Analysis of the Structure of the PsbO Protein and its Implications

Indirect interactions involving the PsbM or PsbT subunits and the PsbO, PsbU and PsbV proteins stabilize assembly and activity of Photosystem II in Synechocystis sp. PCC 6803

The low-molecular-weight PsbM and PsbT proteins of Photosystem II (PS II) are both located at the monomer-monomer interface of the mature PS II dimer. Since the extrinsic proteins are associated with the final step of assembly of an active PS II monomer and, in the case of PsbO, are known to impact the stability of the PS II dimer, we have investigated the potential cooperativity between the PsbM and PsbT subunits and the PsbO, PsbU and PsbV extrinsic proteins. Blue-native polyacrylamide electrophoresis and western blotting detected stable PS II monomers in the ∆PsbM:∆PsbO and ∆PsbT:∆PsbO mutants that retained sufficient oxygen-evolving activity to support reduced photoautotrophic growth. In contrast, the ∆PsbM:∆PsbU and ∆PsbT:∆PsbU mutants assembled dimeric PS II at levels comparable to wild type and supported photoautotrophic growth at rates similar to those obtained with the corresponding ∆PsbM and ∆PsbT cells. Removal of PsbV was more detrimental than removal of PsbO. Only limited levels of dimeric PS II were observed in the ∆PsbM:∆PsbV mutant and the overall reduced level of assembled PS II in this mutant resulted in diminished rates of photoautotrophic growth and PS II activity below those obtained in the ∆PsbM:∆PsbO and ∆PsbT:∆PsbO strains. In addition, the ∆PsbT:∆PsbV mutant did not assemble active PS II centers although inactive monomers could be detected. The inability of the ∆PsbT:∆PsbV mutant to grow photoautotrophically, or to evolve oxygen, suggested a stable oxygen-evolving complex could not assemble in this mutant.

Toxic effects of lead on plants: integrating multi-omics with bioinformatics to develop Pb-tolerant crops

MAIN CONCLUSION

Lead disrupts plant metabolic homeostasis and key structural elements. Utilizing modern biotechnology tools, it's feasible to develop Pb-tolerant varieties by discovering biological players regulating plant metabolic pathways under stress. Lead (Pb) has been used for a variety of purposes since antiquity despite its toxic nature. After arsenic, lead is the most hazardous heavy metal without any known beneficial role in the biological system. It is a crucial inorganic pollutant that affects plant biochemical and morpho-physiological attributes. Lead toxicity harms plants throughout their life cycle and the extent of damage depends on the concentration and duration of exposure. Higher levels of lead exposure disrupt numerous key metabolic activities of plants including oxygen-evolving complex, organelles integrity, photosystem II connectivity, and electron transport chain. This review summarizes the detrimental effects of lead toxicity on seed germination, crop growth, and yield, oxidative and ultra-structural alterations, as well as nutrient absorption, transport, and assimilation. Further, it discusses the Pb-induced toxic modulation of stomatal conductance, photosynthesis, respiration, metabolic-enzymatic activity, osmolytes accumulation, and antioxidant activity. It is a comprehensive review that reports on omics-based studies along with morpho-physiological and biochemical modifications caused by lead stress. With advances in DNA sequencing technologies, genomics and transcriptomics are gradually becoming popular for studying Pb stress effects in plants. Proteomics and metabolomics are still underrated and there is a scarcity of published data, and this review highlights both their technical and research gaps. Besides, there is also a discussion on how the integration of omics with bioinformatics and the use of the latest biotechnological tools can aid in developing Pb-tolerant crops. The review concludes with core challenges and research directions that need to be addressed soon.

Evolutionary diversity of proton and water channels on the oxidizing side of photosystem II and their relevance to function

One of the reasons for the high efficiency and selectivity of biological catalysts arise from their ability to control the pathways of substrates and products using protein channels, and by modulating the transport in the channels using the interaction with the protein residues and the water/hydrogen-bonding network. This process is clearly demonstrated in Photosystem II (PS II), where its light-driven water oxidation reaction catalyzed by the Mn4CaO5 cluster occurs deep inside the protein complex and thus requires the transport of two water molecules to and four protons from the metal center to the bulk water. Based on the recent advances in structural studies of PS II from X-ray crystallography and cryo-electron microscopy, in this review we compare the channels that have been proposed to facilitate this mass transport in cyanobacteria, red and green algae, diatoms, and higher plants. The three major channels (O1, O4, and Cl1 channels) are present in all species investigated; however, some differences exist in the reported structures that arise from the different composition and arrangement of membrane extrinsic subunits between the species. Among the three channels, the Cl1 channel, including the proton gate, is the most conserved among all photosynthetic species. We also found at least one branch for the O1 channel in all organisms, extending all the way from Ca/O1 via the 'water wheel' to the lumen. However, the extending path after the water wheel varies between most species. The O4 channel is, like the Cl1 channel, highly conserved among all species while having different orientations at the end of the path near the bulk. The comparison suggests that the previously proposed functionality of the channels in T. vestitus (Ibrahim et al., Proc Natl Acad Sci USA 117:12624-12635, 2020; Hussein et al., Nat Commun 12:6531, 2021) is conserved through the species, i.e. the O1-like channel is used for substrate water intake, and the tighter Cl1 and O4 channels for proton release. The comparison does not eliminate the potential role of O4 channel as a water intake channel. However, the highly ordered hydrogen-bonded water wire connected to the Mn4CaO5 cluster via the O4 may strongly suggest that it functions in proton release, especially during the S0 → S1 transition (Saito et al., Nat Commun 6:8488, 2015; Kern et al., Nature 563:421-425, 2018; Ibrahim et al., Proc Natl Acad Sci USA 117:12624-12635, 2020; Sakashita et al., Phys Chem Chem Phys 22:15831-15841, 2020; Hussein et al., Nat Commun 12:6531, 2021).

Water Networks in Photosystem II Using Crystalline Molecular Dynamics Simulations and Room-Temperature XFEL Serial Crystallography

Structural dynamics of water and its hydrogen-bonding networks play an important role in enzyme function via the transport of protons, ions, and substrates. To gain insights into these mechanisms in the water oxidation reaction in Photosystem II (PS II), we have performed crystalline molecular dynamics (MD) simulations of the dark-stable S1 state. Our MD model consists of a full unit cell with 8 PS II monomers in explicit solvent (861 894 atoms), enabling us to compute the simulated crystalline electron density and to compare it directly with the experimental density from serial femtosecond X-ray crystallography under physiological temperature collected at X-ray free electron lasers (XFELs). The MD density reproduced the experimental density and water positions with high fidelity. The detailed dynamics in the simulations provided insights into the mobility of water molecules in the channels beyond what can be interpreted from experimental B-factors and electron densities alone. In particular, the simulations revealed fast, coordinated exchange of waters at sites where the density is strong, and water transport across the bottleneck region of the channels where the density is weak. By computing MD hydrogen and oxygen maps separately, we developed a novel Map-based Acceptor-Donor Identification (MADI) technique that yields information which helps to infer hydrogen-bond directionality and strength. The MADI analysis revealed a series of hydrogen-bond wires emanating from the Mn cluster through the Cl1 and O4 channels; such wires might provide pathways for proton transfer during the reaction cycle of PS II. Our simulations provide an atomistic picture of the dynamics of water and hydrogen-bonding networks in PS II, with implications for the specific role of each channel in the water oxidation reaction.

Structure of a dimeric photosystem II complex from a cyanobacterium acclimated to far-red light

Photosystem II (PSII) is the water-splitting enzyme central to oxygenic photosynthesis. To drive water oxidation, light is harvested by accessory pigments, mostly chlorophyll (Chl) a molecules, which absorb visible light (400-700 nm). Some cyanobacteria facultatively acclimate to shaded environments by altering their photosynthetic machinery to additionally absorb far-red light (FRL, 700-800 nm), a process termed far-red light photoacclimation or FaRLiP. During far-red light photoacclimation, FRL-PSII is assembled with FRL-specific isoforms of the subunits PsbA, PsbB, PsbC, PsbD, and PsbH, and some Chl-binding sites contain Chls d or f instead of the usual Chl a. The structure of an apo-FRL-PSII monomer lacking the FRL-specific PsbH subunit has previously been determined, but visualization of the dimeric complex has remained elusive. Here, we report the cryo-EM structure of a dimeric FRL-PSII complex. The site assignments for Chls d and f are consistent with those assigned in the previous apo-FRL-PSII monomeric structure. All sites that bind Chl d or Chl f at high occupancy exhibit a FRL-specific interaction of the formyl moiety of the Chl d or Chl f with the protein environment, which in some cases involves a phenylalanine sidechain. The structure retains the FRL-specific PsbH2 subunit, which appears to alter the energetic landscape of FRL-PSII, redirecting energy transfer from the phycobiliprotein complex to a Chl f molecule bound by PsbB2 that acts as a bridge for energy transfer to the electron transfer chain. Collectively, these observations extend our previous understanding of the structure-function relationship that allows PSII to function using lower energy FRL.

Interplay between local protein interactions and water bridging of a proton antenna carboxylate cluster

Proteins that bind protons at cell membrane interfaces often expose to the bulk clusters of carboxylate and histidine sidechains that capture protons transiently and, in proton transporters, deliver protons to an internal site. The protonation-coupled dynamics of bulk-exposed carboxylate clusters, also known as proton antennas, is poorly described. An essential open question is how water-mediated bridges between sidechains of the cluster respond to protonation change and facilitate transient proton storage. To address this question, here I studied the protonation-coupled dynamics at the proton-binding antenna of PsbO, a small extrinsinc subunit of the photosystem II complex, with atomistic molecular dynamics simulations and systematic graph-based analyses of dynamic protein and protein-water hydrogen-bond networks. The protonation of specific carboxylate groups is found to impact the dynamics of their local protein-water hydrogen-bond clusters. Regardless of the protonation state considered for PsbO, carboxylate pairs that can sample direct hydrogen bonding, or bridge via short hydrogen-bonded water chains, anchor to nearby basic or polar protein sidechains. As a result, carboxylic sidechains of the hypothesized antenna cluster are part of dynamic hydrogen bond networks that may rearrange rapidly when the protonation changes.

Genes encoding the photosystem II proteins are under purifying selection: an insight into the early evolution of oxygenic photosynthesis

The molecular evolution concerns coding sequences (CDSs) of genes and may affect the structure and function of proteins. Non-uniform use of synonymous codons during translation, known as codon usage bias (CUB), depends on the balance between mutations bias and natural selection. We estimated different CUB indices, i.e. the effective number of codons (ENC), G + C content in the 3rd codon positions (GC₃), and codon adaptation index for CDSs of intrinsic proteins of photosystem II (PSII), such as psbA (D1), psbD (D2), psbB (CP47), psbC (CP43), and CDSs of the extrinsic protein psbO (PsbO). These genes occur in all organisms that perform oxygenic photosynthesis (OP) on Earth: cyanobacteria, algae and plants. Comparatively, a similar analysis of codon bias for CDSs of L and M subunits that constitute the core proteins of the type II reaction centre (RCII) in anoxygenic bacteria was performed. Analysis of CUB indices and determination of the number of synonymous (d_S) and nonsynonymous substitutions (d_N) in all analysed CDSs indicated that the crucial PSII and RCII proteins were under strong purifying (negative) selection in course of evolution. Purifying selection was also estimated for CDSs of atpA, the α subunit of ATP synthase, an enzyme that was most likely already present in the last universal common ancestor (LUCA). The data obtained point to an ancient origin of OP, even in the earliest stages of the evolution of life on Earth.

Binding and functions of the two chloride ions in the oxygen-evolving center of photosystem II

Light-driven water oxidation in photosynthesis occurs at the oxygen-evolving center (OEC) of photosystem II (PSII). Chloride ions (Cl^-) are essential for oxygen evolution by PSII, and two Cl^- ions have been found to specifically bind near the Mn₄CaO₅ cluster in the OEC. The retention of these Cl^- ions within the OEC is critically supported by some of the membrane-extrinsic subunits of PSII. The functions of these two Cl^- ions and the mechanisms of their retention both remain to be fully elucidated. However, intensive studies performed recently have advanced our understanding of the functions of these Cl^- ions, and PSII structures from various species have been reported, aiding the interpretation of previous findings regarding Cl^- retention by extrinsic subunits. In this review, we summarize the findings to date on the roles of the two Cl^- ions bound within the OEC. Additionally, together with a short summary of the functions of PSII membrane-extrinsic subunits, we discuss the mechanisms of Cl^- retention by these extrinsic subunits.

Photosynthetic apparatus efficiency, phenolic acid profiling and pattern of chosen phytohormones in pseudometallophyte Alyssum montanum

The present study investigated the response of non-metallicolous (NM) and metallicolous (M) Alyssum montanum shoots cultured in vitro on a medium supplemented simultaneously with heavy metals (HMs) to identify mechanisms involved in alleviating metal-induced damage. Plant status in respect to photosynthetic apparatus efficiency was determined and linked with changes in biochemical composition of shoots, namely phenolic acids' and stress-related phytohormones. Results showed the considerable inter-ecotype differences in (1) the photosynthetic pigments' amount, (2) the functioning of membrane electron transporters as well as (3) the linear and alternative electron transport pathways, whose lower values were reported in NM than in M HM-treated culture. Photosynthetic apparatus protection in M specimens was assured by the activation of cinnamic acid synthesis (by phenylalanine ammonia lyase) and its further transformations to benzoic acid derivatives with high ability to counteract oxidative stress, that was accompanied by the overexpression of jasmonic acid stimulating antioxidant machinery. In turn, detrimental HM effects on NM shoots could result from the diminution of most phenolics' accumulation, and only the content of coumarate (produced by bifunctional phenylalanine/tyrosine ammonia lyase) and rosmarinic acid increased. All these together with an enhanced concentration of abscisic acid might suggest that NM strategy to cope with HMs is based mostly on a restriction of metal movement with transpiration flow and their limited distribution in leaves. Summarizing, our findings for the first time point out the physiological and metabolic adaptation of pseudometallophyte A. montanum to adverse conditions.

Function of PsbO-Asp158 in photosystem II: effects of mutation of this residue on the binding of PsbO and function of PSII in Thermosynechococcus vulcanus

PsbO-D158 is a highly conserved residue of the PsbO protein in photosystem II (PSII), and participates in one of the hydrogen-bonding networks connecting the manganese cluster with the lumenal surface. In order to examine the role of PsbO-D158, we mutated it to E, N or K in Thermosynechococcus vulcanus and characterized photosynthetic properties of the mutants obtained. The growth rates of these three mutants were similar to that of the wild type, whereas the oxygen-evolving activity of the three mutant cells decreased to 60-64% of the wild type. Fluorescence kinetics showed that the mutations did not affect the electron transfer from Q_A to Q_B, but slightly affected the donor side of PSII. Moreover, all of the three mutant cells were more sensitive to high light and became slower to recover from photoinhibition. In the isolated thylakoid membranes from the three mutants, the PsbU subunit was lost and the oxygen-evolving activity was reduced to a lower level compared to that in the respective cells. PSII complexes isolated from these mutants showed no oxygen-evolving activity, which was found to be due to large or complete loss of PsbO, PsbV and PsbU during the process of purification. Moreover, PSII cores purified from the three mutants contained Psb27, an assembly co-factor of PSII. These results suggest that PsbO-D158 is required for the proper binding of the three extrinsic proteins to PSII and plays an important role in maintaining the optimal oxygen-evolving activity, and its mutation caused incomplete assembly of the PSII complex.

Can Ceylon Leadwort (Plumbago zeylanica L.) Acclimate to Lead Toxicity?-Studies of Photosynthetic Apparatus Efficiency

Ceylon leadwort (Plumbago zeylanica) is ornamental plant known for its pharmacological properties arising from the abundant production of various secondary metabolites. It often grows in lead polluted areas. The aim of presented study was to evaluate the survival strategy of P. zeylanica to lead toxicity via photosynthetic apparatus acclimatization. Shoots of P. zeylanica were cultivated on media with different Pb concentrations (0.0, 0.05, and 0.1 g Pb∙l⁻¹). After a four-week culture, the efficiency of the photosynthetic apparatus of plants was evaluated by Chl a fluorescence measurement, photosynthetic pigment, and Lhcb1, PsbA, PsbO, and RuBisCo protein accumulation, antioxidant enzymes activity, and chloroplast ultrastructure observation. Plants from lower Pb concentration revealed no changes in photosynthetic pigments content and light-harvesting complex (LHCII) size, as well as no limitation on the donor side of Photosystem II Reaction Centre (PSII RC). However, the activity and content of antioxidant enzymes indicated a high risk of limitation on the acceptor side of Photosystem I. In turn, plants from 0.1 g Pb∙l⁻¹ showed a significant decrease in pigments content, LHCII size, the amount of active PSII RC, oxygen-evolving complex activity, and significant remodeling of chloroplast ultrastructure indicated limitation of PSII RC donor side. Obtained results indicate that P. zeylanica plants acclimate to lead toxicity by Pb accumulation in roots and, depending on Pb concentration, by adjusting their photosynthetic apparatus via the activation of alternative (cyclic and pseudocyclic) electron transport pathways.

pH-Dependent Protonation of Surface Carboxylate Groups in PsbO Enables Local Buffering and Triggers Structural Changes

Photosystem II (PSII) catalyzes the splitting of water, releasing protons and dioxygen. Its highly conserved subunit PsbO extends from the oxygen‐evolving center (OEC) into the thylakoid lumen and stabilizes the catalytic Mn₄CaO₅ cluster. The high degree of conservation of accessible negatively charged surface residues in PsbO suggests additional functions, as local pH buffer or by affecting the flow of protons. For this discussion, we provide an experimental basis, through the determination of pK_a values of water‐accessible aspartate and glutamate side‐chain carboxylate groups by means of NMR. Their distribution is strikingly uneven, with high pK_a values around 4.9 clustered on the luminal PsbO side and values below 3.5 on the side facing PSII. pH‐dependent changes in backbone chemical shifts in the area of the lumen‐exposed loops are observed, indicating conformational changes. In conclusion, we present a site‐specific analysis of carboxylate group proton affinities in PsbO, providing a basis for further understanding of proton transport in photosynthesis.

Structural basis of light-harvesting in the photosystem II core complex

Photosystem II (PSII) is a membrane-spanning, multi-subunit pigment-protein complex responsible for the oxidation of water and the reduction of plastoquinone in oxygenic photosynthesis. In the present review, the recent explosive increase in available structural information about the PSII core complex based on X-ray crystallography and cryo-electron microscopy is described at a level of detail that is suitable for a future structure-based analysis of light-harvesting processes. This description includes a proposal for a consistent numbering scheme of protein-bound pigment cofactors across species. The structural survey is complemented by an overview of the state of affairs in structure-based modeling of excitation energy transfer in the PSII core complex with emphasis on electrostatic computations, optical properties of the reaction center, the assignment of long-wavelength chlorophylls, and energy trapping mechanisms.

Important roles of glycinebetaine in stabilizing the structure and function of the photosystem II complex under abiotic stresses

The molecular and physiological mechanisms of glycinebetaine stabilizing photosystem II complex under abiotic stresses are discussed, helping to address food shortage problems threatening the survival of growing population. In the backdrop of climate change, the frequency, dimensions and duration of extreme events have increased sharply, which may have unintended consequences for agricultural. The acclimation of plants to a constantly changing environment involves the accumulation of compatible solutes. Various compatible solutes enable plants to tolerate abiotic stresses, and glycinebetaine (GB) is one of the most-studied. The biosynthesis and accumulation of GB appear in numerous plant species, especially under environmental stresses. The exogenous application of GB and GB-accumulating transgenic plants have been proven to further promote plant development under stresses. Early research on GB focused on the maintenance of osmotic potential in plants. Subsequent experimental evidence demonstrated that it also protects proteins including the photosystem II complex (PSII) from denaturation and deactivation. As reviewed here, multiple experimental evidences have indicated considerable progress in the roles of GB in stabilizing PSII under abiotic stresses. Based on these advances, we've concluded two effects of GB on PSII: (1) it stabilizes the structure of PSII by protecting extrinsic proteins from dissociation or by promoting protein synthesize; (2) it enhances the oxygen-evolving activity of PSII or promotes the repair of the photosynthetic damage of PSII.

YR36/WKS1-Mediated Phosphorylation of PsbO, an Extrinsic Member of Photosystem II, Inhibits Photosynthesis and Confers Stripe Rust Resistance in Wheat

Wheat stripe rust, due to infection by Puccinia striiformis f. sp. tritici (Pst), is a devastating disease that causes significant global grain yield losses. Yr36, which encodes Wheat Kinase START1 (WKS1), is an effective high-temperature adult-plant resistance gene and confers resistance to a broad spectrum of Pst races. We previously showed that WKS1 phosphorylates the thylakoid ascorbate peroxidase protein and reduces its ability to detoxify peroxides, which may contribute to the accumulation of reactive oxygen species (ROS). WKS1-mediated Pst resistance is accompanied by leaf chlorosis in Pst-infected regions, but the underlying mechanisms remain elusive. Here, we show that WKS1 interacts with and phosphorylates PsbO, an extrinsic member of photosystem II (PSII), to reduce photosynthesis, regulate leaf chlorosis, and confer Pst resistance. A point mutation in PsbO-A1 or reduction in its transcript levels by RNA interference resulted in chlorosis and reduced Pst sporulation. Biochemical analyses revealed that WKS1 phosphorylates PsbO at two conserved amino acids involved in physical interactions with PSII and reduces the binding affinity of PsbO with PSII. Presumably, phosphorylated PsbO proteins dissociate from the PSII complex and then undergo rapid degradation by cysteine and aspartic proteases. Taken together, these results demonstrate that perturbations of wheat PsbO by point mutation or phosphorylation by WKS1 reduce the rate of photosynthesis and delay the growth of Pst pathogen before the induction of ROS.

Dynamic pH-induced conformational changes of the PsbO protein in the fluctuating acidity of the thylakoid lumen

The PsbO protein is an essential extrinsic subunit of photosystem II, the pigment-protein complex responsible for light-driven water splitting. Water oxidation in photosystem II supplies electrons to the photosynthetic electron transfer chain and is accompanied by proton release and oxygen evolution. While the electron transfer steps in this process are well defined and characterized, the driving forces acting on the liberated protons, their dynamics and their destiny are all largely unknown. It was suggested that PsbO undergoes proton-induced conformational changes and forms hydrogen bond networks that ensure prompt proton removal from the catalytic site of water oxidation, i.e. the Mn₄ CaO₅ cluster. This work reports the purification and characterization of heterologously expressed PsbO from green algae Chlamydomonas reinhardtii and two isoforms from the higher plant Solanum tuberosum (PsbO1 and PsbO2). A comparison to the spinach PsbO reveals striking similarities in intrinsic protein fluorescence and CD spectra, reflecting the near-identical secondary structure of the proteins from algae and higher plants. Titration experiments using the hydrophobic fluorescence probe ANS revealed that eukaryotic PsbO proteins exhibit acid-base hysteresis. This hysteresis is a dynamic effect accompanied by changes in the accessibility of the protein's hydrophobic core and is not due to reversible oligomerization or unfolding of the PsbO protein. These results confirm the hypothesis that pH-dependent dynamic behavior at physiological pH ranges is a common feature of PsbO proteins and causes reversible opening and closing of their β-barrel domain in response to the fluctuating acidity of the thylakoid lumen.

Photosystem II Extrinsic Proteins and Their Putative Role in Abiotic Stress Tolerance in Higher Plants

Abiotic stress remains one of the major challenges in managing and preventing crop loss. Photosystem II (PSII), being the most susceptible component of the photosynthetic machinery, has been studied in great detail over many years. However, much of the emphasis has been placed on intrinsic proteins, particularly with respect to their involvement in the repair of PSII-associated damage. PSII extrinsic proteins include PsbO, PsbP, PsbQ, and PsbR in higher plants, and these are required for oxygen evolution under physiological conditions. Changes in extrinsic protein expression have been reported to either drastically change PSII efficiency or change the PSII repair system. This review discusses the functional role of these proteins in plants and indicates potential areas of further study concerning these proteins.

Effects of PSII Manganese-Stabilizing Protein Succinylation on Photosynthesis in the Model Cyanobacterium Synechococcus sp. PCC 7002

Lysine succinylation is a newly identified protein post-translational modification and plays important roles in various biological pathways in both prokaryotes and eukaryotes, but its extent and function in photosynthetic organisms remain largely unknown. Here, we performed the first systematic studies of lysine succinylation in cyanobacteria, which are the only prokaryotes capable of oxygenic photosynthesis and the established model organisms for studying photosynthetic mechanisms. By using mass spectrometry analysis in combination with the enrichment of succinylated peptides from digested cell lysates, we identified 1,704 lysine succinylation sites on 691 proteins in a model cyanobacterium Synechococcus sp. PCC 7002. Bioinformatic analysis revealed that a large proportion of the succinylation sites were present on proteins in photosynthesis and metabolism. Among all identified succinylated proteins involved in photosynthesis, the PSII manganese-stabilizing protein (PsbO) was found to be succinylated on Lys99 and Lys234. Functional studies of PsbO were performed by site-directed mutagenesis, and mutants mimicking either constitutively succinylated (K99E and K234E) or non-succinylated states (K99R and K234R) were constructed. The succinylation-mimicking K234E mutant exhibited a decreased oxygen evolution rate of the PSII center and the efficiency of energy transfer during the photosynthetic reaction. Molecular dynamics simulations suggested a mechanism that may allow succinylation to influence the efficiency of photosynthesis by altering the conformation of PsbO, thereby hindering the interaction between PsbO and the PSII core. Our findings suggest that reversible succinylation may be an important regulatory mechanism during photosynthesis in Synechococcus, as well as in other photosynthetic organisms.

Dynamics of Long-Distance Hydrogen-Bond Networks in Photosystem II

Photosystem II uses the energy of absorbed light to split water molecules, generating molecular oxygen, electrons, and protons. The four protons generated during each reaction cycle are released to the lumen via mechanisms that are poorly understood. Given the complexity of photosystem II, which consists of multiple protein subunits and cofactor molecules and hosts numerous waters, a fundamental issue is finding transient networks of hydrogen bonds that bridge potential proton donor and acceptor groups. Here, we address this issue by performing all-atom molecular dynamics simulations of wild-type and mutant photosystem II monomers, which we analyze using a new protocol designed to facilitate efficient analysis of hydrogen-bond networks. Our computations reveal that local protein/water hydrogen-bond networks can assemble transiently in photosystem II such that the reaction center connects to the lumen. The dynamics of the hydrogen-bond networks couple to the protonation state of specific carboxylate groups and are altered in a mutant with defective proton transfer. Simulations on photosystem II without its extrinsic PsbO subunit provide a molecular interpretation of the elusive functional role of this subunit.

Acetylome Profiling Reveals Extensive Lysine Acetylation of the Fatty Acid Metabolism Pathway in the Diatom Phaeodactylum tricornutum

N^ε-lysine acetylation represents a highly dynamic and reversibly regulated post-translational modification widespread in almost all organisms, and plays important roles for regulation of protein function in diverse metabolic pathways. However, little is known about the role of lysine acetylation in photosynthetic eukaryotic microalgae. We integrated proteomic approaches to comprehensively characterize the lysine acetylome in the model diatom Phaeodactylum tricornutum In total, 2324 acetylation sites from 1220 acetylated proteins were identified, representing the largest data set of the lysine acetylome in plants to date. Almost all enzymes involved in fatty acid synthesis were found to be lysine acetylated. Six putative lysine acetylation sites were identified in a plastid-localized long-chain acyl-CoA synthetase. Site-directed mutagenesis and site-specific incorporation of N-acetyllysine in acyl-CoA synthetase show that acetylation at K407 and K425 increases its enzyme activity. Moreover, the nonenzymatically catalyzed overall hyperacetylation of acyl-CoA synthetase by acetyl-phosphate can be effectively deacetylated and reversed by a sirtuin-type NAD⁺-dependent deacetylase with subcellular localization of both the plastid and nucleus in Phaeodactylum This work indicates the regulation of acyl-CoA synthetase activity by site-specific lysine acetylation and highlights the potential regulation of fatty acid metabolism by lysine actetylation in the plastid of the diatom Phaeodactylum.

Protein crystallization and initial neutron diffraction studies of the photosystem II subunit PsbO

The PsbO protein of photosystem II stabilizes the active-site manganese cluster and is thought to act as a proton antenna. To enable neutron diffraction studies, crystals of the β-barrel core of PsbO were grown in capillaries. The crystals were optimized by screening additives in a counter-diffusion setup in which the protein and reservoir solutions were separated by a 1% agarose plug. Crystals were cross-linked with glutaraldehyde. Initial neutron diffraction data were collected from a 0.25 mm3 crystal at room temperature using the MaNDi single-crystal diffractometer at the Spallation Neutron Source, Oak Ridge National Laboratory.

Structure and assembly mechanism of plant C2S2M2-type PSII-LHCII supercomplex

In plants, the photosynthetic machinery photosystem II (PSII) consists of a core complex associated with variable numbers of light-harvesting complexes II (LHCIIs). The supercomplex, comprising a dimeric core and two strongly bound and two moderately bound LHCIIs (C2S2M2), is the dominant form in plants acclimated to limited light. Here we report cryo-electron microscopy structures of two forms of C2S2M2 (termed stacked and unstacked) from Pisum sativum at 2.7- and 3.2-angstrom resolution, respectively. In each C2S2M2, the moderately bound LHCII assembles specifically with a peripheral antenna complex CP24-CP29 heterodimer and the strongly bound LHCII, to establish a pigment network that facilitates light harvesting at the periphery and energy transfer into the core. The high mobility of peripheral antennae, including the moderately bound LHCII and CP24, provides insights into functional regulation of plant PSII.

Lysine Acetylome Analysis Reveals Photosystem II Manganese-stabilizing Protein Acetylation is Involved in Negative Regulation of Oxygen Evolution in Model Cyanobacterium Synechococcus sp. PCC 7002

Nε-Acetylation of lysine residues represents a frequently occurring post-translational modification widespread in bacteria that plays vital roles in regulating bacterial physiology and metabolism. However, the role of lysine acetylation in cyanobacteria remains unclear, presenting a hurdle to in-depth functional study of this post-translational modification. Here, we report the lysine acetylome of Synechococcus sp. PCC 7002 (hereafter Synechococcus) using peptide prefractionation, immunoaffinity enrichment, and coupling with high-precision liquid chromatography-tandem mass spectrometry analysis. Proteomic analysis of Synechococcus identified 1653 acetylation sites on 802 acetylproteins involved in a broad range of biological processes. Interestingly, the lysine acetylated proteins were enriched for proteins involved in photosynthesis, for example. Functional studies of the photosystem II manganese-stabilizing protein were performed by site-directed mutagenesis and mutants mimicking either constitutively acetylated (K99Q, K190Q, and K219Q) or nonacetylated states (K99R, K190R, and K219R) were constructed. Mutation of the K190 acetylation site resulted in a distinguishable phenotype. Compared with the K190R mutant, the K190Q mutant exhibited a decreased oxygen evolution rate and an enhanced cyclic electron transport rate in vivo Our findings provide new insight into the molecular mechanisms of lysine acetylation that involved in the negative regulation of oxygen evolution in Synechococcus and creates opportunities for in-depth elucidation of the physiological role of protein acetylation in photosynthesis in cyanobacteria.

Use of Protein Cross-Linking and Radiolytic Labeling To Elucidate the Structure of PsbO within Higher-Plant Photosystem II

We have used protein cross-linking with the zero-length cross-linker 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide, and radiolytic footprinting coupled with high-resolution tandem mass spectrometry, to examine the structure of higher-plant PsbO when it is bound to Photosystem II. Twenty intramolecular cross-linked residue pairs were identified. On the basis of this cross-linking data, spinach PsbO was modeled using the Thermosynechococcus vulcanus PsbO structure as a template, with the cross-linking distance constraints incorporated using the MODELLER program. Our model of higher-plant PsbO identifies several differences between the spinach and cyanobacterial proteins. The N-terminal region is particularly interesting, as this region has been suggested to be important for oxygen evolution and for the specific binding of PsbO to Photosystem II. Additionally, using radiolytic mapping, we have identified regions on spinach PsbO that are shielded from the bulk solvent. These domains may represent regions on PsbO that interact with other components, as yet unidentified, of the photosystem.

Pivotal Role of Iron in the Regulation of Cyanobacterial Electron Transport

Iron-containing metalloproteins are the main cornerstones for efficient electron transport in biological systems. The abundance and diversity of iron-dependent proteins in cyanobacteria makes those organisms highly dependent of this micronutrient. To cope with iron imbalance, cyanobacteria have developed a survey of adaptation strategies that are strongly related to the regulation of photosynthesis, nitrogen metabolism and other central electron transfer pathways. Furthermore, either in its ferrous form or as a component of the haem group, iron plays a crucial role as regulatory signalling molecule that directly or indirectly modulates the composition and efficiency of cyanobacterial redox reactions. We present here the major mechanism used by cyanobacteria to couple iron homeostasis to the regulation of electron transport, making special emphasis in processes specific in those organisms.

Reconstructing the Origin of Oxygenic Photosynthesis: Do Assembly and Photoactivation Recapitulate Evolution?

Due to the great abundance of genomes and protein structures that today span a broad diversity of organisms, now more than ever before, it is possible to reconstruct the molecular evolution of protein complexes at an incredible level of detail. Here, I recount the story of oxygenic photosynthesis or how an ancestral reaction center was transformed into a sophisticated photochemical machine capable of water oxidation. First, I review the evolution of all reaction center proteins in order to highlight that Photosystem II and Photosystem I, today only found in the phylum Cyanobacteria, branched out very early in the history of photosynthesis. Therefore, it is very unlikely that they were acquired via horizontal gene transfer from any of the described phyla of anoxygenic phototrophic bacteria. Second, I present a new evolutionary scenario for the origin of the CP43 and CP47 antenna of Photosystem II. I suggest that the antenna proteins originated from the remodeling of an entire Type I reaction center protein and not from the partial gene duplication of a Type I reaction center gene. Third, I highlight how Photosystem II and Photosystem I reaction center proteins interact with small peripheral subunits in remarkably similar patterns and hypothesize that some of this complexity may be traced back to the most ancestral reaction center. Fourth, I outline the sequence of events that led to the origin of the Mn4CaO5 cluster and show that the most ancestral Type II reaction center had some of the basic structural components that would become essential in the coordination of the water-oxidizing complex. Finally, I collect all these ideas, starting at the origin of the first reaction center proteins and ending with the emergence of the water-oxidizing cluster, to hypothesize that the complex and well-organized process of assembly and photoactivation of Photosystem II recapitulate evolutionary transitions in the path to oxygenic photosynthesis.

Parallel subfunctionalisation of PsbO protein isoforms in angiosperms revealed by phylogenetic analysis and mapping of sequence variability onto protein structure

BACKGROUND

PsbO, the manganese-stabilising protein, is an indispensable extrinsic subunit of photosystem II. It plays a crucial role in the stabilisation of the water-splitting Mn4CaO5 cluster, which catalyses the oxidation of water to molecular oxygen by using light energy. PsbO was also demonstrated to have a weak GTPase activity that could be involved in regulation of D1 protein turnover. Our analysis of psbO sequences showed that many angiosperm species express two psbO paralogs, but the pairs of isoforms in one species were not orthologous to pairs of isoforms in distant species.

RESULTS

Phylogenetic analysis of 91 psbO sequences from 49 land plant species revealed that psbO duplication occurred many times independently, generally at the roots of modern angiosperm families. In spite of this, the level of isoform divergence was similar in different species. Moreover, mapping of the differences on the protein tertiary structure showed that the isoforms in individual species differ from each other on similar positions, mostly on the luminally exposed end of the β-barrel structure. Comparison of these differences with the location of differences between PsbOs from diverse angiosperm families indicated various selection pressures in PsbO evolution and potential interaction surfaces on the PsbO structure.

CONCLUSIONS

The analyses suggest that similar subfunctionalisation of PsbO isoforms occurred parallelly in various lineages. We speculate that the presence of two PsbO isoforms helps the plants to finely adjust the photosynthetic apparatus in response to variable conditions. This might be mediated by diverse GTPase activity, since the isoform differences predominate near the predicted GTP-binding site.

Proteomic approaches to identify substrates of the three Deg/HtrA proteases of the cyanobacterium Synechocystis sp. PCC 6803

The family of Deg/HtrA proteases plays an important role in quality control of cellular proteins in a wide range of organisms. In the genome of the cyanobacterium Synechocystis sp. PCC 6803, a model organism for photosynthetic research and renewable energy products, three Deg proteases are encoded, termed HhoA, HhoB and HtrA. In the present study, we compared wild-type (WT) Synechocystis cells with the single insertion mutants ΔhhoA, ΔhhoB and ΔhtrA. Protein expression of the remaining Deg/HtrA proteases was strongly affected in the single insertion mutants. Detailed proteomic studies using DIGE (difference gel electrophoresis) and N-terminal COFRADIC (N-terminal combined fractional diagonal chromatography) revealed that inactivation of a single Deg protease has similar impact on the proteomes of the three mutants; differences to WT were observed in enzymes involved in the major metabolic pathways. Changes in the amount of phosphate permease system Pst-1 were observed only in the insertion mutant ΔhhoB. N-terminal COFRADIC analyses on cell lysates of ΔhhoB confirmed changed amounts of many cell envelope proteins, including the phosphate permease systems, compared with WT. In vitro COFRADIC studies were performed to identify the specificity profiles of the recombinant proteases rHhoA, rHhoB or rHtrA added to the Synechocystis WT proteome. The combined in vivo and in vitro N-terminal COFRADIC datasets propose RbcS as a natural substrate for HhoA, PsbO for HhoB and HtrA and Pbp8 for HtrA. We therefore suggest that each Synechocystis Deg protease protects the cell through different, but connected mechanisms.

Integrated transcriptomic and proteomic analysis of the global response of Synechococcus to high light stress

Sufficient light is essential for the growth and physiological functions of photosynthetic organisms, but prolonged exposure to high light (HL) stress can cause cellular damage and ultimately result in the death of these organisms. Synechococcus sp. PCC 7002 (hereafter Synechococcus 7002) is a unicellular cyanobacterium with exceptional tolerance to HL intensities. However, the molecular mechanisms involved in HL response by Synechococcus 7002 are not well understood. Here, an integrated RNA sequencing transcriptomic and quantitative proteomic analysis was performed to investigate the cellular response to HL in Synechococcus 7002. A total of 526 transcripts and 233 proteins were identified to be differentially regulated under HL stress. Data analysis revealed major changes in mRNAs and proteins involved in the photosynthesis pathways, resistance to light-induced damage, DNA replication and repair, and energy metabolism. A set of differentially expressed mRNAs and proteins were validated by quantitative RT-PCR and Western blot, respectively. Twelve genes differentially regulated under HL stress were selected for knockout generation and growth analysis of these mutants led to the identification of key genes involved in the response of HL in Synechococcus 7002. Taken altogether, this study established a model for global response mechanisms to HL in Synechococcus 7002 and may be valuable for further studies addressing HL resistance in photosynthetic organisms.

FTIR studies of metal ligands, networks of hydrogen bonds, and water molecules near the active site Mn₄CaO₅ cluster in Photosystem II

The photosynthetic conversion of water to molecular oxygen is catalyzed by the Mn₄CaO₅ cluster in Photosystem II and provides nearly our entire supply of atmospheric oxygen. The Mn₄CaO₅ cluster accumulates oxidizing equivalents in response to light-driven photochemical events within Photosystem II and then oxidizes two molecules of water to oxygen. The Mn₄CaO₅ cluster converts water to oxygen much more efficiently than any synthetic catalyst because its protein environment carefully controls the cluster's reactivity at each step in its catalytic cycle. This control is achieved by precise choreography of the proton and electron transfer reactions associated with water oxidation and by careful management of substrate (water) access and proton egress. This review describes the FTIR studies undertaken over the past two decades to identify the amino acid residues that are responsible for this control and to determine the role of each. In particular, this review describes the FTIR studies undertaken to characterize the influence of the cluster's metal ligands on its activity, to delineate the proton egress pathways that link the Mn₄CaO₅ cluster with the thylakoid lumen, and to characterize the influence of specific residues on the water molecules that serve as substrate or as participants in the networks of hydrogen bonds that make up the water access and proton egress pathways. This information will improve our understanding of water oxidation by the Mn₄CaO₅ catalyst in Photosystem II and will provide insight into the design of new generations of synthetic catalysts that convert sunlight into useful forms of storable energy. This article is part of a Special Issue entitled: Vibrational spectroscopies and bioenergetic systems.

First site-specific incorporation of a noncanonical amino acid into the photosynthetic oxygen-evolving complex

In photosystem II (PSII), water is oxidized at the oxygen-evolving complex. This process occurs through a light-induced cycle that produces oxygen and protons. While coupled proton and electron transfer reactions play an important role in PSII and other proteins, direct detection of internal proton transfer reactions is challenging. Here, we demonstrate that the unnatural amino acid, 7-azatryptophan (7AW), has unique, pH-sensitive vibrational frequencies, which are sensitive markers of proton transfer. The intrinsically disordered, PSII subunit, PsbO, which contains a single W residue (Trp241), was engineered to contain 7AW at position 241. Fluorescence shows that 7AW-241 is buried in a hydrophobic environment. Reconstitution of 7AW(241)PsbO to PSII had no significant impact on oxygen evolution activity or flash-dependent protein dynamics. We conclude that directed substitution of 7AW into other structural domains is likely to provide a nonperturbative spectroscopic probe, which can be used to define internal proton pathways in PsbO.

MS-based cross-linking analysis reveals the location of the PsbQ protein in cyanobacterial photosystem II

PsbQ is a luminal extrinsic protein component that regulates the water splitting activity of photosystem II (PSII) in plants, algae, and cyanobacteria. However, PsbQ is not observed in the currently available crystal structures of PSII from thermophilic cyanobacteria. The structural location of PsbQ within the PSII complex has therefore remained unknown. Here, we report chemical cross-linking followed by immunodetection and liquid chromatography/tandem MS analysis of a dimeric PSII complex isolated from the model cyanobacterium, Synechocystis sp. PCC 6803, to determine the binding site of PsbQ within PSII. Our results demonstrate that PsbQ is closely associated with the PsbO and CP47 proteins, as revealed by cross-links detected between (120)K of PsbQ and (180)K and (59)K of PsbO, and between (102)K of PsbQ and (440)D of CP47. We further show that genetic deletion of the psbO gene results in the complete absence of PsbQ in PSII complexes as well as the loss of the dimeric form of PSII. Overall, our data provide a molecular-level description of the enigmatic binding site of PsbQ in PSII in a cyanobacterium. These results also help us understand the sequential incorporation of the PsbQ protein during the PSII assembly process, as well as its stabilizing effect on the oxygen evolution activity of PSII.

Tat transport of a Sec passenger leads to both completely translocated as well as membrane-arrested passenger proteins

We have studied the membrane transport of the chimeric precursor protein 16/33, which is composed of the Tat(1)-specific transport signal of OEC16 and the Sec passenger protein OEC33, both subunits of the oxygen-evolving system associated with photosystem II. Protein transport experiments performed with isolated pea thylakoids show that the 16/33 chimera is transported in a strictly Tat-dependent manner into the thylakoid vesicles yielding mature OEC33 (mOEC33) in two different topologies. One fraction accumulates in the thylakoid lumen and is thus resistant to externally added protease. A second fraction is arrested during transport in an N-in/C-out topology within the membrane. Chase experiments demonstrate that this membrane-arrested mOEC33 moiety does not represent a translocation intermediate but instead an alternative end product of the transport process. Transport arrest of mOEC33, which is embedded in the membrane with a mildly hydrophobic protein segment, requires more than 26 additional and predominantly hydrophilic residues C-terminal of the membrane-embedded segment. Furthermore, it is stimulated by mutations which potentially affect the conformation of mOEC33 suggesting that at least partial folding of the passenger protein is required for complete membrane translocation.

Proteomic characterization and three-dimensional electron microscopy study of PSII-LHCII supercomplexes from higher plants

In higher plants a variable number of peripheral LHCII trimers can strongly (S), moderately (M) or loosely (L) associate with the dimeric PSII core (C2) complex via monomeric Lhcb proteins to form PSII-LHCII supercomplexes with different structural organizations. By solubilizing isolated stacked pea thylakoid membranes either with the α or β isomeric forms of the detergent n-dodecyl-D-maltoside, followed by sucrose density ultracentrifugation, we previously showed that PSII-LHCII supercomplexes of types C2S2M2 and C2S2, respectively, can be isolated [S. Barera et al., Phil. Trans. R Soc. B 67 (2012) 3389-3399]. Here we analysed their protein composition by applying extensive bottom-up and top-down mass spectrometry on the two forms of the isolated supercomplexes. In this way, we revealed the presence of the antenna proteins Lhcb3 and Lhcb6 and of the extrinsic polypeptides PsbP, PsbQ and PsbR exclusively in the C2S2M2 supercomplex. Other proteins of the PSII core complex, common to the C2S2M2 and C2S2 supercomplexes, including the low molecular mass subunits, were also detected and characterized. To complement the proteomic study with structural information, we performed negative stain transmission electron microscopy and single particle analysis on the PSII-LHCII supercomplexes isolated from pea thylakoid membranes solubilized with n-dodecyl-α-D-maltoside. We observed the C2S2M2 supercomplex in its intact form as the largest PSII complex in our preparations. Its dataset was further analysed in silico, together with that of the second largest identified sub-population, corresponding to its C2S2 subcomplex. In this way, we calculated 3D electron density maps for the C2S2M2 and C2S2 supercomplexes, approaching respectively 30 and 28Å resolution, extended by molecular modelling towards the atomic level. This article is part of a special issue entitled: photosynthesis research for sustainability: keys to produce clean energy.

Structural, functional and auxiliary proteins of photosystem II

Photosystem II (PSII) is the water-splitting enzyme complex of photosynthesis and consists of a large number of protein subunits. Most of these proteins have been structurally and functionally characterized, although there are differences between PSII of plants, algae and cyanobacteria. Here we catalogue all known PSII proteins giving a brief description, where possible of their genetic origin, physical properties, structural relationships and functions. We have also included details of auxiliary proteins known at present to be involved in the in vivo assembly, maintenance and turnover of PSII and which transiently bind to the reaction centre core complex. Finally, we briefly give details of the proteins which form the outer light-harvesting systems of PSII in different types of organisms.

The PsbO homolog from Symbiodinium kawagutii (Dinophyceae) characterized using biochemical and molecular methods

A photosystem II component, the PsbO protein is essential for maximum rates of oxygen production during photosynthesis, and has been extensively characterized in plants and cyanobacteria but not in symbiotic dinoflagellates. Its close interaction with D1 protein has important environmental implications since D1 has been identified as the primary site of damage in endosymbiotic dinoflagellates after thermal stress. We identified and biochemically characterized the PsbO homolog from Symbiodinium kawagutii as a 28-kDa protein, and immunolocalized it to chloroplast membranes. Chloroplast association was further confirmed by western blot on photosynthetic membrane preparations. TX-114 phase partitioning, chromatography, and SDS-PAGE for single band separation and partial peptide sequencing yielded peptides identical or with high identity to PsbO from dinoflagellates. Analysis of a cDNA library revealed three genes differing by only one aminoacid residue in the in silico-translated ORFs despite greater differences at nucleotide level in the untranslated, putative regulatory sequences. The consensus full amino acid sequence displayed all the characteristic domains and features of PsbO from other sources, but changes in functionally critical, highly conserved motifs were detected. Our biochemical, molecular, and immunolocalization data led to the conclusion that the 28-kDa protein from S. kawagutii is the PsbO homolog, thereby named SkPsbO. We discuss the implications of critical amino acid substitutions for a putative regulatory role of this protein.

Environmental pH affects photoautotrophic growth of Synechocystis sp. PCC 6803 strains carrying mutations in the lumenal proteins of PSII

Synechocystis sp. strain PCC 6803 grows photoautotrophically across a broad pH range, but wild-type cultures reach a higher density at elevated pH; however, photoheterotrophic growth is similar at high and neutral pH. A number of PSII mutants each lacking at least one lumenal extrinsic protein, and carrying a second PSII lumenal mutation, are able to grow photoautotrophically in BG-11 medium at pH 10.0, but not pH 7.5. We investigated the basis of this pH effect and observed no pH-specific change in variable fluorescence yield from PSII centers of the wild type or the pH-dependent ΔPsbO:ΔPsbU and ΔPsbV:ΔCyanoQ strains; however, 77 K fluorescence emission spectra indicated increased coupling of the phycobilisome (PBS) antenna at pH 10.0 in all mutants. DNA microarray data showed a cell-wide response to transfer from pH 10.0 to pH 7.5, including decreased mRNA levels of a number of oxidative stress-responsive transcripts. We hypothesize that this transcriptional response led to increased tolerance against reactive oxygen species and in particular singlet oxygen. This response enabled photoautotrophic growth of the PSII mutants at pH 10.0. This hypothesis was supported by increased resistance of all strains to rose bengal at pH 10.0 compared with pH 7.5.

Vectorial charge transfer reactions on the donor side of manganese-depleted and reconstituted photosystem 2 core complexes

The light-induced functioning of photosystem 2 (PS 2) is directly linked to the translocation of both electrons and protons across the membrane, which results in the formation of transmembrane electric potential difference (ΔΨ). Generation of ΔΨ due to S-state transitions of the water oxidation complex was demonstrated for the first time in Mn-depleted and reconstituted PS 2 core complexes incorporated into liposomes. The kinetics and relative amplitudes of the electrogenic reactions in dark-adapted samples during S1→S2, S2→S3, and S4→S0 transitions in response to the first, second and third laser flashes were comparable to those obtained in the intact PS 2 core particles. These results expand current understanding of the nature and mechanisms of electrogenic (vectorial) reactions due to a charge transfer on the donor side of PS 2.