Fast electron transfer from cytochrome c6 and plastocyanin to photosystem I of Chlamydomonas reinhardtii requires PsaF

Decoding Microcystis aeruginosa quorum sensing through AHL-mediated transcriptomic molecular regulation mechanisms

Acyl-homoserine lactone (AHL) serves as a key signaling molecule for quorum sensing (QS) in bacteria. QS-related genes and physiological processes in Microcystis aeruginosa remain elusive. In this study, we elucidated the regulatory role of AHL-mediated QS in M. aeruginosa. Using AHL activity extract and transcriptomic analysis, we revealed significant effects of the AHL on growth and photosynthesis. AHL significantly increased chlorophyll a (Chl-a) content and accelerated photosynthetic rate thereby promoting growth. Transcriptome analysis revealed that AHL stimulated the up-regulation of photosynthesis-related genes (apcABF, petE, psaBFK, psbUV, etc.) as well as nitrogen metabolism and ribosomal metabolism. In addition, AHL-regulated pathways are associated with lipopolysaccharide and phenazine synthesis. Our findings deepen the understanding of the QS system in M. aeruginosa and are important for gaining insights into the role of QS in Microcystis bloom formation. It also provides new insights into the prevalence of M. aeruginosa in water blooms.

Algal photosystem I dimer and high-resolution model of PSI-plastocyanin complex

Photosystem I (PSI) enables photo-electron transfer and regulates photosynthesis in the bioenergetic membranes of cyanobacteria and chloroplasts. Being a multi-subunit complex, its macromolecular organization affects the dynamics of photosynthetic membranes. Here we reveal a chloroplast PSI from the green alga Chlamydomonas reinhardtii that is organized as a homodimer, comprising 40 protein subunits with 118 transmembrane helices that provide scaffold for 568 pigments. Cryogenic electron microscopy identified that the absence of PsaH and Lhca2 gives rise to a head-to-head relative orientation of the PSI-light-harvesting complex I monomers in a way that is essentially different from the oligomer formation in cyanobacteria. The light-harvesting protein Lhca9 is the key element for mediating this dimerization. The interface between the monomers is lacking PsaH and thus partially overlaps with the surface area that would bind one of the light-harvesting complex II complexes in state transitions. We also define the most accurate available PSI-light-harvesting complex I model at 2.3 Å resolution, including a flexibly bound electron donor plastocyanin, and assign correct identities and orientations to all the pigments, as well as 621 water molecules that affect energy transfer pathways.

Structure of cyanobacterial photosystem I complexed with ferredoxin at 1.97 Å resolution

Photosystem I (PSI) is a light driven electron pump transferring electrons from Cytochrome c₆ (Cyt c₆) to Ferredoxin (Fd). An understanding of this electron transfer process is hampered by a paucity of structural detail concerning PSI:Fd interface and the possible binding sites of Cyt c₆. Here we describe the high resolution cryo-EM structure of Thermosynechococcus elongatus BP-1 PSI in complex with Fd and a loosely bound Cyt c₆. Side chain interactions at the PSI:Fd interface including bridging water molecules are visualized in detail. The structure explains the properties of mutants of PsaE and PsaC that affect kinetics of Fd binding and suggests a molecular switch for the dissociation of Fd upon reduction. Calorimetry-based thermodynamic analyses confirms a single binding site for Fd and demonstrates that PSI:Fd complexation is purely driven by entropy. A possible reaction cycle for the efficient transfer of electrons from Cyt c₆ to Fd via PSI is proposed.

In order to aid the understanding of the electron transfer process within the cyanobacterial photosystem I, its structure - when complexed with Ferredoxin - is determined at 1.97 Å resolution.

Electron Transfer in a Bio-Photoelectrode Based on Photosystem I Multilayer Immobilized on the Conducting Glass

A film of ~40 layers of partially oriented photosystem I (PSI) complexes isolated from the red alga Cyanidioschyzon merolae formed on the conducting glass through electrodeposition was investigated by time-resolved absorption spectroscopy and chronoamperometry. The experiments were performed at a range of electric potentials applied to the film and at different compositions of electrolyte solution being in contact with the film. The amount of immobilized proteins supporting light-induced charge separation (active PSI) ranged from ~10%, in the absence of any reducing agents (redox compounds or low potential), to ~20% when ascorbate and 2,6-dichlorophenolindophenol were added, and to ~35% when the high negative potential was additionally applied. The origin of the large fraction of permanently inactive PSI (65–90%) was unclear. Both reducing agents increased the subpopulation of active PSI complexes, with the neutral P700 primary electron donor, by reducing significant fractions of the photo-oxidized P700 species. The efficiencies of light-induced charge separation in the PSI film (10–35%) did not translate into an equally effective generation of photocurrent, whose internal quantum efficiency reached the maximal value of 0.47% at the lowest potentials. This mismatch indicates that the vast majority of the charge-separated states in multilayered PSI complexes underwent charge recombination.

Recent advances in the structural diversity of reaction centers

Photosynthetic reaction centers (RC) catalyze the conversion of light to chemical energy that supports life on Earth, but they exhibit substantial diversity among different phyla. This is exemplified in a recent structure of the RC from an anoxygenic green sulfur bacterium (GsbRC) which has characteristics that may challenge the canonical view of RC classification. The GsbRC structure is analyzed and compared with other RCs, and the observations reveal important but unstudied research directions that are vital for disentangling RC evolution and diversity. Namely, (1) common themes of electron donation implicate a Ca²⁺ site whose role is unknown; (2) a previously unidentified lipid molecule with unclear functional significance is involved in the axial ligation of a cofactor in the electron transfer chain; (3) the GsbRC features surprising structural similarities with the distantly-related photosystem II; and (4) a structural basis for energy quenching in the GsbRC can be gleaned that exemplifies the importance of how exposure to oxygen has shaped the evolution of RCs. The analysis highlights these novel avenues of research that are critical for revealing evolutionary relationships that underpin the great diversity observed in extant RCs.

Transcriptional Analysis of Microcystis aeruginosa Co-Cultured with Algicidal Bacteria Brevibacillus laterosporus

Harmful algal blooms caused huge ecological damage and economic losses around the world. Controlling algal blooms by algicidal bacteria is expected to be an effective biological control method. The current study investigated the molecular mechanism of harmful cyanobacteria disrupted by algicidal bacteria. Microcystis aeruginosa was co-cultured with Brevibacillus laterosporus Bl-zj, and RNA-seq based transcriptomic analysis was performed compared to M. aeruginosa, which was cultivated separately. A total of 1706 differentially expressed genes were identified, which were mainly involved in carbohydrate metabolism, energy metabolism and amino acid metabolism. In the co-cultured group, the expression of genes mainly enriched in photosynthesis and oxidative phosphorylation were significantly inhibited. However, the expression of the genes related to fatty acid synthesis increased. In addition, the expression of the antioxidant enzymes, such as 2-Cys peroxiredoxin, was increased. These results suggested that B. laterosporus could block the electron transport by attacking the PSI system and complex I of M. aeruginosa, affecting the energy acquisition and causing oxidative damage. This further led to the lipid peroxidation of the microalgal cell membrane, resulting in algal death. The transcriptional analysis of algicidal bacteria in the interaction process can be combined to explain the algicidal mechanism in the future.

Structure of plant photosystem I-plastocyanin complex reveals strong hydrophobic interactions

Photosystem I is defined as plastocyanin-ferredoxin oxidoreductase. Taking advantage of genetic engineering, kinetic analyses and cryo-EM, our data provide novel mechanistic insights into binding and electron transfer between PSI and Pc. Structural data at 2.74 Å resolution reveals strong hydrophobic interactions in the plant PSI-Pc ternary complex, leading to exclusion of water molecules from PsaA-PsaB/Pc interface once the PSI-Pc complex forms. Upon oxidation of Pc, a slight tilt of bound oxidized Pc allows water molecules to accommodate the space between Pc and PSI to drive Pc dissociation. Such a scenario is consistent with the six times larger dissociation constant of oxidized as compared with reduced Pc and mechanistically explains how this molecular machine optimized electron transfer for fast turnover.

In vivo electron donation from plastocyanin and cytochrome c6 to PSI in Synechocystis sp. PCC6803

Many cyanobacteria species can use both plastocyanin and cytochrome c₆ as lumenal electron carriers to shuttle electrons from the cytochrome b₆f to either photosystem I or the respiratory cytochrome c oxidase. In Synechocystis sp. PCC6803 placed in darkness, about 60% of the active PSI centres are bound to a reduced electron donor which is responsible for the fast re-reduction of P₇₀₀in vivo after a single charge separation. Here, we show that both cytochrome c₆ and plastocyanin can bind to PSI in the dark and participate to the fast phase of P₇₀₀ reduction, but the fraction of pre-bound PSI is smaller in the case of cytochrome c₆ than with plastocyanin. Because of the inter-connection of respiration and photosynthesis in cyanobacteria, the inhibition of the cytochrome c oxidase results in the over-reduction of the photosynthetic electron transfer chain in the dark that translates into a lag in the kinetics of P₇₀₀ oxidation at the onset of light. We show that this is true both with plastocyanin and cytochrome c₆, indicating that the partitioning of electron transport between respiration and photosynthesis is regulated in the same way independently of which of the two lumenal electron carriers is present, although the mechanisms of such regulation are yet to be understood.

Are plastocyanin and ferredoxin specific electron carriers or generic redox capacitors? Classical and murburn perspectives on two photosynthetic proteins

In the light reaction of oxygenic photosynthesis, plastocyanin (PC) and ferredoxins (Fd) are small/diffusible redox-active proteins playing key roles in electron transfer/transport phenomena. In the Z-scheme mechanistic purview, they are considered as specific affinity binding-based electron-relay agents, linking the functions of Cytochrome b₆f (Cyt. b₆f), Photosystem I (PS I) and Fd:NADPH oxidoreductase (FNR). The murburn explanation for photolytic photophosphorylation deems PC/Fd as generic 'redox capacitors', temporally accepting and releasing one-electron equivalents in reaction milieu. Herein, we explore the two theories with respect to structural, distributional and functional aspects of PC/Fd. Amino acid residues located on the surface loci of key patches of PC/Fd vary in electrostatic/contour (topography) signatures. Crystal structures of four different complexes each of Cyt.f-PC and Fd-FNR show little conservation in the contact-surfaces, thereby discrediting 'affinity binding-based electron transfers (ET)' as an evolutionary logic. Further, thermodynamic and kinetic data of wildtype and mutant proteins interactions do not align with Z-scheme. Furthermore, micromolar physiological concentrations of PC and the non-conducive architecture of chloroplasts render the classical model untenable. In the murburn model, as PC is optional, the observation that plants lacking PC survive and grow is justified. Further, the low physiological concentration/distribution of PC in chloroplast lumen/stroma is supported by murburn equilibriums, as higher concentrations would limit electron transfers. Thus, structural evidence, interactive dynamics with redox partners and physiological distribution/role of PC/Fd support the murburn perspective that these proteins serve as generic redox-capacitors in chloroplasts.Communicated by Ramaswamy H. Sarma.

Evolutionary Loss of the Ability of the Photosystem I Primary Electron Donor for the Redox Interaction with Mn-Bicarbonate Complexes

The structure and functional organization of the photosystem I (PSI) reaction center (RC) donor side has a significant similarity to the reaction centers of purple bacteria (bRCs), despite the fact that they belong to different types of RCs. Moreover, the redox potential values of their primary electron donors are identical (~0.5 V). In our earlier reports [Khorobrykh et al. (2008) Phylos. Trans. R. Soc. B., 363, 1245-1251; Terentyev et al. (2011) Biochemistry (Moscow), 76, 1360-1366; Khorobrykh et al. (2018) ChemBioChem, 14, 1725-1731], we have demonstrated redox interaction of low-potential Mn2+-bicarbonate complexes with bRCs, which might have been one of the first steps in the evolutionary origin of Mn-cluster of the photosystem II water-oxidizing complex that occurred in the Archean (over 3 billion years ago). In this study, we investigated redox interactions between Mn2+-bicarbonate complexes and PSI. Such interactions were almost absent in the original PSI preparations and emerged only in preoxidized PSI preparations containing ~50% oxidized RCs. The interaction between Mn2+-bicarbonate complexes and PSI required increased Mn2+ concentrations, while its dependence on the HCO₃^- concentration indicated involvement of electroneutral low-potential [Mn(HCO₃)₂] complex in the process. Analysis of the PSI crystal structure revealed steric hindrances on the RC donor side, which could block the redox interaction between Mn2+-bicarbonate complexes and oxidized primary electron donor. Comparison of structures of RCs from the PSI and ancient RCs from heliobacteria belonging to the same type of RCs suggested that such hindrances should be absent in the primitive PSI in the Archean and allowed to explain their evolutionary origin as a consequence of PSI RCs into the united electron transport chain (ETC) of the photosynthetic membrane that was accompanied by the evolutionary loss of PSI capacity for the redox interaction with Mn2+-bicarbonate complexes.

Structural basis for the adaptation and function of chlorophyll f in photosystem I

Chlorophylls (Chl) play pivotal roles in energy capture, transfer and charge separation in photosynthesis. Among Chls functioning in oxygenic photosynthesis, Chl f is the most red-shifted type first found in a cyanobacterium Halomicronema hongdechloris. The location and function of Chl f in photosystems are not clear. Here we analyzed the high-resolution structures of photosystem I (PSI) core from H. hongdechloris grown under white or far-red light by cryo-electron microscopy. The structure showed that, far-red PSI binds 83 Chl a and 7 Chl f, and Chl f are associated at the periphery of PSI but not in the electron transfer chain. The appearance of Chl f is well correlated with the expression of PSI genes induced under far-red light. These results indicate that Chl f functions to harvest the far-red light and enhance uphill energy transfer, and changes in the gene sequences are essential for the binding of Chl f.

Chlorophyll f (Chl f) is the most red-shifted Chl in oxygenic photosynthesis but its localization in photosystem I (PSI) has been unknown so far. Here the authors determine the cryo-EM structures of PSI complexes from a Chl f-containing cyanobacterium grown either under white light or far-red light conditions and identify seven Chls f in the far-red light PSI structure, whereas PSI from cells grown under white light contains only Chl a.

Prediction of Intrinsic Disorder with Quality Assessment Using QUARTER

Intrinsically disordered regions (IDRs) are estimated to be highly abundant in nature. While only several thousand proteins are annotated with experimentally derived IDRs, computational methods can be used to predict IDRs for the millions of currently uncharacterized protein chains. Several dozen disorder predictors were developed over the last few decades. While some of these methods provide accurate predictions, unavoidably they also make some mistakes. Consequently, one of the challenges facing users of these methods is how to decide which predictions can be trusted and which are likely incorrect. This practical problem can be solved using quality assessment (QA) scores that predict correctness of the underlying (disorder) predictions at a residue level. We motivate and describe a first-of-its-kind toolbox of QA methods, QUARTER (QUality Assessment for pRotein inTrinsic disordEr pRedictions), which provides the scores for a diverse set of ten disorder predictors. QUARTER is available to the end users as a free and convenient webserver at http://biomine.cs.vcu.edu/servers/QUARTER/ . We briefly describe the predictive architecture of QUARTER and provide detailed instructions on how to use the webserver. We also explain how to interpret results produced by QUARTER with the help of a case study.

Non-detergent isolation of a cyanobacterial photosystem I using styrene maleic acid alternating copolymers

Trimeric Photosystem I (PSI) from the thermophilic cyanobacteriumThermosynechococcus elongatus(Te) is the largest membrane protein complex to be encapsulated within a SMALP to date.

Structure of a PSI-LHCI-cyt b6f supercomplex in Chlamydomonas reinhardtii promoting cyclic electron flow under anaerobic conditions

Photosynthetic linear electron flow (LEF) produces ATP and NADPH, while cyclic electron flow (CEF) exclusively drives photophosphorylation to supply extra ATP. The fine-tuning of linear and cyclic electron transport levels allows photosynthetic organisms to balance light energy absorption with cellular energy requirements under constantly changing light conditions. As LEF and CEF share many electron transfer components, a key question is how the same individual structural units contribute to these two different functional modes. Here, we report the structural identification of a photosystem I (PSI)-light harvesting complex I (LHCI)-cytochrome (cyt) b₆f supercomplex isolated from the unicellular alga Chlamydomonas reinhardtii under anaerobic conditions, which induces CEF. This provides strong evidence for the model that enhanced CEF is induced by the formation of CEF supercomplexes, when stromal electron carriers are reduced, to generate additional ATP. The additional identification of PSI-LHCI-LHCII complexes is consistent with recent findings that both CEF enhancement and state transitions are triggered by similar conditions, but can occur independently from each other. Single molecule fluorescence correlation spectroscopy indicates a physical association between cyt b₆f and fluorescent chlorophyll containing PSI-LHCI supercomplexes. Single particle analysis identified top-view projections of the corresponding PSI-LHCI-cyt b₆f supercomplex. Based on molecular modeling and mass spectrometry analyses, we propose a model in which dissociation of LHCA2 and LHCA9 from PSI supports the formation of this CEF supercomplex. This is supported by the finding that a Δlhca2 knockout mutant has constitutively enhanced CEF.

Absolute quantification of selected photosynthetic electron transfer proteins in Chlamydomonas reinhardtii in the presence and absence of oxygen

The absolute amount of plastocyanin (PC), ferredoxin-NADP⁺-oxidoreductase (FNR), hydrogenase (HYDA1), and ferredoxin 5 (FDX5) were quantified in aerobic and anaerobic Chlamydomonas reinhardtii whole cells using purified (recombinant) proteins as internal standards in a mass spectrometric approach. Quantified protein amounts were related to the estimated amount of PSI. The ratios of PC to FNR to HYDA1 to FDX5 in aerobic cells were determined to be 1.4:1.2:0.003:0. In anaerobic cells, the ratios changed to 1.1:1.3:0.019:0.027 (PC:FNR:HYDA1:FDX5). Employing sodium dithionite and methyl viologen as electron donors, the specific activity of hydrogenase in whole cells was calculated to be 382 ± 96.5 μmolH₂ min^-1 mg^-1. Importantly, these data reveal an about 70-fold lower abundance of HYDA1 compared to FNR. Despite this great disproportion between both proteins, which might further enhance the competition for electrons, the alga is capable of hydrogen production under anaerobic conditions, thus pointing to an efficient channeling mechanism of electrons from FDX1 to the HYDA1.

Extensive gain and loss of photosystem I subunits in chromerid algae, photosynthetic relatives of apicomplexans

In oxygenic photosynthesis the initial photochemical processes are carried out by photosystem I (PSI) and II (PSII). Although subunit composition varies between cyanobacterial and plastid photosystems, the core structures of PSI and PSII are conserved throughout photosynthetic eukaryotes. So far, the photosynthetic complexes have been characterised in only a small number of organisms. We performed in silico and biochemical studies to explore the organization and evolution of the photosynthetic apparatus in the chromerids Chromera velia and Vitrella brassicaformis, autotrophic relatives of apicomplexans. We catalogued the presence and location of genes coding for conserved subunits of the photosystems as well as cytochrome b₆f and ATP synthase in chromerids and other phototrophs and performed a phylogenetic analysis. We then characterised the photosynthetic complexes of Chromera and Vitrella using 2D gels combined with mass-spectrometry and further analysed the purified Chromera PSI. Our data suggest that the photosynthetic apparatus of chromerids underwent unique structural changes. Both photosystems (as well as cytochrome b₆f and ATP synthase) lost several canonical subunits, while PSI gained one superoxide dismutase (Vitrella) or two superoxide dismutases and several unknown proteins (Chromera) as new regular subunits. We discuss these results in light of the extraordinarily efficient photosynthetic processes described in Chromera.

Obstacles in the quantification of the cyclic electron flux around Photosystem I in leaves of C3 plants

Sixty years ago Arnon and co-workers discovered photophosphorylation driven by a cyclic electron flux (CEF) around Photosystem I. Since then understanding the physiological roles and the regulation of CEF has progressed, mainly via genetic approaches. One basic problem remains, however: quantifying CEF in the absence of a net product. Quantification of CEF under physiological conditions is a crucial prerequisite for investigating the physiological roles of CEF. Here we summarize current progress in methods of CEF quantification in leaves and, in some cases, in isolated thylakoids, of C3 plants. Evidently, all present methods have their own shortcomings. We conclude that to quantify CEF in vivo, the best way currently is to measure the electron flux through PS I (ETR1) and that through PS II and PS I in series (ETR2) for the whole leaf tissue under identical conditions. The difference between ETR1 and ETR2 is an upper estimate of CEF, mainly consisting, in C3 plants, of a major PGR5-PGRL1-dependent CEF component and a minor chloroplast NDH-dependent component, where PGR5 stands for Proton Gradient Regulation 5 protein, PGRL1 for PGR5-like photosynthesis phenotype 1, and NDH for Chloroplast NADH dehydrogenase-like complex. These two CEF components can be separated by the use of antimycin A to inhibit the former (major) component. Membrane inlet mass spectrometry utilizing stable oxygen isotopes provides a reliable estimation of ETR2, whilst ETR1 can be estimated from a method based on the photochemical yield of PS I, Y(I). However, some issues for the recommended method remain unresolved.

The structure of plant photosystem I super-complex at 2.8 Å resolution

Most life forms on Earth are supported by solar energy harnessed by oxygenic photosynthesis. In eukaryotes, photosynthesis is achieved by large membrane-embedded super-complexes, containing reaction centers and connected antennae. Here, we report the structure of the higher plant PSI-LHCI super-complex determined at 2.8 Å resolution. The structure includes 16 subunits and more than 200 prosthetic groups, which are mostly light harvesting pigments. The complete structures of the four LhcA subunits of LHCI include 52 chlorophyll a and 9 chlorophyll b molecules, as well as 10 carotenoids and 4 lipids. The structure of PSI-LHCI includes detailed protein pigments and pigment-pigment interactions, essential for the mechanism of excitation energy transfer and its modulation in one of nature's most efficient photochemical machines.

A method aimed at assessing the functional consequences of the supramolecular organization of the respiratory electron transfer chain by time-resolved studies

A steadily increasing number of physiological, biochemical, and structural studies have provided a growing support to the notion that the respiratory electron transfer chain may contain supra-molecular edifices made of the assembly of some, if not all, of its individual links. This structure, usually referred to as the solid state model-in comparison to the liquid state model in which the electron transfer reactions between the membrane bound enzymes are diffusion controlled-is seen as conferring specific kinetic properties to the chain and thus as being highly relevant from a functional point of view. Although the assumption that structural changes are mirrored by functional adjustment is undoubtedly legitimate, experimental evidences supporting it remain scarce. Here we review a recent methodological development aimed at tackling the functional relevance of the supramolecular organization of the respiratory electron transfer chain in intact cells.

Calcium-dependent regulation of cyclic photosynthetic electron transfer by a CAS, ANR1, and PGRL1 complex

Cyclic photosynthetic electron flow (CEF) is crucial to photosynthesis because it participates in the control of chloroplast energy and redox metabolism, and it is particularly induced under adverse environmental conditions. Here we report that down-regulation of the chloroplast localized Ca(2+) sensor (CAS) protein by an RNAi approach in Chlamydomonas reinhardtii results in strong inhibition of CEF under anoxia. Importantly, this inhibition is rescued by an increase in the extracellular Ca(2+) concentration, inferring that CEF is Ca(2+)-dependent. Furthermore, we identified a protein, anaerobic response 1 (ANR1), that is also required for effective acclimation to anaerobiosis. Depletion of ANR1 by artificial microRNA expression mimics the CAS-depletion phenotype, and under anaerobic conditions the two proteins coexist within a large active photosystem I-cytochrome b(6)/f complex. Moreover, we provide evidence that CAS and ANR1 interact with each other as well as with PGR5-Like 1 (PGRL1) in vivo. Overall our data establish a Ca(2+)-dependent regulation of CEF via the combined function of ANR1, CAS, and PGRL1, associated with each other in a multiprotein complex.

Residues PsaB Asp612 and PsaB Glu613 of photosystem I confer pH-dependent binding of plastocyanin and cytochrome c(6)

The binding and electron transfer between plastocyanin (pc) or cytochrome c(6) (cyt c(6)) and photosystem I (PSI) can be described by hydrophobic as well as electrostatic interactions. The two α helices, l and l' in PsaB and PsaA, respectively, are involved in forming the hydrophobic interaction site at the oxidizing site of PSI. To obtain mechanistic insights into the function of the two negatively charged residues D612 and E613, present in α helix l of PsaB, we exchanged both residues by site-directed mutagenesis with His and transformed a PsaB deficient mutant of Chlamydomonas reinhardtii. Flash-induced absorption spectroscopy revealed that PSI harboring the changes D612H and E613H had a high affinity toward binding of the electron donors and possessed an altered pH dependence of electron transfer with pc and cyt c(6). Despite optimized binding and electron transfer between the altered PSI and its electron donors, the mutant strain PsaB-D612H/E613H exhibited a strong light sensitive growth phenotype, indicating that decelerated turnover between pc/cyt c(6) and PSI with respect to electron transfer is deleterious to the cells.

Mutational analysis of photosystem I of Synechocystis sp. PCC 6803: the role of four conserved aromatic residues in the j-helix of PsaB

Photosystem I is the light-driven plastocyanin-ferredoxin oxidoreductase in the photosynthetic electron transfer of cyanobacteria and plants. Two histidyl residues in the symmetric transmembrane helices A-j and B-j provide ligands for the P700 chlorophyll molecules of the reaction center of photosystem I. To determine the role of conserved aromatic residues adjacent to the histidyl molecule in the helix of B-j, we generated six site-directed mutants of the psaB gene in Synechocystis sp. PCC 6803. Three mutant strains with W645C, W643C/A644I and S641C/V642I substitutions could grow photoautotrophically and showed no obvious reduction in the photosystem I activity. Kinetics of P700 re-reduction by plastocyanin remained unaltered in these mutants. In contrast, the strains with H651C/L652M, F649C/G650I and F647C substitutions could not grow under photoautotrophic conditions because those mutants had low photosystem I activity, possibly due to low levels of proteins. A procedure to select spontaneous revertants from the mutants that are incapable to photoautotrophic growth resulted in three revertants that were used in this study. The molecular analysis of the spontaneous revertants suggested that an aromatic residue at F647 and a small residue at G650 may be necessary for maintaining the structural integrity of photosystem I. The (P700⁺-P700) steady-state absorption difference spectrum of the revertant F647Y has a ∼5 nm narrower peak than the recovered wild-type, suggesting that additional hydroxyl group of this revertant may participate in the interaction with the special pair while the photosystem I complexes of the F649C/G650T and H651Q mutants closely resemble the wild-type spectrum. The results presented here demonstrate that the highly conserved residues W645, W643 and F649 are not critical for maintaining the integrity and in mediating electron transport from plastocyanin to photosystem I. Our data suggest that an aromatic residue is required at position of 647 for structural integrity and/or function of photosystem I.

Purification of plastocyanin and cytochrome c6 from plants, green algae, and cyanobacteria

Plastocyanin and cytochrome c6 are widely distributed over the oxygen-evolving photosynthetic organisms. The two proteins are functionally equivalent, but strongly differ in their global electrostatic charge. In fact, they are acidic in eukaryotes, but either neutral or basic in cyanobacteria. Such a difference in their electrostatic features is a critical factor in designing the purification procedure, which must thus be modified and adapted accordingly. This chapter reports the methods for producing (including cell cultures), isolating, and purifying plastocyanin and cytochrome c6--which greatly differ in their isoelectric point--from a number of eukaryotic and prokaryotic organisms.

The structure and function of eukaryotic photosystem I

Eukaryotic photosystem I consists of two functional moieties: the photosystem I core, harboring the components for the light-driven charge separation and the subsequent electron transfer, and the peripheral light-harvesting complex (LHCI). While the photosystem I-core remained highly conserved throughout the evolution, with the exception of the oxidizing side of photosystem I, the LHCI complex shows a high degree of variability in size, subunits composition and bound pigments, which is due to the large variety of different habitats photosynthetic organisms dwell in. Besides summarizing the most current knowledge on the photosystem I-core structure, we will discuss the composition and structure of the LHCI complex from different eukaryotic organisms, both from the red and the green clade. Furthermore, mechanistic insights into electron transfer between the donor and acceptor side of photosystem I and its soluble electron transfer carrier proteins will be given. This article is part of a Special Issue entitled: Regulation of Electron Transport in Chloroplasts.

The composition and structure of photosystem I-associated antenna from Cyanidioschyzon merolae

Red algae contain two types of light-harvesting antenna systems, the phycobilisomes and chlorophyll a binding polypeptides (termed Lhcr), which expand the light-harvesting capacity of the photosynthetic reaction centers. In this study, photosystem I (PSI) and its associated light-harvesting proteins were isolated from the red alga Cyanidioschyzon merolae. The structural and functional properties of the largest PSI particles observed were investigated by biochemical characterization, mass spectrometry, fluorescence emission and excitation spectroscopy, and transmission electron microscopy. Our data provide strong evidence for a stable PSI complex in red algae that possesses two distinct types of functional peripheral light-harvesting antenna complex, comprising both Lhcr and a PSI-linked phycobilisome sub-complex. We conclude that the PSI antennae system of red algae represents an evolutionary intermediate between the prokaryotic cyanobacteria and other eukaryotes, such as green algae and vascular plants.

Isolation of the elusive supercomplex that drives cyclic electron flow in photosynthesis

Photosynthetic light reactions establish electron flow in the chloroplast's thylakoid membranes, leading to the production of the ATP and NADPH that participate in carbon fixation. Two modes of electron flow exist-linear electron flow (LEF) from water to NADP(+) via photosystem (PS) II and PSI in series and cyclic electron flow (CEF) around PSI (ref. 2). Although CEF is essential for satisfying the varying demand for ATP, the exact molecule(s) and operational site are as yet unclear. In the green alga Chlamydomonas reinhardtii, the electron flow shifts from LEF to CEF on preferential excitation of PSII (ref. 3), which is brought about by an energy balancing mechanism between PSII and PSI (state transitions). Here, we isolated a protein supercomplex composed of PSI with its own light-harvesting complex (LHCI), the PSII light-harvesting complex (LHCII), the cytochrome b(6)f complex (Cyt bf), ferredoxin (Fd)-NADPH oxidoreductase (FNR), and the integral membrane protein PGRL1 (ref. 5) from C. reinhardtii cells under PSII-favouring conditions. Spectroscopic analyses indicated that on illumination, reducing equivalents from downstream of PSI were transferred to Cyt bf, whereas oxidised PSI was re-reduced by reducing equivalents from Cyt bf, indicating that this supercomplex is engaged in CEF (Supplementary Fig. 1). Thus, formation and dissociation of the PSI-LHCI-LHCII-FNR-Cyt bf-PGRL1 supercomplex not only controlled the energy balance of the two photosystems, but also switched the mode of photosynthetic electron flow.

Characterization of the key step for light-driven hydrogen evolution in green algae

Under anaerobic conditions, several species of green algae perform a light-dependent hydrogen production catalyzed by a special group of [FeFe] hydrogenases termed HydA. Although highly interesting for biotechnological applications, the direct connection between photosynthetic electron transport and hydrogenase activity is still a matter of speculation. By establishing an in vitro reconstitution system, we demonstrate that the photosynthetic ferredoxin (PetF) is essential for efficient electron transfer between photosystem I and HydA1. To investigate the electrostatic interaction process and electron transfer between PetF and HydA1, we performed site-directed mutagenesis. Kinetic analyses with several site-directed mutagenesis variants of HydA1 and PetF enabled us to localize the respective contact sites. These experiments in combination with in silico docking analyses indicate that electrostatic interactions between the conserved HydA1 residue Lys(396) and the C terminus of PetF as well as between the PetF residue Glu(122) and the N-terminal amino group of HydA1 play a major role in complex formation and electron transfer. Mapping of relevant HydA1 and PetF residues constitutes an important basis for manipulating the physiological photosynthetic electron flow in favor of light-driven H(2) production.

PGRL1 participates in iron-induced remodeling of the photosynthetic apparatus and in energy metabolism in Chlamydomonas reinhardtii

PGRL1 RNA and protein levels are increased in iron-deficient Chlamydomonas reinhardtii cells. In an RNAi strain, which accumulates lower PGRL1 levels in both iron-replete and -starved conditions, the photosynthetic electron transfer rate is decreased, respiratory capacity in iron-sufficient conditions is increased, and the efficiency of cyclic electron transfer under iron-deprivation is diminished. Pgrl1-kd cells exhibit iron deficiency symptoms at higher iron concentrations than wild-type cells, although the cells are not more depleted in cellular iron relative to wild-type cells as measured by mass spectrometry. Thiol-trapping experiments indicate iron-dependent and redox-induced conformational changes in PGRL1 that may provide a link between iron metabolism and the partitioning of photosynthetic electron transfer between linear and cyclic flow. We propose, therefore, that PGRL1 in C. reinhardtii may possess a dual function in the chloroplast; that is, iron sensing and modulation of electron transfer.

Photosystem I gene cassettes are present in marine virus genomes

Cyanobacteria of the Synechococcus and Prochlorococcus genera are important contributors to photosynthetic productivity in the open oceans. Recently, core photosystem II (PSII) genes were identified in cyanophages and proposed to function in photosynthesis and in increasing viral fitness by supplementing the host production of these proteins. Here we show evidence for the presence of photosystem I (PSI) genes in the genomes of viruses that infect these marine cyanobacteria, using pre-existing metagenomic data from the global ocean sampling expedition as well as from viral biomes. The seven cyanobacterial core PSI genes identified in this study, psaA, B, C, D, E, K and a unique J and F fusion, form a cluster in cyanophage genomes, suggestive of selection for a distinct function in the virus life cycle. The existence of this PSI cluster was confirmed with overlapping and long polymerase chain reaction on environmental DNA from the Northern Line Islands. Potentially, the seven proteins encoded by the viral genes are sufficient to form an intact monomeric PSI complex. Projection of viral predicted peptides on the cyanobacterial PSI crystal structure suggested that the viral-PSI components might provide a unique way of funnelling reducing power from respiratory and other electron transfer chains to the PSI.

Light-induced dynamics in photosystem I electron transfer

Protein dynamics are likely to play important, regulatory roles in many aspects of photosynthetic electron transfer, but a detailed description of these coupled protein conformational changes has been unavailable. In oxygenic photosynthesis, photosystem I catalyzes the light-driven oxidation of plastocyanin or cytochrome c and the reduction of ferredoxin. A chlorophyll (chl) a/a' heterodimer, P(700), is the secondary electron donor, and the two P(700) chl, are designated P(A) and P(B). We used specific chl isotopic labeling and reaction-induced Fourier-transform infrared spectroscopy to assign chl keto vibrational bands to P(A) and P(B). In the cyanobacterium, Synechocystis sp. PCC 6803, the chl keto carbon was labeled from (13)C-labeled glutamate, and the chl keto oxygen was labeled from (18)O(2). These isotope-based assignments provide new information concerning the structure of P(A)(+), which is found to give rise to two chl keto vibrational bands, with frequencies at 1653 and 1687 cm(-1). In contrast, P(A) gives rise to one chl keto band at 1638 cm(-1). The observation of two P(A)(+) keto frequencies is consistent with a protein relaxation-induced distribution in P(A)(+) hydrogen bonding. These results suggest a light-induced conformational change in photosystem I, which may regulate the oxidation of soluble electron donors and other electron-transfer reactions. This study provides unique information concerning the role of protein dynamics in oxygenic photosynthesis.

Purification of His6-tagged Photosystem I from Chlamydomonas reinhardtii

We have developed a rapid method for isolation of the Photosystem I (PS1) complex from Chlamydomonas reinhardtii using epitope tagging. Six histidine residues were genetically added to the N-terminus of the PsaA core subunit of PS1. The His(6)-tagged PS1 could be purified with a yield of 80-90% from detergent-solubilized thylakoid membranes within 3 h in a single step using a Ni-nitrilotriacetic acid (Ni-NTA) column. Immunoblots and low-temperature fluorescence analysis indicated that the His(6)-tagged PS1 preparation was highly pure and extremely low in uncoupled pigments. Moreover, the introduced tag appeared to have no adverse effect upon PS1 structure/function, as judged by photochemical assays and EPR spectroscopy of isolated particles, as well as photosynthetic growth tests of the tagged strain.

Knock-out of the chloroplast-encoded PSI-J subunit of photosystem I in Nicotiana tabacum

The plastid-encoded psaJ gene encodes a hydrophobic low-molecular-mass subunit of photosystem I (PSI) containing one transmembrane helix. Homoplastomic transformants with an inactivated psaJ gene were devoid of PSI-J protein. The mutant plants were slightly smaller and paler than wild-type because of a 13% reduction in chlorophyll content per leaf area caused by an approximately 20% reduction in PSI. The amount of the peripheral antenna proteins, Lhca2 and Lhca3, was decreased to the same level as the core subunits, but Lhca1 and Lhca4 were present in relative excess. The functional size of the PSI antenna was not affected, suggesting that PSI-J is not involved in binding of light-harvesting complex I. The specific PSI activity, measured as NADP(+) photoreduction in vitro, revealed a 55% reduction in electron transport through PSI in the mutant. No significant difference in the second-order rate constant for electron transfer from reduced plastocyanin to oxidized P700 was observed in the absence of PSI-J. Instead, a large fraction of PSI was found to be inactive. Immunoblotting analysis revealed a secondary loss of the luminal PSI-N subunit in PSI particles devoid of PSI-J. Presumably PSI-J affects the conformation of PSI-F, which in turn affects the binding of PSI-N. This together renders a fraction of the PSI particles inactive. Thus, PSI-J is an important subunit that, together with PSI-F and PSI-N, is required for formation of the plastocyanin-binding domain of PSI. PSI-J is furthermore important for stability or assembly of the PSI complex.

Late assembly steps and dynamics of the cyanobacterial photosystem I

The dynamics of photosystem I assembly in cyanobacteria have been addressed using in vivo pulse-chase labeling of Synechocystis sp. PCC 6803 proteins in combination with blue native polyacrylamide gel electrophoresis. The analyses indicate the existence of three different monomeric photosystem I complexes and also the high stability of photosystem I trimers. We show that in addition to a complete photosystem I monomer, containing all 11 subunits, we detected a PsaK-less monomer and a short-lived PsaL/PsaK-less complex. The latter two monomers were missing in the ycf37 mutant of Synechocystis sp. PCC 6803 that accumulates also less trimers. Pulse-chase experiments suggest that the three monomeric complexes have different functions in the biogenesis of the trimer. Based on these findings we propose a model where PsaK is incorporated in the latest step of photosystem I assembly. The PsaK-less photosystem I monomer may represent an intermediate complex that is important for the exchange of the two PsaK variants during high light acclimation. Implications of the presented data with respect to Ycf37 function are discussed.

Atmospheric molding of ionic copolymer MALDI-TOF/MS arrays: A new tool for protein identification/profiling

An atmospheric molding protocol has been used to prepare an ionic methacrylate-based copolymer sample support chips for MALDI (pMALDI)-MS by targeting selected groups of various monomers copolymerized during molding, namely, carboxy, sulfo, dimethylalkyamino, and trimethylalkylammonium groups. The new disposable array chips provide analyte-oriented enhancement of protein adsorption to the modified substrates without requiring complicated surface coating or derivatization. The MALDI-MS performance of the new ionic copolymer chips was evaluated for lysozyme, beta-lactoglobulin A, trypsinogen and carbonic anhydrase I using washing with solutions prepared in pH or ionic strength steps. On cationic chips, the proteins are washed out at pH lower than their p/ values, and on anionic chips at pH higher than their p/ values. The ability of the microfabricated pMALDI chip set to selectively adsorb different proteins from real samples and to significantly increase their MS-signal was documented for the transmembrane photosystem I protein complex from the green alga Chlamydomonas reinhardtii. The proteins were almost exclusively adsorbed according to calculated pI values and grand average of hydropathy (GRAVY) indexes. The new disposable chips reduce manipulation times and increase measurement sensitivity for real-world proteomic samples. The simple atmospheric molding procedure enables additional proteomic operations to be incorporated on disposable MALDI-MS integrated platforms.

Identification of precise electrostatic recognition sites between cytochrome c6 and the photosystem I subunit PsaF using mass spectrometry

The reduction of the photo-oxidized special chlorophyll pair P700 of photosystem I (PSI) in the photosynthetic electron transport chain of eukaryotic organisms is facilitated by the soluble copper-containing protein plastocyanin (pc). In the absence of copper, pc is functionally replaced by the heme-containing protein cytochrome c6 (cyt c6) in the green alga Chlamydomonas reinhardtii. Binding and electron transfer between both donors and PSI follows a two-step mechanism that depends on electrostatic and hydrophobic recognition between the partners. Although the electrostatic and hydrophobic recognition sites on pc and PSI are well known, the precise electrostatic recognition site on cyt c6 is unknown. To specify the interaction sites on a molecular level, we cross-linked cyt c6 and PSI using a zero-length cross-linker and obtained a cross-linked complex competent in fast and efficient electron transfer. As shown previously, cyt c6 cross-links specifically with the PsaF subunit of PSI. Mass spectrometric analysis of tryptic peptides from the cross-linked product revealed specific interaction sites between residues Lys27 of PsaF and Glu69 of cyt c6 and between Lys23 of PsaF and Glu69/Glu70 of cyt c6. Using these new data, we present a molecular model of the intermolecular electron transfer complex between eukaryotic cyt c6 and PSI.

Ultrafast transient absorption studies on photosystem I reaction centers from Chlamydomonas reinhardtii. 2: mutations near the P700 reaction center chlorophylls provide new insight into the nature of the primary electron donor

The energy transfer and charge separation kinetics in several core Photosystem I particles of Chlamydomonas reinhardtii with point mutations around the PA and PB reaction center chlorophylls (Chls) have been studied using ultrafast transient absorption spectroscopy in the femtosecond to nanosecond time range to characterize the influence on the early electron transfer processes. The data have been analyzed in terms of kinetic compartment models. The adequate description of the transient absorption kinetics requires three different radical pairs in the time range up to approximately 100 ps. Also a charge recombination process from the first radical pair back to the excited state is present in all the mutants, as already shown previously for the wild-type (Müller, M. G., J. Niklas, W. Lubitz, and A. R. Holzwarth. 2003. Biophys. J. 85:3899-3922; and Holzwarth, A. R., M. G. Müller, J. Niklas, and W. Lubitz. 2005. J. Phys. Chem. B. 109:5903-59115). In all mutants, the primary charge separation occurs with the same effective rate constant within the error limits as in the wild-type (>>350 ns(-1)), which implies an intrinsic rate constant of charge separation of <1 ps(-1). The rate constant of the secondary electron transfer process is slowed down by a factor of approximately 2 in the mutant B-H656C, which lacks the ligand to the central metal of Chl PB. For the mutant A-T739V, which breaks the hydrogen bond to the keto carbonyl of Chl PA, only a slight slowing down of the secondary electron transfer is observed. Finally for mutant A-W679A, which has the Trp near the PA Chl replaced, either no pronounced effect or, at best, a slight increase on the secondary electron transfer rate constants is observed. The effective charge recombination rate constant is modified in all mutants to some extent, with the strongest effect observed in mutant B-H656C. Our data strongly suggest that the Chls of the PA and PB pair, constituting what is traditionally called the "primary electron donor P700", are not oxidized in the first electron transfer process, but rather only in the secondary electron transfer step. We thus propose a new electron transfer mechanism for Photosystem I where the accessory Chl(s) function as the primary electron donor(s) and the A0 Chl(s) are the primary electron acceptor(s). This new mechanism also resolves in a straightforward manner the difficulty with the previous mechanism, where an electron would have to overcome a distance of approximately 14 A in <1 ps in a single step. If interpreted within a scheme of single-sided electron transfer, our data suggest that the B-branch is the active branch, although parallel A-branch activity cannot be excluded. All the mutations do affect to a varying extent the energy difference between the reaction center excited state RC* and the first radical pair and thus affect the rate constant of charge recombination. It is interesting to note that the new mechanism proposed is in fact analogous to the electron transfer mechanism in Photosystem II, where the accessory Chl also plays the role of the primary electron donor, rather than the special Chl pair P680 (Prokhorenko, V. and A. R. Holzwarth. 2000. J. Phys. Chem. B. 104:11563-11578).

In vivo photosystem I reduction in thermophilic and mesophilic cyanobacteria: The thermal resistance of the process is limited by factors other than the unfolding of the partners

Photosystem I reduction by plastocyanin and cytochrome c(6) in cyanobacteria has been extensively studied in vitro, but much less information is provided on this process inside the cell. Here, we report an analysis of the electron transfer from both plastocyanin and cytochrome c(6) to photosystem I in intact cells of several cyanobacterial species, including a comparative study of the temperature effect in mesophilic and thermophilic organisms. Our data show that cytochrome c(6) reduces photosystem I by following a reaction mechanism involving complex formation, whereas the copper-protein follows a simpler collisional mechanism. These results contrast with previous kinetic studies in vitro. The effect of temperature on photosystem I reduction leads us to conclude that the thermal resistance of this process is determined by factors other than the proper stability of the protein partners.

Release of oxidized plastocyanin from photosystem I limits electron transfer between photosystem I and cytochrome b6f complex in vivo

We used fast absorbance spectroscopy to investigate in vivo binding dynamics and electron transfer between plastocyanin (pc) and photosystem I (PSI), and cytochrome (cyt) f oxidation kinetics in Chlamydomonas reinhardtii mutants in which either the binding or the release of pc from PSI was diminished. Under single flash-excitation conditions, electron flow between PSI and the cyt complex was not affected by a 5-fold lowering of the binding affinity of pc to PSI, as induced by a mutation replacing the tryptophan-651 of the PsaA subunit by a serine residue (PsaA-W651S). On the other hand, electron flow from PSI to the cyt b(6)f complex was very sensitive to a 2- to 3-fold decrease in the rate of pc release from PSI, obtained by replacing the glutamic acid residue 613 of the PsaB subunit with glutamine (PsaB-E613N). Thus, our data indicate that under these experimental conditions the release of oxidized pc limits electron transfer between cyt b(6)f complex and PSI in vivo.

N-terminal Processing of Lhca3 Is a Key Step in Remodeling of the Photosystem I-Light-harvesting Complex Under Iron Deficiency in Chlamydomonas reinhardtii

Iron deficiency induces a remodeling of the photosynthetic apparatus in Chlamydomonas reinhardtii. In this study we showed that a key mechanistic event in the remodeling process of photosystem I (PSI) and its associated light-harvesting proteins (LHCI) is the N-terminal processing of Lhca3. N-terminal processing of Lhca3 is documented independently by two-dimensional gel electrophoresis and tandem mass spectrometric (MS/MS) analysis as well as by quantitative comparative MS/MS peptide profiling using isotopic labeling of proteins. Dynamic remodeling of the LHCI complex under iron deficiency is further exemplified by depletion of Lhca5 and up-regulation of Lhca4 and Lhca9 polypeptides in respect to photosystem I. Most importantly, the induction of N-terminal processing of Lhca3 by progression of iron deficiency correlates with the functional drop in excitation energy transfer efficiency between LHCI and PSI as assessed by low temperature fluorescence emission spectroscopy. Using an RNA interference (RNAi) strategy, we showed that the truncated form of Lhca3 is essential for the structural stability of LHCI. Depletion of Lhca3 by RNAi strongly impacted the efficiency of excitation energy transfer between PSI and LHCI, as is the case for iron deficiency. However, in contrast to iron deficiency, comparative MS/MS peptide profiling using isotopic labeling of proteins demonstrated that RNAi depletion of Lhca3 caused strong reduction of almost all Lhca proteins in isolated PSI particles.

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.

Chlamydomonas reinhardtii proteomics

Proteomics, based on the expanding genomic resources, has begun to reveal new details of Chlamydomonas reinhardtii biology. In particular, analyses focusing on subproteomes have already provided new insight into the dynamics and composition of the photosynthetic apparatus, the chloroplast ribosome, the oxidative phosphorylation machinery of the mitochondria, and the flagellum. It assisted to discovered putative new components of the circadian clockwork as well as shed a light on thioredoxin protein-protein interactions. In the future, quantitative techniques may allow large scale comparison of protein expression levels. Advances in software algorithms will likely improve the use of genomic databases for mass spectrometry (MS) based protein identification and validation of gene models that have been predicted from the genomic DNA sequences. Although proteomics has only been recently applied for exploring C. reinhardtii biology, it will likely be utilized extensively in the near future due to the already existing genetic, genomic, and biochemical tools.

Ultrafast transient absorption studies on Photosystem I reaction centers from Chlamydomonas reinhardtii. 1. A new interpretation of the energy trapping and early electron transfer steps in Photosystem I

The energy transfer and charge separation kinetics in core Photosystem I (PSI) particles of Chlamydomonas reinhardtii has been studied using ultrafast transient absorption in the femtosecond-to-nanosecond time range. Although the energy transfer processes in the antenna are found to be generally in good agreement with previous interpretations, we present evidence that the interpretation of the energy trapping and electron transfer processes in terms of both kinetics and mechanisms has to be revised substantially as compared to current interpretations in the literature. We resolved for the first time i), the transient difference spectrum for the excited reaction center state, and ii), the formation and decay of the primary radical pair and its intermediate spectrum directly from measurements on open PSI reaction centers. It is shown that the dominant energy trapping lifetime due to charge separation is only 6-9 ps, i.e., by a factor of 3 shorter than assumed so far. The spectrum of the first radical pair shows the expected strong bleaching band at 680 nm which decays again in the next electron transfer step. We show furthermore that the early electron transfer processes up to approximately 100 ps are more complex than assumed so far. Several possibilities are discussed for the intermediate redox states and their sequence which involve oxidation of P700 in the first electron transfer step, as assumed so far, or only in the second electron transfer step, which would represent a fundamental change from the presently assumed mechanism. To explain the data we favor the inclusion of an additional redox state in the electron transfer scheme. Thus we distinguish three different redox intermediates on the timescale up to 100 ps. At this level no final conclusion as to the exact mechanism and the nature of the intermediates can be drawn, however. From comparison of our data with fluorescence kinetics in the literature we also propose a reversible first charge separation step which has been excluded so far for open PSI reaction centers. For the first time an ultrafast 150-fs equilibration process, occurring among exciton states in the reaction center proper, upon direct excitation of the reaction center at 700 nm, has been resolved. Taken together the data call for a fundamental revision of the present understanding of the energy trapping and early electron transfer kinetics in the PSI reaction center. Due to the fact that it shows the fastest trapping time observed so far of any intact PSI particle, the PSI core of C. reinhardtii seems to be best suited to further characterize the electron transfer steps and mechanisms in the reaction center of PSI.

Proteomics of Chlamydomonas reinhardtii light-harvesting proteins

With the recent development of techniques for analyzing transmembrane thylakoid proteins by two-dimensional gel electrophoresis, systematic approaches for proteomic analyses of membrane proteins became feasible. In this study, we established detailed two-dimensional protein maps of Chlamydomonas reinhardtii light-harvesting proteins (Lhca and Lhcb) by extensive tandem mass spectrometric analysis. We predicted eight distinct Lhcb proteins. Although the major Lhcb proteins were highly similar, we identified peptides which were unique for specific lhcbm gene products. Interestingly, lhcbm6 gene products were resolved as multiple spots with different masses and isoelectric points. Gene tagging experiments confirmed the presence of differentially N-terminally processed Lhcbm6 proteins. The mass spectrometric data also revealed differentially N-terminally processed forms of Lhcbm3 and phosphorylation of a threonine residue in the N terminus. The N-terminal processing of Lhcbm3 leads to the removal of the phosphorylation site, indicating a potential novel regulatory mechanism. At least nine different lhca-related gene products were predicted by comparison of the mass spectrometric data against Chlamydomonas expressed sequence tag and genomic databases, demonstrating the extensive variability of the C. reinhardtii Lhca antenna system. Out of these nine, three were identified for the first time at the protein level. This proteomic study demonstrates the complexity of the light-harvesting proteins at the protein level in C. reinhardtii and will be an important basis of future functional studies addressing this diversity.