The plastocyanin binding domain of photosystem I

Three structures of PSI-LHCI from Chlamydomonas reinhardtii suggest a resting state re-activated by ferredoxin

Photosystem I (PSI) from the green alga Chlamydomonas reinhardtii, with various numbers of membrane bound antenna complexes (LHCI), has been described in great detail. In contrast, structural characterization of soluble binding partners is less advanced. Here, we used X-ray crystallography and single particle cryo-EM to investigate three structures of the PSI-LHCI supercomplex from Chlamydomonas reinhardtii. An X-ray structure demonstrates the absence of six chlorophylls from the luminal side of the LHCI belts, suggesting these pigments were either physically absent or less stably associated with the complex, potentially influencing excitation transfer significantly. CryoEM revealed extra densities on luminal and stromal sides of the supercomplex, situated in the vicinity of the electron transfer sites. These densities disappeared after the binding of oxidized ferredoxin to PSI-LHCI. Based on these structures, we propose the existence of a PSI-LHCI resting state with a reduced active chlorophyll content, electron donors docked in waiting positions and regulatory binding partners positioned at the electron acceptor site. The resting state PSI-LHCI supercomplex would be recruited to its active form by the availability of oxidized ferredoxin.

Structural insights into photosynthetic cyclic electron transport

During photosynthesis, light energy is utilized to drive sophisticated biochemical chains of electron transfers, converting solar energy into chemical energy that feeds most life on earth. Cyclic electron transfer/flow (CET/CEF) plays an essential role in efficient photosynthesis, as it balances the ATP/NADPH ratio required in various regulatory and metabolic pathways. Photosystem I, cytochrome b₆f, and NADH dehydrogenase (NDH) are large multisubunit protein complexes embedded in the thylakoid membrane of the chloroplast and key players in NDH-dependent CEF pathway. Furthermore, small mobile electron carriers serve as shuttles for electrons between these membrane protein complexes. Efficient electron transfer requires transient interactions between these electron donors and acceptors. Structural biology has been a powerful tool to advance our knowledge of this important biological process. A number of structures of the membrane-embedded complexes, soluble electron carrier proteins, and transient complexes composed of both have now been determined. These structural data reveal detailed interacting patterns of these electron donor-acceptor pairs, thus allowing us to visualize the different parts of the electron transfer process. This review summarizes the current state of structural knowledge of three membrane complexes and their interaction patterns with mobile electron carrier proteins.

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.

The Evolution and Functional Roles of miR408 and Its Targets in Plants

MicroRNA408 (miR408) is an ancient and highly conserved miRNA, which is involved in the regulation of plant growth, development and stress response. However, previous research results on the evolution and functional roles of miR408 and its targets are relatively scattered, and there is a lack of a systematic comparison and comprehensive summary of the detailed evolutionary pathways and regulatory mechanisms of miR408 and its targets in plants. Here, we analyzed the evolutionary pathway of miR408 in plants, and summarized the functions of miR408 and its targets in regulating plant growth and development and plant responses to various abiotic and biotic stresses. The evolutionary analysis shows that miR408 is an ancient and highly conserved microRNA, which is widely distributed in different plants. miR408 regulates the growth and development of different plants by down-regulating its targets, encoding blue copper (Cu) proteins, and by transporting Cu to plastocyanin (PC), which affects photosynthesis and ultimately promotes grain yield. In addition, miR408 improves tolerance to stress by down-regulating target genes and enhancing cellular antioxidants, thereby increasing the antioxidant capacity of plants. This review expands and promotes an in-depth understanding of the evolutionary and regulatory roles of miR408 and its targets in plants.

New Insights into the Evolution of the Electron Transfer from Cytochrome f to Photosystem I in the Green and Red Branches of Photosynthetic Eukaryotes

In cyanobacteria and most green algae of the eukaryotic green lineage, the copper-protein plastocyanin (Pc) alternatively replaces the heme-protein cytochrome c6 (Cc6) as the soluble electron carrier from cytochrome f (Cf) to photosystem I (PSI). The functional and structural equivalence of 'green' Pc and Cc6 has been well established, representing an example of convergent evolution of two unrelated proteins. However, plants only produce Pc, despite having evolved from green algae. On the other hand, Cc6 is the only soluble donor available in most species of the red lineage of photosynthetic organisms, which includes, among others, red algae and diatoms. Interestingly, Pc genes have been identified in oceanic diatoms, probably acquired by horizontal gene transfer from green algae. However, the mechanisms that regulate the expression of a functional Pc in diatoms are still unclear. In the green eukaryotic lineage, the transfer of electrons from Cf to PSI has been characterized in depth. The conclusion is that in the green lineage, this process involves strong electrostatic interactions between partners, which ensure a high affinity and an efficient electron transfer (ET) at the cost of limiting the turnover of the process. In the red lineage, recent kinetic and structural modeling data suggest a different strategy, based on weaker electrostatic interactions between partners, with lower affinity and less efficient ET, but favoring instead the protein exchange and the turnover of the process. Finally, in diatoms the interaction of the acquired green-type Pc with both Cf and PSI may not yet be optimized.

The Plasticity of Photosystem I

Most of life's energy comes from sunlight, and thus, photosynthesis underpins the survival of virtually all life forms. The light-driven electron transfer at photosystem I (PSI) is certainly the most important generator of reducing power at the cellular level and thereby largely determines the global amount of enthalpy in living systems (Nelson 2011). The PSI is a light-driven plastocyanin:ferredoxin oxidoreductase, which is embedded into thylakoid membranes of cyanobacteria and chloroplasts of eukaryotic photosynthetic organism. Structural determination of complexes of the photosynthetic machinery is vital for the understanding of its mode of action. Here, we describe new structural and functional insights into PSI and associated light-harvesting proteins, with a focus on the plasticity of PSI.

Phylloquinone is the principal Mehler reaction site within photosystem I in high light

Photosynthesis is a vital process, responsible for fixing carbon dioxide, and producing most of the organic matter on the planet. However, photosynthesis has some inherent limitations in utilizing solar energy, and a part of the energy absorbed is lost in the reduction of O2 to produce the superoxide radical (O2•-) via the Mehler reaction, which occurs principally within photosystem I (PSI). For decades, O2 reduction within PSI was assumed to take place solely in the distal iron-sulfur clusters rather than within the two asymmetrical cofactor branches. Here, we demonstrate that under high irradiance, O2 photoreduction by PSI primarily takes place at the phylloquinone of one of the branches (the A-branch). This conclusion derives from the light dependency of the O2 photoreduction rate constant in fully mature wild-type PSI from Chlamydomonas reinhardtii, complexes lacking iron-sulfur clusters, and a mutant PSI, in which phyllosemiquinone at the A-branch has a significantly longer lifetime. We suggest that the Mehler reaction at the phylloquinone site serves as a release valve under conditions where both the iron-sulfur clusters of PSI and the mobile ferredoxin pool are highly reduced.

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.

The structure of a triple complex of plant photosystem I with ferredoxin and plastocyanin

The ability of photosynthetic organisms to use sunlight as a sole source of energy is endowed by two large membrane complexes-photosystem I (PSI) and photosystem II (PSII). PSI and PSII are the fundamental components of oxygenic photosynthesis, providing oxygen, food and an energy source for most living organisms on Earth. Currently, high-resolution crystal structures of these complexes from various organisms are available. The crystal structures of megadalton complexes have revealed excitation transfer and electron-transport pathways within the various complexes. PSI is defined as plastocyanin-ferredoxin oxidoreductase but a high-resolution structure of the entire triple supercomplex is not available. Here, using a new cryo-electron microscopy technique, we solve the structure of native plant PSI in complex with its electron donor plastocyanin and the electron acceptor ferredoxin. We reveal all of the contact sites and the modes of interaction between the interacting electron carriers and PSI.

The structure of a triple complex of plant photosystem I with ferredoxin and plastocyanin

The ability of photosynthetic organisms to use sunlight as a sole source of energy is endowed by two large membrane complexes-photosystem I (PSI) and photosystem II (PSII). PSI and PSII are the fundamental components of oxygenic photosynthesis, providing oxygen, food and an energy source for most living organisms on Earth. Currently, high-resolution crystal structures of these complexes from various organisms are available. The crystal structures of megadalton complexes have revealed excitation transfer and electron-transport pathways within the various complexes. PSI is defined as plastocyanin-ferredoxin oxidoreductase but a high-resolution structure of the entire triple supercomplex is not available. Here, using a new cryo-electron microscopy technique, we solve the structure of native plant PSI in complex with its electron donor plastocyanin and the electron acceptor ferredoxin. We reveal all of the contact sites and the modes of interaction between the interacting electron carriers and PSI.

Structure and function of photosystem I in Cyanidioschyzon merolae

The evolution of photosynthesis from primitive photosynthetic bacteria to higher plants has been driven by the need to adapt to a wide range of environmental conditions. The red alga Cyanidioschyzon merolae is a primitive organism, which is capable of performing photosynthesis in extreme acidic and hot environments. The study of its photosynthetic machinery may provide new insight on the evolutionary path of photosynthesis and on light harvesting and its regulation in eukaryotes. With that aim, the structural and functional properties of the PSI complex were investigated by biochemical characterization, mass spectrometry, and X-ray crystallography. PSI was purified from cells grown at 25 and 42 °C, crystallized and its crystal structure was solved at 4 Å resolution. The structure of C. merolae reveals a core complex with a crescent-shaped structure, formed by antenna proteins. In addition, the structural model shows the position of PsaO and PsaM. PsaG and PsaH are present in plant complex and are missing from the C. merolae model as expected. This paper sheds new light onto the evolution of photosynthesis, which gives a strong indication for the chimerical properties of red algae PSI. The subunit composition of the PSI core from C. merolae and its associated light-harvesting antennae suggests that it is an evolutionary and functional intermediate between cyanobacteria and plants.

Crystal Structure of Photosystem I Monomer From Synechocystis PCC 6803

A single histidine addition to the C-terminus of PsaL of Synechocystis sp. PCC 6803 was previously reported by our lab to shift the trimer-to-monomer ratio of PSI in favor of the monomeric form. P700 re-reduction and NADP⁺ photo-reduction measurements of the PsaL^HIS strain show no effect on PSI activity in comparison to the WT trimeric PSI. Crystal structure of the PsaL^HIS monomeric PSI reveals several alterations that occurred in the trimerisation site of PSI, primarily a deformation of the C-terminus of PsaL and loss of chlorophyll a and β-carotene molecules.

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.

Biofunctionalisation of p-doped silicon with cytochrome c553minimises charge recombination and enhances photovoltaic performance of the all-solid-state photosystem I-based biophotoelectrode

Passivation of p-doped silicon substrate was achieved by its biofunctionalisation with hexahistidine-tagged cytochrome c₅₅₃, a soluble electroactive photosynthetic protein responsible for electron donation to photooxidised photosystem I.

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.

Physicochemical nature of interfaces controlling ferredoxin NADP(+) reductase activity through its interprotein interactions with ferredoxin

Although acidic residues of ferredoxin (Fd) are known to be essential for activities of various Fd-dependent enzymes, including ferredoxin NADP(+) reductase (FNR) and sulfite reductase (SiR), through electrostatic interactions with basic residues of partner enzymes, non-electrostatic contributions such as hydrophobic forces remain largely unknown. We herein demonstrated that intermolecular hydrophobic and charge-charge interactions between Fd and enzymes were both critical for enzymatic activity. Systematic site-directed mutagenesis, which altered physicochemical properties of residues on the interfaces of Fd for FNR /SiR, revealed various changes in activities of both enzymes. The replacement of serine 43 of Fd to a hydrophobic residue (S43W) and charged residue (S43D) increased and decreased FNR activity, respectively, while S43W showed significantly lower SiR activity without affecting SiR activity by S43D, suggesting that hydrophobic and electrostatic interprotein forces affected FNR activity. Enzyme kinetics revealed that changes in FNR activity by mutating Fd correlated with Km, but not with kcat or activation energy, indicating that interprotein interactions determined FNR activity. Calorimetry-based binding thermodynamics between Fd and FNR showed different binding modes of FNR to wild-type, S43W, or S43D, which were controlled by enthalpy and entropy, as shown by the driving force plot. Residue-based NMR spectroscopy of (15)N FNR with Fds also revealed distinct binding modes of each complex based on different directions of NMR peak shifts with similar overall chemical shift differences. We proposed that subtle adjustments in both hydrophobic and electrostatic forces were critical for enzymatic activity, and these results may be applicable to protein-based electron transfer systems.

Hybrid system of semiconductor and photosynthetic protein

Photosynthetic protein has the potential to be a new attractive material for solar energy absorption and conversion. The development of semiconductor/photosynthetic protein hybrids is an example of recent progress toward efficient, clean and nanostructured photoelectric systems. In the review, two biohybrid systems interacting through different communicating methods are addressed: (1) a photosynthetic protein immobilized semiconductor electrode operating via electron transfer and (2) a hybrid of semiconductor quantum dots and photosynthetic protein operating via energy transfer. The proper selection of materials and functional and structural modification of the components and optimal conjugation between them are the main issues discussed in the review. In conclusion, we propose the direction of future biohybrid systems for solar energy conversion systems, optical biosensors and photoelectric devices.

The evolution of photosystem I in light of phage-encoded reaction centres

Recent structural determinations and metagenomic studies shed light on the evolution of photosystem I (PSI) from the homodimeric reaction centre of primitive bacteria to plant PSI at the top of the evolutionary development. The evolutionary scenario of over 3.5 billion years reveals an increase in the complexity of PSI. This phenomenon of ever-increasing complexity is common to all evolutionary processes that in their advanced stages are highly dependent on fine-tuning of regulatory processes. On the other hand, the recently discovered virus-encoded PSI complexes contain a minimal number of subunits. This may reflect the unique selection scenarios associated with viral replication. It may be beneficial for future engineering of productive processes to utilize 'primitive' complexes that disregard the cellular regulatory processes and to avoid those regulatory constraints when our goal is to divert the process from its original route. In this article, we discuss the evolutionary forces that act on viral reaction centres and the role of the virus-carried photosynthetic genes in the evolution of photosynthesis.

Deletion of Synechocystis sp. PCC 6803 leader peptidase LepB1 affects photosynthetic complexes and respiration

The cyanobacterium Synechocystis sp. PCC 6803 possesses two leader peptidases, LepB1 (Sll0716) and LepB2 (Slr1377), responsible for the processing of signal peptide-containing proteins. Deletion of the gene for LepB1 results in an inability to grow photoautotrophically and an extreme light sensitivity. Here we show, using a combination of Blue Native/SDS-PAGE, Western blotting and iTRAQ analysis, that lack of LepB1 strongly affects the cell's ability to accumulate wild-type levels of both photosystem I (PSI) and cytochrome (Cyt) b6f complexes. The impaired assembly of PSI and Cyt b6f is considered to be caused by the no or slow processing of the integral subunits PsaF and Cyt f respectively. In particular, PsaF, one of the PSI subunits, was found incorporated into PSI in its unprocessed form, which could influence the assembly and/or stability of PSI. In contrast to these results, we found the amount of assembled photosystem II (PSII) unchanged, despite a slower processing of PsbO. Thus, imbalance in the ratios of PSI and Cyt b6f to photosystem II leads to an imbalanced photosynthetic electron flow up- and down-stream of the plastoquinone pool, resulting in the observed light sensitivity of the mutant. We conclude that LepB1 is the natural leader peptidase for PsaF, PsbO, and Cyt f. The maturation of PsbO and Cyt f can be partially performed by LepB2, whereas PsaF processing is completely dependent on LepB1. iTRAQ analysis also revealed a number of indirect effects accompanying the mutation, primarily a strong induction of the CydAB oxidase as well as a significant decrease in phycobiliproteins and chlorophyll/heme biosynthesis enzymes.

Crystal structures of virus-like photosystem I complexes from the mesophilic cyanobacterium Synechocystis PCC 6803

Oxygenic photosynthesis supports virtually all life forms on earth. Light energy is converted by two photosystems—photosystem I (PSI) and photosystem II (PSII). Globally, nearly 50% of photosynthesis takes place in the Ocean, where single cell cyanobacteria and algae reside together with their viruses. An operon encoding PSI was identified in cyanobacterial marine viruses. We generated a PSI that mimics the salient features of the viral complex, named PSI^PsaJF. PSI^PsaJF is promiscuous for its electron donors and can accept electrons from respiratory cytochromes. We solved the structure of PSI^PsaJF and a monomeric PSI, with subunit composition similar to the viral PSI, providing for the first time a detailed description of the reaction center and antenna system from mesophilic cyanobacteria, including red chlorophylls and cofactors of the electron transport chain. Our finding extends the understanding of PSI structure, function and evolution and suggests a unique function for the viral PSI.

DOI:http://dx.doi.org/10.7554/eLife.01496.001

Photosynthesis—the process by which plants and other organisms harness the energy in sunlight—is the source of almost all oxygen, food and fuel on earth. Oxygenic photosynthesis in living cells involves a series of reactions catalyzed by large protein complexes, various other soluble chemicals, and the transfer of electrons from so-called donors to acceptors. The energy in the sunlight is captured by two membrane-embedded protein complexes—photosystem I, which is the most powerful electron donor in nature, and photosystem II—and converted into chemical energy.

Almost half of the world’s photosynthesis occurs in the oceans, and is performed by single-celled cyanobacteria and algae. Interestingly, some of the genes that encode photosynthetic enzymes in cyanobacteria are also found in the genomes of viruses that infect these bacteria. It is thought that these viruses can alter photosynthetic pathways in their hosts, but the interactions between these viruses and their hosts are not fully understood.

Now, Mazor et al. have created a photosystem I complex that mimics the viral version of this complex, and have gone on to solve its three-dimensional structure. This mimetic virus-encoded complex was shown to be a ‘promiscuous’ electron acceptor: this means that, unlike most electron acceptors, it can accept electrons from more than one electron donor.

Further, Mazor et al. solved the structure of photosystem I from Synechocystis, a cyanobacterium that lives in fresh water; and found some surprising differences between it and the only other published structure for photosystem I from a cyanobacterium (which was from a species that lives in hot water springs). These included differences in components involved in the electron transfer chain—a series of chemical reactions in which electrons are passed from donor to acceptor molecules—that were thought to be highly conserved. Other differences in the structures allowed Mazor et al. to identify the location of a unique chlorophyll pigment group in the Synechocystis photosystem I.

Since Synechocystis is commonly used as a model to study photosynthesis, an improved understanding of its photosystem I should lead to further improvements in our knowledge of photosynthesis.

DOI:http://dx.doi.org/10.7554/eLife.01496.002

Structure and function of photosystem I and its application in biomimetic solar-to-fuel systems

Photosystem I (PSI) is one of the most efficient biological macromolecular complexes that converts solar energy into condensed energy of chemical bonds. Despite high structural complexity, PSI operates with a quantum yield close to 1.0 and to date, no man-made synthetic system approached this remarkable efficiency. This review highlights recent developments in dissecting molecular structure and function of the prokaryotic and eukaryotic PSI. It also overviews progress in the application of this complex as a natural photocathode for production of hydrogen within the biomimetic solar-to-fuel nanodevices.

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.

Structure and Function of Photosynthetic Reaction Centres

Extensive biochemical, biophysical, molecular biological and structural studies on a wide range of prokaryotic and eukaryotic photosynthetic organisms has revealed common features of their reaction centres where light induced charge separation and stabilization occurs. There is little doubt that all reaction centres have evolved from a common ancestor and have been optimized to maximum efficiency. As such they provide principles that can be used as a blueprint for developing artificial photo-electrochemical catalytic systems to generate solar fuels. This chapter summarises the common features of the organization of cofactors, electron transfer pathways and protein environments of reaction centres of anoxygenic and oxygenic phototrophs. In particular, the latest molecular details derived from X-ray crystallography are discussed in context of the specific catalytic functions of the Type I and Type II reaction centres.

A phytocyanin-related early nodulin-like gene, BcBCP1, cloned from Boea crassifolia enhances osmotic tolerance in transgenic tobacco

Using the mRNA differential display combined with 5' rapid amplification of cDNA ends (RACE) technique, an early nodulin-like protein gene (BcBCP1) (accession no. AY243047.1) was isolated from drought-treated Boea crassifolia leaves. The full-length cDNA of BcBCP1 consists of 844 bp nucleotides and has an open reading frame of 606 bp, encoding a putative polypeptide of 201 amino acids with a predicted molecular mass of 22 kDa and a pI of 5.13. The putative protein precursor contains four sequence domains, including a 27 amino acid hydrophobic N-terminal transit peptide, a 100 amino acid phytocyanin-homologous globular domain, a 51 amino acid hydroxyproline-rich cell wall structural protein domain, and a 22 amino acid hydrophobic extension domain. Sequence alignment defined the encoded protein as an early nodulin-like protein, and the absence of key ligands implies that it is unlikely to bind copper. BcBCP1 expression was strongly induced by dehydration, salinity and abscisic acid (ABA), slightly induced by moderate heat shock, and weakly inhibited by low temperature, methyl jasmonic acid (MeJA), and a low concentration of salicylic acid (SA). Overexpression of BcBCP1 in tobacco under the control of CaMV 35S promoter enhanced tolerance to osmotic stress, as indicated by the less impaired growth, less damaged membrane integrity and lower lipid peroxidation levels after osmotic stress. Transgenic tobacco lines overexpressing BcBCP1 showed higher photosynthetic rates, higher antioxidant enzyme activities and higher cytosyl ascorbic peroxidase transcription levels than non-transgenic tobacco plants, both under normal conditions and under osmotic stress.

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.

Identification and characterization of an assembly intermediate subcomplex of photosystem I in the green alga Chlamydomonas reinhardtii

Photosystem I (PSI) is a multiprotein complex consisting of the PSI core and peripheral light-harvesting complex I (LHCI) that together form the PSI-LHCI supercomplex in algae and higher plants. The supercomplex is synthesized in steps during which 12-15 core and 4-9 LHCI subunits are assembled. Here we report the isolation of a PSI subcomplex that separated on a sucrose density gradient from the thylakoid membranes isolated from logarithmic growth phase cells of the green alga Chlamydomonas reinhardtii. Pulse-chase labeling of total cellular proteins revealed that the subcomplex was synthesized de novo within 1 min and was converted to the mature PSI-LHCI during the 2-h chase period, indicating that the subcomplex was an assembly intermediate. The subcomplex was functional; it photo-oxidized P700 and demonstrated electron transfer activity. The subcomplex lacked PsaK and PsaG, however, and it bound PsaF and PsaJ weakly and was not associated with LHCI. It seemed likely that LHCI had been integrated into the subcomplex unstably and was dissociated during solubilization and/or fractionation. We, thus, infer that PsaK and PsaG stabilize the association between PSI core and LHCI complexes and that PsaK and PsaG bind to the PSI core complex after the integration of LHCI in one of the last steps of PSI complex assembly.

Structure of Photosystems I and II

Photosynthesis is the major process that converts solar energy into chemical energy on Earth. Two and a half billion years ago, the ancestors of cyanobacteria were able to use water as electron source for the photosynthetic process, thereby evolving oxygen and changing the atmosphere of our planet Earth. Two large membrane protein complexes, Photosystems I and II, catalyze the primary step in this energy conversion, the light-induced charge separation across the photosynthetic membrane. This chapter describes and compares the structure of two Photosystems and discusses their function in respect to the mechanism of light harvesting, electron transfer and water splitting.

Functional organization of a plant Photosystem I: evolution of a highly efficient photochemical machine

Despite its enormous complexity, a plant Photosystem I (PSI) is arguably the most efficient nano-photochemical machine in Nature. It emerged as a homodimeric structure containing several chlorophyll molecules over 3.5 billion years ago, and has perfected its photoelectric properties ever since. The recently determined structure of plant PSI, which is at the top of the evolutionary tree of this kind of complexes, provided the first relatively high-resolution structural model of the supercomplex containing a reaction center (RC) and a peripheral antenna (LHCI) complexes. The RC is highly homologous to that of the cyanobacterial PSI and maintains the position of most transmembrane helices and chlorophylls during 1.5 years of separate evolution. The LHCI is composed of four nuclear gene products (Lhca1-Lhca4) that are unique among the chlorophyll a/b binding proteins in their pronounced long-wavelength absorbance and their assembly into dimers. In this respect, we describe structural elements, which establish the biological significance of a plant PSI and discuss structural variance from the cyanobacterial version. The present comprehensive structural analysis summarizes our current state of knowledge, providing the first glimpse at the architecture of this highly efficient photochemical machine at the atomic level.

Proteomic analysis of photosystem I components from different plant species

In this study, the photosystem I (PSI) highly hydrophobic proteins present within stroma lamellae of the thylakoid membrane were separated by RP-HPLC and identified either by in-solution trypsin digestion peptide fragment fingerprinting or by the close correspondence between the intact mass measurements (IMMs) and those expected from the DNA sequence. Protein identification performed by MS/MS was as reliable as IMMs. Thus, IMM is an easy and valid method for identifying proteins that have no PTMs. This paper reports the M(r) for all PSI proteins in ten different species, including those whose genes have not yet been cloned. Lhca5 was revealed unequivocally in four species, corroborating that it is indeed a protein belonging to the light-harvesting antenna of PSI. In all species examined, the product of the Lhca6 gene has never been revealed. Concerning core proteins, Psa-O has been revealed in three species; isoforms of Psa-D and Psa-E have been found in both monocots and dicots. Small proteins like Psa-I and Psa-J are well separated and identified. RP-HPLC produces reliable fingerprints and reveals that the relative amounts of PSI proteins appear to be markedly different.

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.

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.

Crystal structure of oxidized cytochromec6AfromArabidopsis thaliana

Compared with algal and cyanobacterial cytochrome c(6), cytochrome c(6A) from higher plants contains an additional loop of 12 amino acid residues. We have determined the first crystal structure of cytochrome c(6A) from Arabidopsis thaliana at 1.5 Angstrom resolution in order to help elucidate its function. The overall structure of cytochrome c(6A) follows the topology of class I c-type cytochromes in which the heme prosthetic group covalently binds to Cys16 and Cys19, and the iron has octahedral coordination with His20 and Met60 as the axial ligands. Two cysteine residues (Cys67 and Cys73) within the characteristic 12 amino acids loop form a disulfide bond, contributing to the structural stability of cytochrome c(6A). Our model provides a chemical basis for the known low redox potential of cytochrome c(6A) which makes it an unsuitable electron carrier between cytochrome b(6)f and PSI.

Structure and function of photosystems I and II

Oxygenic photosynthesis, the principal converter of sunlight into chemical energy on earth, is catalyzed by four multi-subunit membrane-protein complexes: photosystem I (PSI), photosystem II (PSII), the cytochrome b(6)f complex, and F-ATPase. PSI generates the most negative redox potential in nature and largely determines the global amount of enthalpy in living systems. PSII generates an oxidant whose redox potential is high enough to enable it to oxidize H(2)O, a substrate so abundant that it assures a practically unlimited electron source for life on earth. During the last century, the sophisticated techniques of spectroscopy, molecular genetics, and biochemistry were used to reveal the structure and function of the two photosystems. The new structures of PSI and PSII from cyanobacteria, algae, and plants has shed light not only on the architecture and mechanism of action of these intricate membrane complexes, but also on the evolutionary forces that shaped oxygenic photosynthesis.

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.

Biochemical and biophysical characterization of photosystem I from phytoene desaturase and zeta-carotene desaturase deletion mutants of Synechocystis Sp. PCC 6803: evidence for PsaA- and PsaB-side electron transport in cyanobacteria

In photosystem I, oxidation of reduced acceptor A(1)(-) through iron-sulfur cluster F(X) is biphasic with half-times of approximately 5-30 ns ("fast" phase) and approximately 150-300 ns ("slow" phase). Whether these biphasic kinetics reflect unidirectional electron transfer, involving only the PsaA-side phylloquinone or bi-directional electron transfer, involving both the PsaA- and PsaB-side phylloquinones, has been the source of some controversy. Brettel (Brettel, K. (1988) FEBS Lett. 239, 93-98) and Joliot and Joliot (Joliot, P., and Joliot, A. (1999) Biochemistry 38, 11130-11136) have attributed to nearby carotenoids electrochromic band shifts, accompanying A(1) reduction, centered at approximately 450 and 500-510 nm. As a test of these assignments, we separately deleted in Synechocystis sp. PCC 6803 the genes that encode phytoene desaturase (encoded by crtP (pds)) and zeta-carotene desaturase (encoded by crtQ (zds)). The pds(-) and zds(-) strains synthesize phytoene and zeta-carotene, respectively, both of which absorb to shorter wavelength than beta-carotene. Compared with wild type, the mutant A(1)(-) (FeS) - A(1)(FeS)(-) difference spectra, measured in cells and photosystem I complexes, retain the electrochromic band shift centered at 450 nm but show a complete loss of the electrochromic band shifts centered at 500-510 nm. Thus, the latter clearly arise from beta-carotene. In the wild type, the electrochromic band shift of the slow phase (centered at 500 nm) is shifted by 6 nm to the blue compared with the fast phase (centered at 506 nm). Thus, the carotenoid pigments acting as electrochromic markers during the fast and slow phases of A(1)(-) oxidation are different, indicating the involvement of both the PsaA- and the PsaB-side phylloquinones in photosystem I electron transport.

Photosystem I lacking the PSI-G subunit has a higher affinity for plastocyanin and is sensitive to photodamage

PSI-G is an 11 kDa subunit of PSI in photosynthetic eukaryotes. Arabidopsis thaliana plants devoid of PSI-G have a decreased PSI content and an increased activity of NADP(+) photoreduction in vitro but otherwise no obvious phenotype. To investigate the biochemical basis for the increased activity, the kinetic parameters of the reaction between PSI and plastocyanin were determined. PSI-G clearly plays a role in the affinity for plastocyanin since the dissociation constant (K(D)) is only 12 muM in the absence of PSI-G compared to 32 muM for the wild type. On the physiological level, plants devoid of PSI-G have a more reduced Q(A). This indicates that the decreased PSI content is due to unstable PSI rather than an adaptation to the increased activity. In agreement with this indication of decreased stability, plants devoid of PSI-G were found to be more photo-inhibited both at low temperature and after high light treatment. The decreased PSI stability was confirmed in vitro by measuring PSI activity after illumination of a thylakoid suspension which clearly showed a faster decrease in PSI activity in the thylakoids lacking PSI-G. Light response of the P700 redox state in vivo showed that in the absence of PSI-G, P700 is more reduced at low light intensities. We conclude that PSI-G is involved in the binding dynamics of plastocyanin to PSI and that PSI-G is important for the stability of the PSI complex.

Transient homodimer interactions studied using the electron self-exchange reaction

Transient homodimer protein interactions have been investigated by analyzing the influence of ionic strength (NaCl) on the electron self-exchange (the bimolecular reaction whereby the two oxidation states of a redox protein interconvert) rate constant (k(ese)) of four plastocyanins. The k(ese) values for the plastocyanins from spinach, Dryopteris crassirhizoma (a fern), and the green alga Ulva pertusa, which possess acidic patches of varying size and locations, increase 190-, 29-, and 21-fold, respectively, at elevated ionic strength (I = 2.03 M). In contrast, the k(ese) for the almost neutral cyanobacterial plastocyanin from Anabaena variabilis exhibits very little dependence on ionic strength. The temperature dependence of the k(ese) for spinach plastocyanin (I = 0.28 M) provides evidence for poor packing at the homodimer interface. Representative structures of the transient homodimers involved in electron self-exchange, which are consistent with fits of the ionic strength dependence of k(ese) to van Leeuwen theory, have been obtained from protein modeling and docking simulations. The Coulombic energy of the docked homodimers follows the order spinach > D. crassirhizoma > U. pertusa > A. variabilis, which matches that of the overall influence of ionic strength on k(ese). Analysis of the homodimer structures indicates that poor packing and high planarity are features of the interface that favor transient interactions. The physiologically relevant Mg2+ ion has a much more pronounced influence on the k(ese) of spinach plastocyanin, which along with the known properties of the thylakoid lumen suggests a biological role for electron self-exchange.