Investigating the photoprotective role of cytochrome b-559 in photosystem II in a mutant with altered ligation of the haem

The lumenal domain of Cyt b559 interacting with extrinsic subunits is crucial for accumulation of functional photosystem II

Cytochrome b559 (Cyt b559) is an essential component of the photosystem II (PSII) reaction center core. It consists of two subunits, PsbE and PsbF, which together coordinate a redox-active heme. While extensive studies have revealed the importance of Cyt b559, its structural and functional roles are not fully understood. Previous studies have implied that the lumenal region of Cyt b559, interacting with the PSII extrinsic subunit PsbP in green plant PSII, may have important roles. However, few studies have investigated its lumenal region. Here, we have focused on a well-conserved lumenal region of PsbE, which was found to interact with the N-terminal region of PsbP in green-lineage PSII (from green algae and land plants). In red-lineage PSII (from red algae and algae possessing red algal-derived plastids), very similar interactions were observed between the same lumenal region of PsbE and the N-terminal region of PsbQ'. We generated Arabidopsis thaliana mutants harboring mutations in the well-conserved lumenal region of PsbE through targeted base editing of the plastid genome by ptpTALECD. The mutations led to strong growth defects and extremely low Fv/Fm. This study suggests the importance of the lumenal regions of Cyt b559, and gives insight into possible structural and functional compensation between the N-terminal regions of PsbP in green-lineage PSII and PsbQ' in red-lineage PSII.

Antimycin A induces light hypersensitivity of PSII in the presence of quinone QB-site binding herbicides

Photosynthetic electron transport consists of linear electron flow and 2 cyclic electron flow (CEF) pathways around PSI (CEF-PSI). PROTON GRADIENT REGULATION 5 (PGR5)-dependent CEF-PSI is thought to be the major CEF-PSI pathway and an important regulator of photosynthetic electron transfer. Antimycin A (AA) is commonly recognized as an inhibitor of PGR5-dependent CEF-PSI in photosynthesis. Although previous findings imply that AA may also affect PSII, which does not participate in CEF-PSI, these "secondary effects" tend to be neglected, and AA is often used for inhibition of PGR5-dependent CEF-PSI as if it were a specific inhibitor. Here, we investigated the direct effects of AA on PSII using isolated spinach (Spinacia oleracea) PSII membranes and thylakoid membranes isolated from spinach, Arabidopsis thaliana (wild-type Columbia-0 and PGR5-deficient mutant pgr5hope1), and Chlamydomonas reinhardtii. Measurements of quinone QA- reoxidation kinetics showed that AA directly affects the acceptor side of PSII and inhibits electron transport within PSII. Furthermore, repetitive Fv/Fm measurements revealed that, in the presence of quinone QB-site binding inhibitors, AA treatment results in severe photodamage even from a single-turnover flash. The direct effects of AA on PSII are nonnegligible, and caution is required when using AA as an inhibitor of PGR5-dependent CEF-PSI. Meanwhile, we found that the commercially available compound AA3, which is a component of the AA complex, inhibits PGR5-dependent CEF-PSI without having notable effects on PSII. Thus, we propose that AA3 should be used instead of AA for physiological studies of the PGR5-dependent CEF-PSI.

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.

RESISTANCE TO PHYTOPHTHORA1 promotes cytochrome b559 formation during early photosystem II biogenesis in Arabidopsis

As an essential intrinsic component of photosystem II (PSII) in all oxygenic photosynthetic organisms, heme-bridged heterodimer cytochrome b559 (Cyt b559) plays critical roles in the protection and assembly of PSII. However, the underlying mechanisms of Cyt b559 assembly are largely unclear. Here, we characterized the Arabidopsis (Arabidopsis thaliana) rph1 (resistance to Phytophthora1) mutant, which was previously shown to be susceptible to the oomycete pathogen Phytophthora brassicae. Loss of RPH1 leads to a drastic reduction in PSII accumulation, which can be primarily attributed to the defective formation of Cyt b559. Spectroscopic analyses showed that the heme level in PSII supercomplexes isolated from rph1 is significantly reduced, suggesting that RPH1 facilitates proper heme assembly in Cyt b559. Due to the loss of RPH1-mediated processes, a covalently bound PsbE-PsbF heterodimer is formed during the biogenesis of PSII. In addition, rph1 is highly photosensitive and accumulates elevated levels of reactive oxygen species under photoinhibitory-light conditions. RPH1 is a conserved intrinsic thylakoid protein present in green algae and terrestrial plants, but absent in Synechocystis, and it directly interacts with the subunits of Cyt b559. Thus, our data demonstrate that RPH1 represents a chloroplast acquisition specifically promoting the efficient assembly of Cyt b559, probably by mediating proper heme insertion into the apo-Cyt b559 during the initial phase of PSII biogenesis.

The biogenesis and maintenance of PSII: Recent advances and current challenges

The growth of plants, algae, and cyanobacteria relies on the catalytic activity of the oxygen-evolving PSII complex, which uses solar energy to extract electrons from water to feed into the photosynthetic electron transport chain. PSII is proving to be an excellent system to study how large multi-subunit membrane-protein complexes are assembled in the thylakoid membrane and subsequently repaired in response to photooxidative damage. Here we summarize recent developments in understanding the biogenesis of PSII, with an emphasis on recent insights obtained from biochemical and structural analysis of cyanobacterial PSII assembly/repair intermediates. We also discuss how chlorophyll synthesis is synchronized with protein synthesis and suggest a possible role for PSI in PSII assembly. Special attention is paid to unresolved and controversial issues that could be addressed in future research.

Membrane protein provision controls prothylakoid biogenesis in tobacco etioplasts

The cytochrome b559 heterodimer is a conserved component of photosystem II whose physiological role in photosynthetic electron transfer is enigmatic. A particularly puzzling aspect of cytochrome b559 has been its presence in etiolated seedlings, where photosystem II is absent. Whether or not the cytochrome has a specific function in etioplasts is unknown. Here, we have attempted to address the function of cytochrome b559 by generating transplastomic tobacco (Nicotiana tabacum) plants that overexpress psbE and psbF, the plastid genes encoding the two cytochrome b559 apoproteins. We show that strong overaccumulation of the PsbE apoprotein can be achieved in etioplasts by suitable manipulations of the promoter and the translation signals, while the cytochrome b559 level is only moderately elevated. The surplus PsbE protein causes striking ultrastructural alterations in etioplasts; most notably, it causes a condensed prolamellar body and a massive proliferation of prothylakoids, with multiple membrane layers coiled into spiral-like structures. Analysis of plastid lipids revealed that increased PsbE biosynthesis strongly stimulated plastid lipid biosynthesis, suggesting that membrane protein abundance controls prothylakoid membrane biogenesis. Our data provide evidence for a structural role of PsbE in prolamellar body formation and prothylakoid biogenesis, and indicate that thylakoid membrane protein abundance regulates lipid biosynthesis in etioplasts.

A possible relationship between the effect of factors on photoactivation of photosystem II depleted of functional Mn and cytochrome b559

The photoassembly of the Mn4CaO5 cluster in Mn-depleted photosystem II preparations (photoactivation) was studied under the influence of oxidants, reductants and pH. New data on the effect of these factors on the photoactivation yield are presented. The presence of the oxidant, ferricyanide, negatively affected the photoactivation yield over the entire concentration range studied (0-1 mM). In contrast to ferricyanide, the addition of the reductant, ferrocyanide, up to 1 mM resulted in an increase in the photoactivation yield. Other reductants either did not significantly affect (diphenylcarbazide) or suppressed (ascorbate) the photoactivation yield. The effect of ferrocyanide on photoactivation were found to be similar dichlorophenolindophenol. Investigation of the photoactivation yield as a function of pH revealed that the maximum yield was observed at pH 6.5 in the presence of ferrocyanide and DCPIP, and at pH 5.5 without additives. In addition, the photoactivation yield at pH 5.5 was the same without and with the addition of ferrocyanide or dichlorophenolindophenol. Although ferricyanide suppressed the photoactivation, the photoactivation yield increased in the presence of ferricyanide by shifting the pH to the acidic region. The samples contained approximately 25 % of the HP cyt b559, which was in the reduced state, as the absorbance at 559 nm was decreased upon addition of ferricyanide and subsequent addition of ferrocyanide returned the spectrum to the baseline. A possible relationship between the effect of factors on the photoactivation and the involvement of cyt b559 in the protection of PSII from oxidative damage on the donor side is discussed.

Microcystin aids in cold temperature acclimation: Differences between a toxic Microcystis wildtype and non-toxic mutant

For Microcystis aeruginosa PCC 7806, temperature decreases from 26 °C to 19 °C double the microcystin quota per cell during growth in continuous culture. Here we tested whether this increase in microcystin provided M. aeruginosa PCC 7806 with a fitness advantage during colder-temperature growth by comparing cell concentration, cellular physiology, reactive oxygen species damage, and the transcriptomics-inferred metabolism to a non-toxigenic mutant strain M. aeruginosa PCC 7806 ΔmcyB. Photo-physiological data combined with transcriptomic data revealed metabolic changes in the mutant strain during growth at 19 °C, which included increased electron sinks and non-photochemical quenching. Increased gene expression was observed for a glutathione-dependent peroxiredoxin during cold treatment, suggesting compensatory mechanisms to defend against reactive oxygen species are employed in the absence of microcystin in the mutant. Our observations highlight the potential selective advantages of a longer-term defensive strategy in management of oxidative stress (i.e., making microcystin) vs the shorter-term proactive strategy of producing cellular components to actively dissipate or degrade oxidative stress agents.

Cool temperature acclimation in toxigenic Microcystis aeruginosa PCC 7806 and its non-toxigenic mutant

For Microcystis aeruginosa PCC 7806, temperature decreases from 26° C to 19° C double the microcystin quota per cell during growth in continuous culture. Here we tested whether this increase in microcystin provided M. aeruginosa PCC 7806 with a fitness advantage during colder-temperature growth by comparing cell concentration, cellular physiology, and the transcriptomics-inferred metabolism to a non-toxigenic mutant strain M. aeruginosa PCC 7806 ΔmcyB. Photo-physiological data combined with transcriptomic data revealed metabolic changes in the mutant strain during growth at 19° C, which included increased electron sinks and non-photochemical quenching. Increased gene expression was observed for a glutathione-dependent peroxiredoxin during cold treatment, suggesting compensatory mechanisms to defend against reactive oxygen species are employed in the absence of microcystin in the mutant. Our observations highlight the potential selective advantages of a longer-term defensive strategy in management of oxidative stress (i.e., making microcystin) vs the shorter-term proactive strategy of producing cellular components to actively dissipate or degrade oxidative stress agents.

Action of 2,6-Dichloro-1,4-benzoquinone on the O2-Evolving Activity of Photosystem II in Chlamydomonas reinhardtii Cells with and without Cell Wall: Inhibitory Effect of Its Oxidized Form

Chlamydomonas reinhardtii is a widely used object in studies on green algae concerning both photosynthesis aspects and possible biotechnological approaches. The measurement of the maximum O2 evolution by photosystem II (PSII) in living algal cells in the presence of artificial acceptors is one of the commonly used methods for determining the photosynthetic apparatus state or its change as compared to a control, parent strain, etc., because PSII is the most sensitive component of the thylakoid membrane. The present study shows the need to use low concentrations of 2,6-dichloro-1,4-benzoquinone (DCBQ) paired with potassium ferricyanide (FeCy) for achieving the maximum O2 evolution rate, while a DCBQ concentration above certain threshold results in strong suppression of O2 evolution. The required DCBQ concentration depends on the presence of the cell wall and should be exactly ~0.1 mM or in the range of 0.2–0.4 mM for cells with and without a cell wall, respectively. The inhibition effect is caused, probably, by a higher content of DCBQ in the oxidized form inside cells; this depends on the presence of the cell wall, which influences the efficiency of DCBQ diffusion into and out of the cell, where it is maintained by FeCy in the oxidized state. The possible mechanism of DCBQ inhibition action is discussed.

Rajput V, Kumar Singh R, Guo Y, Kosolapov A, Usova E, Varduny T, Chalenko E, Yadronova O, Dmitriev P, Zaruba T. Chlorophyll fluorometry in evaluating photosynthetic performance: key limitations, possibilities, perspectives and alternatives

Non-destructive methods for the assessment of photosynthetic parameters of plants are widely applied to evaluate rapidly the photosynthetic performance, plant health, and shifts in plant productivity induced by environmental and cultivation conditions. Most of these methods are based on measurements of chlorophyll fluorescence kinetics, particularly on pulse modulation (PAM) fluorometry. In this paper, fluorescence methods are critically discussed in regard to some their possibilities and limitations inherent to vascular plants and microalgae. Attention is paid to the potential errors related to the underestimation of thylakoidal cyclic electron transport and anoxygenic photosynthesis. PAM-methods are also observed considering the color-addressed measurements. Photoacoustic methods are discussed as an alternative and supplement to fluorometry. Novel Fourier modifications of PAM-fluorometry and photoacoustics are noted as tools allowing simultaneous application of a dual or multi frequency measuring light for one sample.

New Structural and Mechanistic Insights Into Functional Roles of Cytochrome b 559 in Photosystem II

Cytochrome (Cyt) b ₅₅₉ is a key component of the photosystem II (PSII) complex for its assembly and proper function. Previous studies have suggested that Cytb ₅₅₉ has functional roles in early assembly of PSII and in secondary electron transfer pathways that protect PSII against photoinhibition. In addition, the Cytb ₅₅₉ in various PSII preparations exhibited multiple different redox potential forms. However, the precise functional roles of Cytb ₅₅₉ in PSII remain unclear. Recent site-directed mutagenesis studies combined with functional genomics and biochemical analysis, as well as high-resolution x-ray crystallography and cryo-electron microscopy studies on native, inactive, and assembly intermediates of PSII have provided important new structural and mechanistic insights into the functional roles of Cytb ₅₅₉. This mini-review gives an overview of new exciting results and their significance for understanding the structural and functional roles of Cytb ₅₅₉ in PSII.

Loss of carbonic anhydrase in the thylakoid lumen causes unusual moderate-light-induced rearrangement of the chloroplast in Chlamydomonas reinhardtii as a way of photosystem II photoprotection

Chlamydomonas reinhardtii cells have a single large cup-shaped chloroplast that can lose lobes under high light to prevent photodamage of the photosynthetic apparatus, including photosystem II (PSII). Here, under moderate light treatment, the development of the unusual morphology of the chloroplast is shown for mutant cia3, which is deficient in carbonic anhydrase (EC 4.2.1.1) CAH3 in the thylakoid lumen, while such light intensity is harmless for wild type (WT) cells for hours. Cia3 cells had more activated PSII photoprotective mechanisms and showed a tendency to shift in the balance of the PSII damage-repair cycle, whereas PSII retained the same photosynthetic efficiency as in the WT. These findings allow speculation about the unique PSII photoprotection strategy by rearranging the chloroplast in the absence of CAH3. CAH3, in turn, is suggested to be an important participant of the C. reinhardtii photosynthetic apparatus operation, functioning in close connection with PSII.

A thylakoid membrane-bound and redox-active rubredoxin (RBD1) functions in de novo assembly and repair of photosystem II

Photosystem II (PSII) catalyzes the light-driven oxidation of water in photosynthesis, supplying energy and oxygen to many life-forms on earth. During PSII assembly and repair, PSII intermediate complexes are prone to photooxidative damage, requiring mechanisms to minimize this damage. Here, we report the functional characterization of RBD1, a PSII assembly factor that interacts with PSII intermediate complexes to ensure their functional assembly and repair. We propose that the redox activity of RBD1 participates together with the cytochrome b₅₅₉ to protect PSII from photooxidation. This work not only improves our understanding of cellular protection mechanisms for the vital PSII complex but also informs genetic engineering strategies for protection of PSII repair to increase agricultural productivity.

Photosystem II (PSII) undergoes frequent photooxidative damage that, if not repaired, impairs photosynthetic activity and growth. How photosynthetic organisms protect vulnerable PSII intermediate complexes during de novo assembly and repair remains poorly understood. Here, we report the genetic and biochemical characterization of chloroplast-located rubredoxin 1 (RBD1), a PSII assembly factor containing a redox-active rubredoxin domain and a single C-terminal transmembrane α-helix (TMH) domain. RBD1 is an integral thylakoid membrane protein that is enriched in stroma lamellae fractions with the rubredoxin domain exposed on the stromal side. RBD1 also interacts with PSII intermediate complexes containing cytochrome b₅₅₉. Complementation of the Chlamydomonas reinhardtii (hereafter Chlamydomonas) RBD1-deficient 2pac mutant with constructs encoding RBD1 protein truncations and site-directed mutations demonstrated that the TMH domain is essential for de novo PSII assembly, whereas the rubredoxin domain is involved in PSII repair. The rubredoxin domain exhibits a redox midpoint potential of +114 mV and is proficient in 1-electron transfers to a surrogate cytochrome c in vitro. Reduction of oxidized RBD1 is NADPH dependent and can be mediated by ferredoxin-NADP⁺ reductase (FNR) in vitro. We propose that RBD1 participates, together with the cytochrome b₅₅₉, in the protection of PSII intermediate complexes from photooxidative damage during de novo assembly and repair. This role of RBD1 is consistent with its evolutionary conservation among photosynthetic organisms and the fact that it is essential in photosynthetic eukaryotes.

Early Archean origin of Photosystem II

Photosystem II is a photochemical reaction center that catalyzes the light-driven oxidation of water to molecular oxygen. Water oxidation is the distinctive photochemical reaction that permitted the evolution of oxygenic photosynthesis and the eventual rise of eukaryotes. At what point during the history of life an ancestral photosystem evolved the capacity to oxidize water still remains unknown. Here, we study the evolution of the core reaction center proteins of Photosystem II using sequence and structural comparisons in combination with Bayesian relaxed molecular clocks. Our results indicate that a homodimeric photosystem with sufficient oxidizing power to split water had already appeared in the early Archean about a billion years before the most recent common ancestor of all described Cyanobacteria capable of oxygenic photosynthesis, and well before the diversification of some of the known groups of anoxygenic photosynthetic bacteria. Based on a structural and functional rationale, we hypothesize that this early Archean photosystem was capable of water oxidation to oxygen and had already evolved protection mechanisms against the formation of reactive oxygen species. This would place primordial forms of oxygenic photosynthesis at a very early stage in the evolutionary history of life.

Consequences of structural modifications in cytochrome b559 on the electron acceptor side of Photosystem II

Cytb₅₅₉ in Photosystem II is a heterodimeric b-type cytochrome. The subunits, PsbE and PsbF, consist each in a membrane α-helix. Mutants were previously designed and studied in Thermosynechococcus elongatus (Sugiura et al., Biochim Biophys Acta 1847:276-285, 2015) either in which an axial histidine ligand of the haem-iron was substituted for a methionine, the PsbE/H23M mutant in which the haem was lacking, or in which the haem environment was modified, the PsbE/Y19F and PsbE/T26P mutants. All these mutants remained active showing that the haem has no structural role provided that PsbE and PsbF subunits are present. Here, we have carried on the characterization of these mutants. The following results were obtained: (i) the Y19F mutation hardly affect the E_m of Cytb₅₅₉, whereas the T26P mutation converts the haem into a form with a E_m much below 0 mV (so low that it is likely not reducible by Q_B^-) even in an active enzyme; (ii) in the PsbE/H23M mutant, and to a less extent in PsbE/T26P mutant, the electron transfer efficiency from Q_A^- to Q_B is decreased; (iii) the lower E_m of the Q_A/Q_A^- couple in the PsbE/H23M mutant correlates with a higher production of singlet oxygen; (iv) the superoxide and/or hydroperoxide formation was not increased in the PsbE/H23M mutant lacking the haem, whereas it was significantly larger in the PsbE/T26P. These data are discussed in view of the literature to discriminate between structural and redox roles for the haem of Cytb₅₅₉ in the production of reactive oxygen species.

A tribute to Ulrich Heber (1930-2016) for his contribution to photosynthesis research: understanding the interplay between photosynthetic primary reactions, metabolism and the environment

The dynamic and efficient coordination of primary photosynthetic reactions with leaf energization and metabolism under a wide range of environmental conditions is a fundamental property of plants involving processes at all functional levels. The present historical perspective covers 60 years of research aiming to understand the underlying mechanisms, linking major breakthroughs to current progress. It centers on the contributions of Ulrich Heber who had pioneered novel concepts, fundamental methods, and mechanistic understanding of photosynthesis. An important first step was the development of non-aqueous preparation of chloroplasts allowing the investigation of chloroplast metabolites ex vivo (meaning that the obtained results reflect the in vivo situation). Later on, intact chloroplasts, retaining their functional envelope membranes, were isolated in aqueous media to investigate compartmentation and exchange of metabolites between chloroplasts and external medium. These studies elucidated metabolic interaction between chloroplasts and cytoplasm during photosynthesis. Experiments with isolated intact chloroplasts clarified that oxygenation of ribulose-1.5-bisphosphate generates glycolate in photorespiration. The development of non-invasive optical methods enabled researchers identifying mechanisms that balance electron flow in the photosynthetic electron transport system avoiding its over-reduction. Recording chlorophyll a (Chl a) fluorescence allowed one to monitor, among other parameters, thermal energy dissipation by means of 'nonphotochemical quenching' of the excited state of Chl a. Furthermore, studies both in vivo and in vitro led to basic understanding of the biochemical mechanisms of freezing damage and frost tolerance of plant leaves, to SO₂ tolerance of tree leaves and dehydrating lichens and mosses.

The N-terminal sequence of the extrinsic PsbP protein modulates the redox potential of Cyt b559 in photosystem II

The PsbP protein, an extrinsic subunit of photosystem II (PSII) in green plants, is known to induce a conformational change around the catalytic Mn₄CaO₅ cluster securing the binding of Ca²⁺ and Cl^– in PSII. PsbP has multiple interactions with the membrane subunits of PSII, but how these affect the structure and function of PSII requires clarification. Here, we focus on the interactions between the N-terminal residues of PsbP and the α subunit of Cytochrome (Cyt) b₅₅₉ (PsbE). A key observation was that a peptide fragment formed of the first N-terminal 15 residues of PsbP, ‘pN15’, was able to convert Cyt b₅₅₉ into its HP form. Interestingly, addition of pN15 to NaCl-washed PSII membranes decreased PSII’s oxygen-evolving activity, even in the presence of saturating Ca²⁺ and Cl^– ions. In fact, pN15 reversibly inhibited the S₁ to S₂ transition of the OEC in PSII. These data suggest that pN15 can modulate the redox property of Cyt b₅₅₉ involved in the side-electron pathway in PSII. This potential change of Cyt b₅₅₉, in the absence of the C-terminal domain of PsbP, however, would interfere with any electron donation from the Mn₄CaO₅ cluster, leading to the possibility that multiple interactions of PsbP, binding to PSII, have distinct roles in regulating electron transfer within PSII.

The Roles of Cytochrome b 559 in Assembly and Photoprotection of Photosystem II Revealed by Site-Directed Mutagenesis Studies

Cytochrome b 559 (Cyt b 559) is one of the essential components of the Photosystem II reaction center (PSII). Despite recent accomplishments in understanding the structure and function of PSII, the exact physiological function of Cyt b 559 remains unclear. Cyt b 559 is not involved in the primary electron transfer pathway in PSII but may participate in secondary electron transfer pathways that protect PSII against photoinhibition. Site-directed mutagenesis studies combined with spectroscopic and functional analysis have been used to characterize Cyt b 559 mutant strains and their mutant PSII complex in higher plants, green algae, and cyanobacteria. These integrated studies have provided important in vivo evidence for possible physiological roles of Cyt b 559 in the assembly and stability of PSII, protecting PSII against photoinhibition, and modulating photosynthetic light harvesting. This mini-review presents an overview of recent important progress in site-directed mutagenesis studies of Cyt b 559 and implications for revealing the physiological functions of Cyt b 559 in PSII.

Assembly of oxygen-evolving Photosystem II efficiently occurs with the apo-Cytb559 but the holo-Cytb559 accelerates the recovery of a functional enzyme upon photoinhibition

Cytb559 in Photosystem II is a heterodimeric b-type cytochrome. The subunits, PsbE and PsbF, consist each in a membrane α-helix. Roles for Cytb559 remain elusive. In Thermosynechococcus elongatus, taking advantage of the robustness of the PSII variant with PsbA3 as the D1 subunit (WT*3), 4 mutants were designed hoping to get mutants nevertheless the obligatory phototrophy of this cyanobacterium. In two of them, an axial histidine ligand of the haem-iron was substituted for either a methionine, PsbE/H23M, which could be potentially a ligand or for an alanine, PsbE/H23A, which cannot. In the other mutants, PsbE/Y19F and PsbE/T26P, the environment around PsbE/H23 was expected to be modified. From EPR, MALDI-TOF and O2 evolution activity measurements, the following results were obtained: Whereas the PsbE/H23M and PsbE/H23A mutants assemble only an apo-Cytb559 the steady-state level of active PSII was comparable to that in WT*3. The lack of the haem or, in PsbE/T26P, conversion of the high-potential into a lower potential form, slowed-down the recovery rate of the O2 activity after high-light illumination but did not affect the photoinhibition rate. This resulted in the following order for the steady-state level of active PSII centers under high-light conditions: PsbE/H23M≈PsbE/H23A<< PsbE/Y19F≤PsbE/T26P≤WT*3. These data show i) that the haem has no structural role provided that PsbE and PsbF are present, ii) a lack of correlation between the rate of photoinhibition and the Em of the haem and iii) that the holo-Cytb559 favors the recovery of a functional enzyme upon photoinhibition.