlvd was identical to that of the native flavodoxin of the cyanobacteria. Photoreduction of rFlvd by PSI particles and thylakoid membranes was determined directly by monitoring the absorption change at 467 nm. The optimal conditions for rFlvd photoreduction were determined. Compared to other methods currently employed to measure PSI activity such as oxygen uptake in the presence of methyl viologen and NADP+ photoreduction in the presence of ferredoxin and ferredoxin:NADP+ oxidoreductase, measurement of PSI activity with flavodoxin as an electron acceptor has several advantages. It measures the full-chain electron transfer chain of PSI since flavodoxin accepts electrons from FA/FB and it is much simpler than the method with NADP+ photoreduction. With this method, we found that the affinity of wild-type PSI for rFlvd was 35% higher than that of the PsaE-less PSI, showing that this method is sensitive to structural changes of PSI. Our results demonstrate that rFlvd photoreduction is an effective and simple method for PSI activity measurement.

Surface analysis of the photosystem I complex by electron and atomic force microscopy

Two-dimensional (2D) crystals of the photosystem I (PSI) reaction center from Synechococcus sp. OD24 were analyzed by electron and atomic force microscopy. Surface relief reconstructions from electron micrographs of freeze-dried unidirectionally shadowed samples and topographs recorded with the atomic force microscope (AFM) provided a precise definition of the lumenal and stromal PSI surfaces. The lumenal surface was composed of four protrusions that surrounded an indentation. One of the protrusions, the PsaF subunit, was often missing. Removal of the extrinsic proteins with the AFM stylus exposed the stromal side of the PSI core, whose surface structure could then be imaged at a resolution better than 1.4 nm. This interfacial surface between core and extrinsic subunits, had a pseudo-2-fold symmetry and protrusions that correlated with the surface helices e and e' or were at the sites of putative alpha-helix-connecting loops estimated from the 4 A map of the complex. The molecular dissection achieved with the AFM, opens new possibilities to unveil the interfaces between subunits of supramolecular assemblies.

Characterization of psaI and psaL mutants of Synechococcus sp. strain PCC 7002: a new model for state transitions in cyanobacteria

The psaI and psaL genes were characterized from the cyanobacterium Synechococcus sp. strain PCC 7002. The gene organization was different from that reported for other cyanobacteria with psaI occurring upstream and being divergently transcribed from the psaL gene. Mutants lacking PsaI or PsaL were generated by interposon mutagenesis and characterized physiologically and biochemically. Mutant strains PR6307 (delta psaI), PR6308 (psaI-) and PR6309 (psaL-) had doubling times similar to that of the wild type under both high- and low-intensity white light, but all grew more slowly than the wild type in green light. Only monomeric photosystem I (PS I) complexes could be isolated from each mutant strain when Triton X-100 was used to solubilize thylakoid membranes; however, approximately 10% of the PS I complexes from the psaI mutants, but not the psaL mutant, could be isolated as trimers when n-dodecyl beta-D-maltoside was used. Compositional analyses of the mutant PS I complexes indicate that the presence of PsaL is required for trimer formation or stabilization and that PsaI plays a role in stabilizing the binding of both PsaL and PsaM to the PS I complex. Strain PR6309 (psaL-) was capable of performing a state 2 to state 1 transition approximately three times more rapidly than the wild type. Because the monomeric PS I complexes of this mutant should be capable of diffusing more rapidly than trimeric complexes, these data suggest that PS I complexes rather than phycobilisomes might move during state transitions. A "mobile-PS I" model for state transitions that incorporates these ideas is discussed.

Widespread use of the glu-tRNAGln transamidation pathway among bacteria. A member of the alpha purple bacteria lacks glutaminyl-trna synthetase

The expression of the Rhizobium meliloti glutamyl-tRNA synthetase gene in Escherichia coli under the control of a trc promoter results in a toxic effect upon isopropyl-beta-D-thiogalactopyranoside induction, which is probably caused by a misacylation activity. To further investigate this unexpected result, we looked at the pathway of Gln-tRNAGln formation in R. meliloti. No glutaminyl-tRNA synthetase activity has been found in R. meliloti crude extract, but we detected a specific aminotransferase activity that changes Glu-tRNAGln to Gln-tRNAGln. Our results show that R. meliloti, a member of the alpha-subdivision of the purple bacteria, is the first Gram-negative bacteria reported to use a transamidation pathway for Gln-tRNAGln synthesis. A phylogenetic analysis of the contemporary glutamyl-tRNA synthetase and glutaminyl-tRNA synthetase amino acid sequences reveals that a close evolutionary relationship exists between R. meliloti and yeast mitochondrial glutamyl-tRNA synthetases, which is consistent with an origin of mitochondria in the alpha-subdivision of Gram-negative purple bacteria. A 256-amino acid open reading frame closely related to bacterial glutamyl-tRNA synthetases, which probably originates from a glutamyl-tRNA synthetase gene duplication, was found in the 4-min region of the E. coli chromosome. We suggest that this open reading frame is a relic of an ancient transamidation pathway that occurred in an E. coli ancestor before the horizontal transfer of a eukaryotic glutaminyl-tRNA synthetase (Lamour, V., Quevillon, S., Diriong, S., N'Guyen, V. C., Lipinski, M., and Mirande, M.(1994) Proc. Natl. Acad. Sci. U. S. A. 91, 8670-8674) and that it favored its stable acquisition. From these observations, a revisited model for the evolution of the contemporary glutamyl-tRNA synthetases and glutaminyl-tRNA synthetases that differs from the generally accepted model for the evolution of aminoacyl-tRNA synthetases is proposed.

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.

Interaction between photosystem I and flavodoxin from the cyanobacterium Synechococcus sp. PCC 7002 as revealed by chemical cross-linking

The interaction between photosystem I (PS I) and flavodoxin from the cyanobacterium Synechococcus sp. PCC 7002 was investigated by covalent cross-linking in the presence of a hydrophilic cross-linker, N- ethyl-3-(3-diaminopropyl)carbodiimide. Under the experimental conditions employed, five distinct cross-linking products of flavodoxin and PS I subunits are formed. Immunoblot analyses show that these species are the result of cross-linking of flavodoxin to PsaC, PsaD, an unidentified low-molecular-mass PS I polypeptide, and a 15-kDa subunit. The latter has been indirectly identified as the PsaF subunit. Analysis of the interaction of flavodoxin with PS I from a psaE mutant indicates that the PsaE subunit is required for correct complex formation between flavodoxin and PS I, although this subunit is not directly cross-linked to flavodoxin. In addition, the cross-linking products of PsaD with PsaC and PsaL, and PsaE with PsaF, are observed. The covalent complex of flavodoxin and PS I is shown to be fully inhibited with respect to electron transfer to soluble flavodoxin, ferredoxin or ferredoxin:NADP+ oxidoreductase.

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.

Light adaptation of cyclic electron transport through Photosystem I in the cyanobacterium Synechococcus sp. PCC 7942

Photosystem I-driven cyclic electron transport was measured in intact cells of Synechococcus sp PCC 7942 grown under different light intensities using photoacoustic and spectroscopic methods. The light-saturated capacity for PS I cyclic electron transport increased relative to chlorophyll concentration, PS I concentration, and linear electron transport capacity as growth light intensity was raised. In cells grown under moderate to high light intensity, PS I cyclic electron transport was nearly insensitive to methyl viologen, indicating that the cyclic electron supply to PS I derived almost exclusively from a thylakoid dehydrogenase. In cells grown under low light intensity, PS I cyclic electron transport was partially inhibited by methyl viologen, indicating that part of the cyclic electron supply to PS I derived directly from ferredoxin. It is proposed that the increased PSI cyclic electron transport observed in cells grown under high light intensity is a response to chronic photoinhibition.

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)

Characterization of a Synechococcus sp. strain PCC 7002 mutant lacking Photosystem I. Protein assembly and energy distribution in the absence of the Photosystem I reaction center core complex

A Synechococcus sp. strain PCC 7002 ΔpsaAB::cat mutant has been constructed by deletional interposon mutagenesis of the psaA and psaB genes through selection and segregation under low-light conditions. This strain can grow photoheterotrophically with glycerol as carbon source with a doubling time of 25 h at low light intensity (10 μE m(-2) s(-1)). No Photosystem I (PS I)-associated chlorophyll fluorescence emission peak was detected in the ΔpsaAB::cat mutant. The chlorophyll content of the ΔpsaAB::cat mutant was approximately 20% that of the wild-type strain on a per cell basis. In the absence of the PsaA and PsaB proteins, several other PS I proteins do not accumulate to normal levels. Assembly of the peripheral PS I proteins PsaC,PsaD, PsaE, and PsaL is dependent on the presence of the PsaA and PsaB heterodimer core. The precursor form of PsaF may be inserted into the thylakoid membrane but is not processed to its mature form in the absence of PsaA and PsaB. The absence of PS I reaction centers has no apparent effect on Photosystem II (PS II) assembly and activity. Although the mutant exhibited somewhat greater fluorescence emission from phycocyanin, most of the light energy absorbed by phycobilisomes was efficiently transferred to the PS II reaction centers in the absence of the PS I. No light state transition could be detected in the ΔpsaAB::cat strain; in the absence of PS I, cells remain in state 1. Development of this relatively light-tolerant strain lacking PS I provides an important new tool for the genetic manipulation of PS I and further demonstrates the utility of Synechococcus sp. PCC 7002 for structural and functional analyses of the PS I reaction center.

Function and organization of Photosystem I polypeptides

Photosystem I functions as a plastocyanin:ferredoxin oxidoreductase in the thylakoid membranes of chloroplasts and cyanobacteria. The PS I complex contains the photosynthetic pigments, the reaction center P700, and five electron transfer centers (A0, A1, FX, FA, and FB) that are bound to the PsaA, PsaB, and PsaC proteins. In addition, PS I complex contains at least eight other polypeptides that are accessory in their functions. Recent use of cyanobacterial molecular genetics has revealed functions of the accessory subunits of PS I. Site-directed mutagenesis is now being used to explore structure-function relations in PS I. The overall architecture of PSI complex has been revealed by X-ray crystallography, electron microscopy, and biochemical methods. The information obtained by different techniques can be used to propose a model for the organization of PS I. Spectroscopic and molecular genetic techniques have deciphered interaction of PS I proteins with the soluble electron transfer partners. This review focuses on the recent structural, biochemical and molecular genetic studies that decipher topology and functions of PS I proteins, and their interactions with soluble electron carriers.