Directed mutagenesis of the transmembrane domain of the PsbL subunit of photosystem II in Synechocystis sp. PCC 6803

Arg24 and 26 of the D2 protein are important for photosystem II assembly and plastoquinol exchange in Synechocystis sp. PCC 6803

Photosystem II (PS II) assembly is a stepwise process involving preassembly complexes or modules focused around four core PS II proteins. The current model of PS II assembly in cyanobacteria is derived from studies involving the deletion of one or more of these core subunits. Such deletions may destabilize other PS II assembly intermediates, making constructing a clear picture of the intermediate events difficult. Information on plastoquinone exchange pathways operating within PS II is also unclear and relies heavily on computer-aided simulations. Deletion of PsbX in [S. Biswas, J.J. Eaton-Rye, Biochim. Biophys. Acta - Bioenerg. 1863 (2022) 148519] suggested modified QB binding in PS II lacking this subunit. This study has indicated the phenotype of the ∆PsbX mutant arose by disrupting a conserved hydrogen bond between PsbX and the D2 (PsbD) protein. We mutated two conserved arginine residues (D2:Arg24 and D2:Arg26) to further understand the observations made with the ∆PsbX mutant. Mutating Arg24 disrupted the interaction between PsbX and D2, replicating the high-light sensitivity and altered fluorescence decay kinetics observed in the ∆PsbX strain. The Arg26 residue, on the other hand, was more important for either PS II assembly or for stabilizing the fully assembled complex. The effects of mutating both arginine residues to alanine or aspartate were severe enough to render the corresponding double mutants non-photoautotrophic. Our study furthers our knowledge of the amino-acid interactions stabilizing plastoquinone-exchange pathways while providing a platform to study PS II assembly and repair without the actual deletion of any proteins.

Lys264 of the D2 Protein Performs a Dual Role in Photosystem II Modifying Assembly and Electron Transfer through the Quinone-Iron Acceptor Complex

Bicarbonate (HCO3-) binding regulates electron flow between the primary (QA) and secondary (QB) plastoquinone electron acceptors of Photosystem II (PS II). Lys264 of the D2 subunit of PS II contributes to a hydrogen-bond network that stabilizes HCO3- ligation to the non-heme iron in the QA-Fe-QB complex. Using the cyanobacterium Synechocystis sp. PCC 6803, alanine and glutamate were introduced to create the K264A and K264E mutants. Photoautotrophic growth was slowed in K264E cells but not in the K264A strain. Both mutants accumulated an unassembled CP43 precomplex as well as the CP43-lacking RC47 assembly intermediate, indicating weakened binding of the CP43 precomplex to RC47. Assembly was impeded more in K264E cells than in the K264A strain, but K264A cells were more susceptible to high-light-induced photodamage when assayed using PS II-specific electron acceptors. Furthermore, an impaired repair mechanism was observed in the K264A mutant in protein labeling experiments. Unexpectedly, unlike the K264A strain, the K264E mutant displayed inhibited oxygen evolution following high-light exposure when HCO3- was added to support whole chain electron transport. In both mutants, the decay of chlorophyll fluorescence was slowed, indicating impaired electron transfer between QA and QB. Furthermore, the fluorescence decay kinetics in the K264E strain were insensitive to addition of either formate or HCO3-, whereas HCO3--reversible formate-induced inhibition in the K264A mutant was observed. Exchange of plastoquinol with the membrane plastoquinone pool at the QB-binding site was also retarded in both mutants. Hence, D2-Lys264 possesses key roles in both assembly and activity of PS II.

Tyr244 of the D2 Protein Is Required for Correct Assembly and Operation of the Quinone-Iron-Bicarbonate Acceptor Complex of Photosystem II

Two plastoquinone electron acceptors, Q_A and Q_B, are present in Photosystem II (PS II) with their binding sites formed by the D2 and D1 proteins, respectively. A hexacoordinate non-heme iron is bound between Q_A and Q_B by D2 and D1, each providing two histidine ligands, and a bicarbonate that is stabilized via hydrogen bonds with D2-Tyr244 and D1-Tyr246. Both tyrosines and bicarbonate are conserved in oxygenic photosynthetic organisms but absent from the corresponding quinone-iron electron acceptor complex of anoxygenic photosynthetic bacteria. We investigated the role of D2-Tyr244 by introducing mutations in the cyanobacterium Synechocystis sp. PCC 6803. Alanine, histidine, and phenylalanine substitutions were introduced creating the Y244A, Y244H, and Y244F mutants. Electron transfer between Q_A and Q_B was impaired, the back-reaction with the S2 state of the oxygen-evolving complex was modified, and PS II assembly was disrupted, with the Y244A strain being more affected than the Y244F and Y244H mutants. The strains were also highly susceptible to photodamage in the presence of PS II-specific electron acceptors. Thermoluminescence and chlorophyll a fluorescence decay measurements indicated that the redox potential of the Q_A/Q_A^- couple became more positive in the Y244F and Y244H mutants, consistent with bicarbonate binding being impacted. The replacement of Tyr244 by alanine also led to an insertion of two amino acid repeats from Gln239 to Ala249 within the DE loop of D2, resulting in an inactive PS II complex that lacked PS II-specific variable fluorescence. The 66 bp insertion giving rise to the inserted amino acids therefore created an obligate photoheterotrophic mutant.

PsbX maintains efficient electron transport in Photosystem II and reduces susceptibility to high light in Synechocystis sp. PCC 6803

PsbX is a 4.1 kDa intrinsic Photosystem II (PS II) protein, found together with the low-molecular-weight proteins, PsbY and PsbJ, in proximity to cytochrome b559. The function of PsbX is not yet fully characterized but PsbX may play a role in the exchange of the secondary plastoquinone electron acceptor QB with the quinone pool in the thylakoid membrane. To study the role of PsbX, we have constructed a PsbX-lacking strain of Synechocystis sp. PCC 6803. Our studies indicate that the absence of PsbX causes sensitivity to high light and impairs electron transport within PS II. In addition to a change in the QB-binding pocket, PsbX-lacking cells exhibited sensitivity to sodium formate, suggesting altered binding of the bicarbonate ligand to the non-heme iron between the sequential plastoquinone electron acceptors QA and QB. Experiments using 35S-methionine revealed high-light-treated PsbX-lacking cells restore PS II activity during recovery under low light by an increase in the turnover of PS II-associated core proteins. These labeling experiments indicate the recovery after exposure to high light requires both selective removal and replacement of the D1 protein and de novo PS II assembly.

Slr0320 Is Crucial for Optimal Function of Photosystem II during High Light Acclimation in Synechocystis sp. PCC 6803

Upon exposure of photosynthetic organisms to high light (HL), several HL acclimation responses are triggered. Herein, we identified a novel gene, slr0320, critical for HL acclimation in Synechocystis sp. PCC 6803. The growth rate of the Δslr0320 mutant was similar to wild type (WT) under normal light (NL) but severely declined under HL. Net photosynthesis of the mutant was lower under HL, but maximum photosystem II (PSII) activity was higher under NL and HL. Immunodetection revealed the accumulation and assembly of PSII were similar between WT and the mutant. Chlorophyll fluorescence traces showed the stable fluorescence of the mutant under light was much higher. Kinetics of single flash-induced chlorophyll fluorescence increase and decay revealed the slower electron transfer from Q_A to Q_B in the mutant. These data indicate that, in the Δslr0320 mutant, the number of functional PSIIs was comparable to WT even under HL but the electron transfer between Q_A and Q_B was inefficient. Quantitative proteomics and real-time PCR revealed that expression profiles of psbL, psbH and psbI were significantly altered in the Δslr0320 mutant. Thus, Slr0320 protein plays critical roles in optimizing PSII activity during HL acclimation and is essential for PSII electron transfer from Q_A to Q_B.

The Interaction between PsbT and the DE Loop of D1 in Photosystem II Stabilizes the Quinone-Iron Electron Acceptor Complex

The X-ray-derived Photosystem II (PS II) structure from the thermophilic cyanobacterium Thermosynechococcus vulcanus (Protein Data Bank entry 4UB6) indicates Phe239 of the DE loop of the D1 protein forms a hydrophobic interaction with Pro27 and Ile29 at the C-terminus of the 5 kDa PsbT protein found at the monomer-monomer interface of the PS II dimer. To investigate the importance of this interaction, we created the F239A and F239L mutants in Synechocystis sp. PCC 6803 through targeted mutagenesis of the D1:Phe239 residue into either an alanine or a leucine. Under moderate-light conditions, the F239A strain displayed reduced rates of oxygen evolution and impaired rates of fluorescence decay following a single-turnover actinic flash, while the F239L strain behaved like the control; however, under high-light conditions, the F239A and F239L strains grew more slowly than the control. Our results indicate the quinone-iron acceptor complex becomes more accessible to exogenously added electron acceptors in the F239A mutant and a ΔPsbT strain when compared with the control and F239L strains. This led to the hypothesis that the interaction between D1:Phe239 and the PsbT subunit is required to restrict movement of the DE loop of the D1 subunit, and we suggest disruption of this interaction may perturb the binding of bicarbonate to the non-heme iron and contribute to the signal for PS II to undergo repair following photodamage.

Stabilization of Photosystem II by the PsbT protein impacts photodamage, repair and biogenesis

Photosystem II (PS II) catalyzes the light-driven process of water splitting in oxygenic photosynthesis. Four core membrane-spanning proteins, including D1 that binds the majority of the redox-active co-factors, are surrounded by 13 low-molecular-weight (LMW) proteins. We previously observed that deletion of the LMW PsbT protein in the cyanobacterium Synechocystis sp. PCC 6803 slowed electron transfer between the primary and secondary plastoquinone electron acceptors Q_A and Q_B and increased the susceptibility of PS II to photodamage. Here we show that photodamaged ∆PsbT cells exhibit unimpaired rates of oxygen evolution if electron transport is supported by HCO₃^- even though the cells exhibit negligible variable fluorescence. We find that the protein environment in the vicinity of Q_A and Q_B is altered upon removal of PsbT resulting in inhibition of Q_A^- oxidation in the presence of 2,5-dimethyl-1,4-benzoquinone, an artificial PS II-specific electron acceptor. Thermoluminescence measurements revealed an increase in charge recombination between the S₂ oxidation state of the water-oxidizing complex and Q_A^- by the indirect radiative pathway in ∆PsbT cells and this is accompanied by increased ¹O₂ production. At the protein level, both D1 removal and replacement, as well as PS II biogenesis, were accelerated in the ∆PsbT strain. Our results demonstrate that PsbT plays a key role in optimizing the electron acceptor complex of the acceptor side of PS II and support the view that repair and biogenesis of PS II share an assembly pathway that incorporates both de novo synthesis and recycling of the assembly modules associated with the core membrane-spanning proteins.

The importance of the hydrophilic region of PsbL for the plastoquinone electron acceptor complex of Photosystem II

The PsbL protein is a 4.5kDa subunit at the monomer-monomer interface of Photosystem II (PS II) consisting of a single membrane-spanning domain and a hydrophilic stretch of ~15 residues facing the cytosolic (or stromal) side of the photosystem. Deletion of conserved residues in the N-terminal region has been used to investigate the importance of this hydrophilic extension. Using Synechocystis sp. PCC 6803, three deletion strains: ∆(N6-N8), ∆(P11-V12) and ∆(E13-N15), have been created. The ∆(N6-N8) and ∆(P11-V12) strains remained photoautotrophic but were more susceptible to photodamage than the wild type; however, the ∆(E13-N15) cells had the most severe phenotype. The Δ(E13-N15) mutant showed decreased photoautotrophic growth, a reduced number of PS II centers, impaired oxygen evolution in the presence of PS II-specific electron acceptors, and was highly susceptible to photodamage. The decay kinetics of chlorophyll a variable fluorescence after a single turnover saturating flash and the sensitivity to low concentrations of PS II-directed herbicides in the Δ(E13-N15) strain indicate that the binding of plastoquinone to the QB-binding site had been altered such that the affinity of QB is reduced. In addition, the PS II-specific electron acceptor 2,5-dimethyl-p-benzoquinone was found to inhibit electron transfer through the quinone-acceptor complex of the ∆(E13-N15) strain. The PsbL Y20A mutant was also investigated and it exhibited increased susceptibility to photodamage and increased herbicide sensitivity. Our data suggest that the N-terminal hydrophilic region of PsbL influences forward electron transfer from QA through indirect interactions with the D-E loop of the D1 reaction center protein. Our results further indicate that disruption of interactions between the N-terminal region of PsbL and other PS II subunits or lipids destabilizes PS II dimer formation. 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.

A unique regulation of the expression of the psbA, psbD, and psbE genes, encoding the 01, 02 and cytochrome b559 subunits of the Photosystem II complex in the chlorophyll d containing cyanobacterium Acaryochloris marina

Photosynthetic electron transport, chromatic photoacclirnation and expression of the genes encoding the 01, 02, and cytochrome b559 subunits of the Photosystem II complex were studied in the chlorophyll d containing cyanobacterium Acaryochloris marina MBIC11017 under various environmental conditions. During oxygen deprivation and inhibition of photosynthetic electron transport by dibromothymoquinone the psbA1 gene encoding a 01' isoform was induced. All of the three psbA and one of the three psbD (psbD2) genes, encoding two different isoforms of the 01 and the abundant isoform of the 02 proteins, respectively were induced under exposure to UV-B radiation and high intensity visible light. Under far red light the amount of Photosystem II complexes increased, and expression of the psbE2 gene encoding the alpha-subunit of cytochrome b559 was enhanced. However, the psbF and psbE1 genes encoding the beta- and another isoform of alpha-cytochrome b559, respectively remained lowly expressed under all conditions. Far red light also induced the psbD3 gene encoding a 02' isoform whose primary structure is different from the abundant 02 isoform. psbD3 was also induced under low intensity visible light, when chromatic photoacclimation was indicated by a red-shifted absorption of chlorophyll d. Our results show that differential expression of multigene families encoding different isoforms of 01 and 02 plays an important role in the acclimation of A. marina to contrasting environmental conditions. Moreover, the disproportionate quantity of transcripts of the alpha and beta subunits of cytochrome b559 implies the existence of an alpha-alpha homodimer organization of cytochrome b559 in Photosystem II complexes.

High-throughput pyrosequencing of the chloroplast genome of a highly neutral-lipid-producing marine pennate diatom, Fistulifera sp. strain JPCC DA0580

The chloroplast genome of the highly neutral-lipid-producing marine pennate diatom Fistulifera sp. strain JPCC DA0580 was fully sequenced using high-throughput pyrosequencing. The general features and gene content were compared with three other complete diatom chloroplast genomes. The chloroplast genome is 134,918 bp with an inverted repeat of 13,330 bp and is slightly larger than the other diatom chloroplast genomes due to several low gene-density regions lacking similarity to the other diatom chloroplast genomes. Protein-coding genes were nearly identical to those from Phaeodactylum tricornutum. On the other hand, we found unique sequence variations in genes of photosystem II which differ from the consensus in other diatom chloroplasts. Furthermore, five functional unknown ORFs and a putative serine recombinase gene, serC2, are located in the low gene-density regions. SerC2 was also identified in the plasmids of another pennate diatom, Cylindrotheca fusiformis, and in the plastid genome of the diatom endosymbiont of Kryptoperidinium foliaceum. Exogenous plasmids might have been incorporated into the chloroplast genome of Fistulifera sp. by lateral gene transfer. Chloroplast genome sequencing analysis of this novel diatom provides many important insights into diatom evolution.

Photosystem II, a growing complex: updates on newly discovered components and low molecular mass proteins

Photosystem II is a unique complex capable of absorbing light and splitting water. The complex has been thoroughly studied and to date there are more than 40 proteins identified, which bind to the complex either stably or transiently. Another special feature of this complex is the unusually high content of low molecular mass proteins that represent more than half of the proteins. In this review we summarize the recent findings on the low molecular mass proteins (<15kDa) and present an overview of the newly identified components as well. We have also performed co-expression analysis of the genes encoding PSII proteins to see if the low molecular mass proteins form a specific sub-group within the Photosystem II complex. Interestingly we found that the chloroplast-localized genes encoding PSII proteins display a different response to environmental and stress conditions compared to the nuclear localized genes. This article is part of a Special Issue entitled: Photosystem II.