The function of D1-H332 in Photosystem II electron transport studied by thermoluminescence and chlorophyll fluorescence in site-directed mutants of Synechocystis 6803

Investigation of the effect of salt stress on photosynthetic electron transport pathways in the Synechocystis PCC 6803 cyanobacterium

Cyanobacteria are important model organisms for studying the process of photosynthesis and the effects of environmental stress factors. This study aimed to identify the inhibitory sites of NaCl in the whole photosynthetic electron transport in Synechocystis sp. PCC 6803 WT cells by using multiple biophysical tools. Exposure of cells to various NaCl concentrations (200 mM to 1 M) revealed the inhibition of Photosystem II (PSII) activity at the water oxidizing complex and between the QA and QB electron acceptors. In contrast to the inhibition of PSII, electron flow through Photosystem I (PSI) was accelerated, indicating enhanced cyclic electron flow. The oxygen-evolving capacity of the cells was inhibited to a larger extent when only CO2 was the final electron acceptor in the Calvin-Benson-Bassham (CBB) cycle than in the presence of the PSII electron acceptor DMBQ, suggesting important NaCl inhibitory site(s) downstream of PSI. Measurements of NADPH kinetics revealed NaCl-induced inhibition of light-induced production of NADPH as well as retardation of NADPH consumption both in the light and in the initial dark period after switching off the light. Chlorophyll fluorescence kinetics, measured in parallel with NADPH fluorescence, showed the enhancement of post-illumination fluorescence rise up to 500 mM NaCl, which was however inhibited at higher NaCl concentrations. Our results show, for the first time, that NaCl inhibits the activity of the CBB cycle at least at two different sites, and confirm earlier results about the NaCl-induced inhibition of the PSII donor and acceptor side and the enhancement of electron flow through PSI.

Redesign of the Chlamydomonas reinhardtii QB binding niche reveals photosynthesis works in the absence of a driving force for QA-QB electron transfer

An in silico redesign of the secondary quinone electron acceptor (QB) binding pocket of the D1 protein of Photosystem II (PSII) suggested that mutations of the F265 residue would affect atrazine binding. Chlamydomonas reinhardtii mutants F265T and F265S were produced to obtain atrazine-hypersensitive strains for biosensor applications, and the mutants were indeed found to be more atrazine-sensitive than the reference strain IL. Fluorescence and thermoluminescence data agree with a weak driving force and confirm slow electron transfer but cannot exclude an additional effect on protonation of the secondary quinone. Both mutants grow autotrophically, indicating that PSII requires strong light for optimal function, as was the case in the ancestral homodimeric reaction center.

Conformational changes in a Photosystem II hydrogen bond network stabilize the oxygen-evolving complex

The Mn4CaO5 oxygen-evolving complex (OEC) in Photosystem II (PSII) is assembled in situ and catalyzes water oxidation. After OEC assembly, the PsbO extrinsic subunit docks to the lumenal face of PSII and both stabilizes the OEC and facilitates efficient proton transfer to the lumen. D1 residue R334 is part of a hydrogen bond network involved in proton release during catalysis and interacts directly with PsbO. D1-R334 has recently been observed in different conformations in apo- and holo-OEC PSII structures. We generated a D1-R334G point mutant in Synechocystis sp. PCC 6803 to better understand this residue's function. D1-R334G PSII is active under continuous light, but the OEC is unstable in darkness. Isolated D1-R334G core complexes have little bound PsbO and less manganese as the wild type control. The S2 intermediate is stabilized in D1-R334G indicating that the local environment around the OEC has been altered. These results suggest that the hydrogen bond network that includes D1-R334 exists in a different functional conformation during PSII biogenesis in the absence of PsbO.

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.

Nitric oxide represses photosystem II and NDH-1 in the cyanobacterium Synechocystis sp. PCC 6803

Photosynthetic electron transfer comprises a series of light-induced redox reactions catalysed by multiprotein machinery in the thylakoid. These protein complexes possess cofactors susceptible to redox modifications by reactive small molecules. The gaseous radical nitric oxide (NO), a key signalling molecule in green algae and plants, has earlier been shown to bind to Photosystem (PS) II and obstruct electron transfer in plants. The effects of NO on cyanobacterial bioenergetics however, have long remained obscure. In this study, we exposed the model cyanobacterium Synechocystis sp. PCC 6803 to NO under anoxic conditions and followed changes in whole-cell fluorescence and oxidoreduction of P700 in vivo. Our results demonstrate that NO blocks photosynthetic electron transfer in cells by repressing PSII, PSI, and likely the NDH dehydrogenase-like complex 1 (NDH-1). We propose that iron‑sulfur clusters of NDH-1 complex may be affected by NO to such an extent that ferredoxin-derived electron injection to the plastoquinone pool, and thus cyclic electron transfer, may be inhibited. These findings reveal the profound effects of NO on Synechocystis cells and demonstrate the importance of controlled NO homeostasis in cyanobacteria.

Analysis of the changes of electron transfer and heterogeneity of photosystem II in Deg1-reduced Arabidopsis plants

Deg1 protease functions in protease and chaperone of PSII complex components, but few works were performed to study the effects of Deg1 on electron transport activities on the donor and acceptor side of PSII and its correlation with the photoprotection of PSII during photoinhibition. Therefore, we performed systematic and comprehensive investigations of electron transfers on the donor and acceptor sides of photosystem II (PSII) in the Deg1-reduced transgenic lines deg1-2 and deg1-4. Both the maximal quantum efficiency of PSII photochemistry (F_v/F_m) and the actual PSII efficiency (Φ_PSII) decreased significantly in the transgenic plants. Increases in nonphotochemical quenching (NPQ) and the dissipated energy flux per reaction center (DI₀/RC) were also shown in the transgenic plants. Along with the decreased D1, CP47, and CP43 content, these results suggested photoinhibition under growth light conditions in transgenic plants. Decreased Deg1 caused inhibition of electron transfer on the PSII reducing side, leading to a decline in the number of Q_B-reducing centers and accumulation of Q_B-nonreducing centers. The Tm of the Q band shifted from 5.7 °C in the wild-type plant to 10.4 °C and 14.2 °C in the deg1-2 and deg1-4 plants, respectively, indicating an increase in the stability of S₂Q_A^¯ in transgenic plants. PSIIα in the transgenic plants largely reduced, while PSIIβ and PSIIγ increased with the decline in the Deg1 levels in transgenic plants suggesting PSIIα centers gradually converted into PSIIβ and PSIIγ centers in the transgenic plants. Besides, the connectivity of PSIIα and PSIIβ was downregulated in transgenic plants. Our results reveal that downregulation of Deg1 protein levels induced photoinhibition in transgenic plants, leading to loss of PSII activities on both the donor and acceptor sides in transgenic plants. These results give a new insight into the regulation role of Deg1 in PSII electron transport.

Role of PsbV-Tyr137 in photosystem II studied by site-directed mutagenesis in the thermophilic cyanobacterium Thermosynechococcus vulcanus

PsbV (cytochrome c₅₅₀) is one of the three extrinsic proteins of photosystem II (PSII) and functions to maintain the stability and activity of the Mn₄CaO₅ cluster, the catalytic center for water oxidation. PsbV-Y137 is the C-terminal residue of PsbV and is located at the exit of a hydrogen-bond network mediated by the D1-Y161-H190 residue pair. In order to examine the function of PsbV-Y137, four mutants, PsbV-Y137A, PsbV-Y137F, PsbV-Y137G, and PsbV-Y137W, were generated with Thermosynechococcus vulcanus (T. vulcanus). These mutants showed growth rates similar to that of the wild-type strain (WT); however, their oxygen-evolving activities were different. At pH 6.5, the oxygen evolution rates of Y137F and Y137W were almost identical to that of WT, whereas the oxygen evolution rates of the Y137A, Y137G mutants were 64% and 61% of WT, respectively. However, the oxygen evolution in the latter two mutants decreased less at higher pHs, suggesting that higher pHs facilitated oxygen evolution probably by facilitating proton egress in these two mutants. Furthermore, thylakoid membranes isolated from the PsbV-Y137A, PsbV-Y137G mutants exhibited much lower levels of oxygen-evolving activity than that of WT, which was found to be caused by the release of PsbV. In addition, PSII complexes purified from the PsbV-Y137A and PsbV-Y137G mutants lost all of the three extrinsic proteins but instead bind Psb27, an assembly cofactor of PSII. These results demonstrate that the PsbV-Tyr137 residue is required for the stable binding of PsbV to PSII, and the hydrogen-bond network mediated by D1-Y161-H190 is likely to function in proton egress during water oxidation.

Simulation of chlorophyll fluorescence rise and decay kinetics, and P700-related absorbance changes by using a rule-based kinetic Monte-Carlo method

A model of primary photosynthetic reactions in the thylakoid membrane was developed and its validity was tested by simulating three types of experimental kinetic curves: (1) the light-induced chlorophyll a fluorescence rise (OJIP transients) reflecting the stepwise transition of the photosynthetic electron transport chain from the oxidized to the fully reduced state; (2) the dark relaxation of the flash-induced fluorescence yield attributed to the Q_A^- oxidation kinetics in PSII; and (3) the light-induced absorbance changes near 820 or 705 nm assigned to the redox transitions of P₇₀₀ in PSI. A model was implemented by using a rule-based kinetic Monte-Carlo method and verified by simulating experimental curves under different treatments including photosynthetic inhibitors, heat stress, anaerobic conditions, and very high light intensity.

Parallel expression of alternate forms of psbA2 gene provides evidence for the existence of a targeted D1 repair mechanism in Synechocystis sp. PCC 6803

The D1 protein of Photosystem II (PSII) is recognized as the main target of photoinhibitory damage and exhibits a high turnover rate due to its degradation and replacement during the PSII repair cycle. Damaged D1 is replaced by newly synthesized D1 and, although reasonable, there is no direct evidence for selective replacement of damaged D1. Instead, it remains possible that increased turnover of D1 subunits occurs in a non-selective manner due for example, to a general up-regulation of proteolytic activity triggered during damaging environmental conditions, such as high light. To determine if D1 degradation is targeted to damaged D1 or generalized to all D1, we developed a genetic system involving simultaneous dual expression of wild type and mutant versions of D1 protein. Dual D1 strains (nS345P:eWT and nD170A:eWT) expressed a wild type (WT) D1 from ectopic and a damage prone mutant (D1-S345P, D1-D170A) from native locus on the chromosome. Characterization of strains showed that all dual D1 strains restore WT like phenotype with high PSII activity. Higher PSII activity indicates increased population of PSII reaction centers with WT D1. Analysis of steady state levels of D1 in nS345P:eWT by immunoblot showed an accumulation of WT D1 only. But, in vivo pulse labeling confirmed the synthesis of both S345P (exists as iD1) and WT D1 in the dual strain. Expression of nS345P:eWT in FtsH2 knockout background showed accumulation of both iD1 and D1 proteins. This demonstrates that dual D1 strains express both forms of D1, yet only damage prone PSII complexes are selected for repair providing evidence that the D1 degradation process is targeted towards damaged PSII complexes. Since the N-terminus has been previously shown to be important for the degradation of damaged D1, the possibility that the highly conserved cysteine 18 residue situated in the N-terminal domain of D1 is involved in the targeted repair process was tested by examining site directed mutants of this and the other cysteines of the D1 protein. This article is part of a special issue entitled: photosynthesis research for sustainability: keys to produce clean energy.

Characterization of wave phenomena in the relaxation of flash-induced chlorophyll fluorescence yield in cyanobacteria

Fluorescence yield relaxation following a light pulse was studied in various cyanobacteria under aerobic and microaerobic conditions. In Synechocystis PCC 6803 fluorescence yield decays in a monotonous fashion under aerobic conditions. However, under microaerobic conditions the decay exhibits a wave feature showing a dip at 30-50 ms after the flash followed by a transient rise, reaching maximum at ~1s, before decaying back to the initial level. The wave phenomenon can also be observed under aerobic conditions in cells preilluminated with continuous light. Illumination preconditions cells for the wave phenomenon transiently: for few seconds in Synechocystis PCC 6803, but up to one hour in Thermosynechocystis elongatus BP-1. The wave is eliminated by inhibition of plastoquinone binding either to the QB site of Photosystem-II or the Qo site of cytochrome b6f complex by 3-(3',4'-dichlorophenyl)-1,1-dimethylurea or 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone, respectively. The wave is also absent in mutants, which lack either Photosystem-I or the NAD(P)H-quinone oxidoreductase (NDH-1) complex. Monitoring the redox state of the plastoquinone pool revealed that the dip of the fluorescence wave corresponds to transient oxidation, whereas the following rise to re-reduction of the plastoquinone pool. It is concluded that the unusual wave feature of fluorescence yield relaxation reflects transient oxidation of highly reduced plastoquinone pool by Photosystem-I followed by its re-reduction from stromal components via the NDH-1 complex, which is transmitted back to the fluorescence yield modulator primary quinone electron acceptor via charge equilibria. Potential applications of the wave phenomenon in studying photosynthetic and respiratory electron transport are discussed. This article is part of a special issue entitled: photosynthesis research for sustainability: keys to produce clean energy.

Spectroscopic and functional characterization of cyanobacterium Synechocystis PCC 6803 mutants on the cytoplasmic-side of cytochrome b559 in photosystem II

We performed spectroscopic and functional characterization on cyanobacterium Synechocystis PCC6803 with mutations of charged residues of the cytoplasmic side of cytochrome (Cyt) b559 in photosystem II (PSII). All of the mutant cells grew photoautotrophically and assembled stable PSII. However, R7Eα, R17Eα and R17Lβ mutant cells grew significantly slower and were more susceptible to photoinhibition than wild-type cells. The adverse effects of the arginine mutations on the activity and the stability of PSII were in the following order (R17Lβ>R7Eα>R17Eα and R17Aα). All these arginine mutants exhibited normal period-four oscillation in oxygen yield. Thermoluminescence characteristics indicated a slight decrease in the stability of the S3QB(-)/S2QB(-) charge pairs in the R7Eα and R17Lβ mutant cells. R7Eα and R17Lβ PSII core complexes contained predominantly the low potential form of Cyt b559. EPR results indicated the displacement of one of the two axial ligands to the heme of Cyt b559 in R7Eα and R17Lβ mutant reaction centers. Our results demonstrate that the electrostatic interactions between these arginine residues and the heme propionates of Cyt b559 are important to the structure and redox properties of Cyt b559. In addition, the blue light-induced nonphotochemical quenching was significantly attenuated and its recovery was accelerated in the R7Lα and R17Lβ mutant cells. Furthermore, ultra performance liquid chromatography-mass spectrometry results showed that the PQ pool was more reduced in the R7Eα and R17Lβ mutant cells than wild-type cells in the dark. Our data support a functional role of Cyt b559 in protection of PSII under photoinhibition conditions in vivo.

Ligation of D1-His332 and D1-Asp170 to the manganese cluster of photosystem II from Synechocystis assessed by multifrequency pulse EPR spectroscopy

Multifrequency electron spin-echo envelope modulation (ESEEM) spectroscopy is used to ascertain the nature of the bonding interactions of various active site amino acids with the Mn ions that compose the oxygen-evolving cluster (OEC) in photosystem II (PSII) from the cyanobacterium Synechocystis sp. PCC 6803 poised in the S(2) state. Spectra of natural isotopic abundance PSII ((14)N-PSII), uniformly (15)N-labeled PSII ((15)N-PSII), and (15)N-PSII containing (14)N-histidine ((14)N-His/(15)N-PSII) are compared. These complementary data sets allow for a precise determination of the spin Hamiltonian parameters of the postulated histidine nitrogen interaction with the Mn ions of the OEC. These results are compared to those from a similar study on PSII isolated from spinach. Upon mutation of His332 of the D1 polypeptide to a glutamate residue, all isotopically sensitive spectral features vanish. Additional K(a)- and Q-band ESEEM experiments on the D1-D170H site-directed mutant give no indication of new (14)N-based interactions.

A synthetic DNA and fusion PCR approach to the ectopic expression of high levels of the D1 protein of photosystem II in Synechocystis sp. PCC 6803

A hybrid approach involving synthetic DNA, fusion PCR, and ectopic expression has been used to genetically manipulate the expression of the D1 protein of photosystem II (PSII) in the model cyanobacterium Synechocystis sp. PCC6803. Due to the toxicity of the full-length psbA gene in E. coli, a chimeric psbA2 gene locus was commercially synthesised and cloned in two halves. High-fidelity fusion PCR utilizing sequence overlap between the two synthetic gene halves allowed the production of a DNA fragment that was able to recombine the full-length psbA2 gene into the Synechocystis chromosome at an ectopic (non-native) location. This was accomplished by designing the synthetic DNA/fusion PCR product to have the psbA2 gene, with control sequences, interposed between chimeric sequences corresponding to an ectopic target chromosomal location. Additionally, a recipient strain of Synechocystis lacking all three psbA genes was produced by a combination of traditional marker replacement and markerless replacement techniques. Transformation of this multiple deletion strain by the synthetic DNA/fusion PCR product faithfully restored D1 expression in terms of its expression and PSII repair capacity. The advantages and potential issues for using this approach to rapidly introduce chimeric sequence characteristics as a general tool to produce novel genetic constructs are discussed.

Altered structure of the Mn4Ca cluster in the oxygen-evolving complex of photosystem II by a histidine ligand mutation

The effect of replacing a histidine ligand on the properties of the oxygen-evolving complex (OEC) and the structure of the Mn(4)Ca cluster in Photosystem II (PSII) is studied by x-ray absorption spectroscopy using PSII core complexes from the Synechocystis sp. PCC 6803 D1 polypeptide mutant H332E. In the x-ray crystallographic structures of PSII, D1-His(332) has been assigned as a direct ligand of a manganese ion, and the mutation of this histidine ligand to glutamate has been reported to prevent the advancement of the OEC beyond the S(2)Yz(•) intermediate state. The manganese K-edge (1s core electron to 4p) absorption spectrum of D1-H332E shifts to a lower energy compared with that of the native WT samples, suggesting that the electronic structure of the manganese cluster is affected by the presence of the additional negative charge on the OEC of the mutant. The extended x-ray absorption spectrum shows that the geometric structure of the cluster is altered substantially from that of the native WT state, resulting in an elongation of manganese-ligand and manganese-manganese interactions in the mutant. The strontium-H332E mutant, in which calcium is substituted by strontium, confirms that strontium (calcium) is a part of the altered cluster. The structural perturbations caused by the D1-H332E mutation are much larger than those produced by any biochemical treatment or mutation examined previously with x-ray absorption spectroscopy. The substantial structural changes provide an explanation not only for the altered properties of the D1-H332E mutant but also the importance of the histidine ligand for proper assembly of the Mn(4)Ca cluster.

Acclimation of photosynthesis to nitrogen deficiency in Phaseolus vulgaris

Nitrogen deficiency diminishes consumption of photosynthates in anabolic metabolism. We studied adjustments of the photosynthetic machinery in nitrogen-deficient bean plants and found four phenomena. First, the number of chloroplasts per cell decreased. Chloroplasts of nitrogen starved leaves contained less pigments than those of control leaves, but the in vitro activities of light reactions did not change when measured on chlorophyll basis. Second, nitrogen deficiency induced cyclic electron transfer. The amounts of Rubisco and ferredoxin-NADP(+) reductase decreased in nitrogen starved plants. Low activities of these enzymes are expected to lead to increase in reduction of oxygen by photosystem I. However, diaminobenzidine staining did not reveal hydrogen peroxide production in nitrogen starved plants. Measurements of far-red-light-induced redox changes of the primary donor of photosystem I suggested that instead of producing oxygen radicals, nitrogen starved plants develop a high activity of cyclic electron transport that competes with oxygen for electrons. Nitrogen starvation led to decrease in photochemical quenching and increase in non-photochemical quenching, indicating that cyclic electron transport reduces the plastoquinone pool and acidifies the lumen. A third effect is redistribution of excitation energy between the photosystems in favor of photosystem I. Thus, thylakoids of nitrogen starved plants appeared to be locked in state 2, which further protects photosystem II by decreasing its absorption cross-section. As a fourth response, the proportion of non-Q(B)-reducing photosystem II reaction centers increased and the redox potential of the Q(B)/Q(B)(-) pair decreased by 25 mV in a fraction of photosystem II centers of nitrogen starved plants.

Phosphatidylglycerol depletion affects photosystem II activity in Synechococcus sp. PCC 7942 cells

The role of phosphatidylglycerol (PG) in photosynthetic membranes of cyanobacteria was analyzed in a Synechococcus sp. PCC 7942 mutant produced by inactivating its cdsA gene presumably encoding cytidine 5'-diphosphate-diacylglycerol synthase, a key enzyme in PG synthesis. In a medium supplemented with PG the Synechococcus sp. PCC 7942/DeltacdsA cells grew photoautotrophically. Depletion of PG in the medium resulted (a) in an arrest of cell growth and division, (b) in a suppression of O(2) evolving activity, and (c) in a modification of Chl fluorescence induction curves. Two-dimensional PAGE showed that in the absence of PG (a) the amount of the PSI monomers increased at the expense of the PSI trimers and (b) PSII dimers were decomposed into monomers. [(35)S]methionine labeling confirmed that PG depletion did not block the de novo synthesis of PSII proteins but slowed down the assembly of the newly synthesized D1 protein into PSII core complexes. Retailoring of PG was observed during PG depletion: the exogenously added artificial dioleoyl PG was transformed into photosynthetically more essential PG derivatives. Concomitantly with a decrease in PG content, SQDG content increased, but it could not restore photosynthetic activity.

Evidence that D1-His332 in photosystem II from Thermosynechococcus elongatus interacts with the S3-state and not with the S2-state

Oxygen evolution by Photosystem II (PSII) is catalyzed by a Mn(4)Ca cluster. Thus far, from the crystallographic three-dimensional (3D) structures, seven amino acid residues have been identified as possible ligands of the Mn(4)Ca cluster. Among them, there is only one histidine, His332, which belongs to the D1 polypeptide. The relationships of the D1-His332 amino acid with kinetics and thermodynamic properties of the Mn(4)Ca cluster in the S(2)- and S(3)-states of the catalytic cycle were investigated in purified PSII from Thermosynechococcus elongatus. This was done by examining site-directed D1-His332Gln and D1-His332Ser mutants by a variety of spectroscopic techniques such as time-resolved UV-visible absorption change spectroscopy, cw- and pulse-EPR, thermoluminescence, and measurement of substrate water exchange. Both mutants grew photo-autotrophically and active PSII could be purified. On the basis of the parameters assessed in this work, the D1-His332(Gln, Ser) mutations had no effect in the S(2)-state. Electron spin-echo envelope modulation (ESEEM) spectroscopy also showed that possible interactions between the nuclear spin of the nitrogen(s) of D1-His332 with the electronic spin S = 1/2 of the Mn(4)Ca cluster in the S(2)-state were not detectable and that the D1-His332Ser mutation did not affect the detected hyperfine couplings. In contrast, the following changes were observed in the S(3)-state of the D1-His332 mutants: (1) The redox potential of the S(3)/S(2) couple was slightly increased by < or = 20 meV, (2) The S(3)-EPR spectrum was slightly modified, (3) The D1-His332Gln mutation resulted in a approximately 3 fold decrease of the slow (tightly bound) exchange rate and a approximately 2 fold increase of the fast exchange rate of the water substrate molecules. All these results suggest that the D1-His332 would be more involved in S(3) than in S(2). This could be one element of the conformational changes put forward in the S(2) to S(3) transition.

Comparison of the electron transport properties of the psbo1 and psbo2 mutants of Arabidopsis thaliana

Genome sequence of Arabidopsis thaliana (Arabidopsis) revealed two psbO genes (At5g66570 and At3g50820) which encode two distinct PsbO isoforms: PsbO1 and PsbO2, respectively. To get insights into the function of the PsbO1 and PsbO2 isoforms in Arabidopsis we have performed systematic and comprehensive investigations of the whole photosynthetic electron transfer chain in the T-DNA insertion mutant lines, psbo1 and psbo2. The absence of the PsbO1 isoform and presence of only the PsbO2 isoform in the psbo1 mutant results in (i) malfunction of both the donor and acceptor sides of Photosystem (PS) II and (ii) high sensitivity of PSII centers to photodamage, thus implying the importance of the PsbO1 isoform for proper structure and function of PSII. The presence of only the PsbO2 isoform in the PSII centers has consequences not only to the function of PSII but also to the PSI/PSII ratio in thylakoids. These results in modification of the whole electron transfer chain with higher rate of cyclic electron transfer around PSI, faster induction of NPQ and a larger size of the PQ-pool compared to WT, being in line with apparently increased chlororespiration in the psbo1 mutant plants. The presence of only the PsbO1 isoform in the psbo2 mutant did not induce any significant differences in the performance of PSII under standard growth conditions as compared to WT. Nevertheless, under high light illumination, it seems that the presence of also the PsbO2 isoform becomes favourable for efficient repair of the PSII complex.

Flavodiiron proteins in oxygenic photosynthetic organisms: photoprotection of photosystem II by Flv2 and Flv4 in Synechocystis sp. PCC 6803

Background

Flavodiiron proteins (FDPs) comprise a group of modular enzymes that function in oxygen and nitric oxide detoxification in Bacteria and Archaea. The FDPs in cyanobacteria have an extra domain as compared to major prokaryotic enzymes. The physiological role of cyanobacteria FDPs is mostly unknown. Of the four putative flavodiiron proteins (Flv1–4) in the cyanobacterium Synechocystis sp. PCC 6803, a physiological function in Mehler reaction has been suggested for Flv1 and Flv3.

Principal Findings

We demonstrate a novel and crucial function for Flv2 and Flv4 in photoprotection of photosystem II (PSII) in Synechocystis. It is shown that the expression of Flv2 and Flv4 is high under air level of CO₂ and negligible at elevated CO₂. Moreover, the rate of accumulation of flv2 and flv4 transcripts upon shift of cells from high to low CO₂ is strongly dependent on light intensity. Characterization of FDP inactivation mutants of Synechocystis revealed a specific decline in PSII centers and impaired translation of the D1 protein in Δflv2 and Δflv4 when grown at air level CO₂ whereas at high CO₂ the Flvs were dispensable. Δflv2 and Δflv4 were also more susceptible to high light induced inhibition of PSII than WT or Δflv1 and Δflv3.

Significance

Analysis of published sequences revealed the presence of cyanobacteria-like FDPs also in some oxygenic photosynthetic eukaryotes like green algae, mosses and lycophytes. Our data provide evidence that Flv2 and Flv4 have an important role in photoprotection of water-splitting PSII against oxidative stress when the cells are acclimated to air level CO₂. It is conceivable that the function of FDPs has changed during evolution from protection against oxygen in anaerobic microbes to protection against reactive oxygen species thus making the sustainable function of oxygen evolving PSII possible. Higher plants lack FDPs and distinctly different mechanisms have evolved for photoprotection of PSII.

Functional complementation of the Arabidopsis thaliana psbo1 mutant phenotype with an N-terminally His6-tagged PsbO-1 protein in photosystem II

The Arabidopsis thaliana mutant psbo1 has recently been described and characterized. Loss of expression of the PsbO-1 protein leads to a variety of functional perturbations including elevated levels of the PsbO-2 protein and defects on both the oxidizing- and reducing-sides of Photosystem II. In this communication, two plant lines were produced using the psbo1 mutant as transgenic host, which contained an N-terminally histidine(6)-tagged PsbO-1 protein. This protein was expressed and correctly targeted into the thylakoid lumen. Immunological analysis indicated that different levels of expression of the modified PsbO-1 protein were obtained in different transgenic plant lines and that the level of expression in each line was stable over several generations. Examination of the Photosystem II closure kinetics demonstrated that the defective double reduction of Q(B) and the delayed exchange of Q(B)H(2) with the plastoquinone pool which were observed during the characterization of the psbo1 mutant were effectively restored to wild-type levels by the His(6)-tagged PsbO-1 protein. Flash fluorescence induction and decay were also examined. Our results indicated that high expression of the modified PsbO-1 was required to increase the ratio of PS II(alpha)/PS II(beta) reaction centers to wild-type levels. Fluorescence decay kinetics in the absence of DCMU indicated that the expression of the His(6)-tagged PsbO-1 protein restored efficient electron transfer to Q(B), while in the presence of DCMU, charge recombination between Q(A)(-) and the S(2) state of the oxygen-evolving complex occurred at near wild-type rates. Our results indicate that high expression of the His(6)-tagged PsbO-1 protein efficiently complements nearly all of the photochemical defects observed in the psbo1 mutant. Additionally, this study establishes a platform on which the in vivo consequences of site-directed mutagenesis of the PsbO-1 protein can be examined.

The effects of simultaneous RNAi suppression of PsbO and PsbP protein expression in photosystem II of Arabidopsis

Interfering RNA was used to suppress simultaneously the expression of the four genes which encode the PsbO and PsbP proteins of Photosystem II in Arabidopsis (PsbO: At5g66570, At3g50820 and PsbP: At1g06680, At2g30790). A phenotypic series of transgenic plants was obtained that expressed variable amounts of the PsbO proteins and undetectable amounts of the PsbP proteins. Immunological studies indicated that the loss of PsbP expression was correlated with the loss of expression of the PsbQ, D2, and CP47 proteins, while the loss of PsbO expression was correlated with the loss of expression of the D1 and CP43 proteins. Q(A)(-) reoxidation kinetics in the absence of DCMU indicated that the slowing of electron transfer from Q(A)(-) to Q(B) was correlated with the loss of the PsbP protein. Q(A)(-) reoxidation kinetics in the presence of DCMU indicated that charge recombination between Q(A)(-) and donor side components of the photosystem was retarded in all of the mutants. Decreasing amounts of the PsbO protein in the absence of the PsbP component also led to a progressive loss of variable fluorescence yield (F(V)/F(M)). During fluorescence induction, the loss of PsbP was correlated with a more rapid O to J transition and a loss of the J to I transition. These results indicate that the losses of the PsbO and PsbP proteins differentially affect separate protein components and different PS II functions and can do so, apparently, in the same plant.

Energetics of Photosystem II charge recombination in Acaryochloris marina studied by thermoluminescence and flash-induced chlorophyll fluorescence measurements

We studied the charge recombination characteristics of Photosystem II (PSII) redox components in whole cells of the chlorophyll (Chl) d-dominated cyanobacterium, Acaryochloris marina, by flash-induced chlorophyll fluorescence and thermoluminescence measurements. Flash-induced chlorophyll fluorescence decay was retarded in the mus and ms time ranges and accelerated in the s time range in Acaryochloris marina relative to that in the Chl a-containing cyanobacterium, Synechocystis PCC 6803. In the presence of 3-(3,4-dichlorophenyl)-1, 1-dimethylurea, which blocks the Q(B) site, the relaxation of fluorescence decay arising from S(2)Q(A)(-) recombination was somewhat faster in Acaryochloris marina than in Synechocystis PCC 6803. Thermoluminescence intensity of the so called B band, arising from the recombination of the S(2)Q(B)(-) charge separated state, was enhanced significantly (2.5 fold) on the basis of equal amounts of PSII in Acaryochloris marina as compared with Synechocystis 6803. Our data show that the energetics of charge recombination is modified in Acaryochloris marina leading to a approximately 15 meV decrease of the free energy gap between the Q(A) and Q(B) acceptors. In addition, the total free energy gap between the ground state and the excited state of the reaction center chlorophyll is at least approximately 25-30 meV smaller in Acaryochloris marina, suggesting that the primary donor species cannot consist entirely of Chl a in Acaryochloris marina, and there is a contribution from Chl d as well.

The D1 and D2 proteins of dinoflagellates: unusually accumulated mutations which influence on PSII photoreaction

Plastid encoded genes of the dinoflagellates are rapidly evolving and most divergent. The importance of unusually accumulated mutations on structure of PSII core protein and photosynthetic function was examined in the dinoflagellates, Symbiodinium sp. and Alexandrium tamarense. Full-length cDNA sequences of psbA (D1 protein) and psbD (D2 protein) were obtained and compared with the other oxygen-evolving photoautotrophs. Twenty-three amino acid positions (7%) for the D1 protein and 34 positions (10%) for the D2 were mutated in the dinoflagellates, although amino acid residues at these positions were conserved in cyanobacteria, the other algae, and plant. Many mutations were likely to distribute in the N-terminus and the D-E interhelical loop of the D1 protein and helix B of D2 protein, while the remaining regions were well conserved. The different structural properties in these mutated regions were supported by hydropathy profiles. The chlorophyll fluorescence kinetics of the dinoflagellates was compared with Synechocystis sp. PCC6803 in relation to the altered protein structure.

AtCYP38 ensures early biogenesis, correct assembly and sustenance of photosystem II

AtCYP38 is a thylakoid lumen protein comprising the immunophilin domain and the phosphatase inhibitor module. Here we show the association of AtCYP38 with the photosystem II (PSII) monomer complex and address its functional role using AtCYP38-deficient mutants. The dynamic greening process of etiolated leaves failed in the absence of AtCYP38, due to specific problems in the biogenesis of PSII complexes. Also the development of leaves under short-day conditions was severely disturbed. Detailed biophysical and biochemical analysis of mature AtCYP38-deficient plants from favorable growth conditions (long photoperiod) revealed: (i) intrinsic malfunction of PSII, which (ii) occurred on the donor side of PSII and (iii) was dependent on growing light intensity. AtCYP38 mutant plants also showed decreased accumulation of PSII, which was shown not to originate from impaired D1 synthesis or assembly of PSII monomers, dimers and supercomplexes as such but rather from the incorrect fine-tuning of the oxygen-evolving side of PSII. This, in turn, rendered PSII centers extremely susceptible to photoinhibition. AtCYP38 deficiency also drastically decreased the in vivo phosphorylation of PSII core proteins, probably related to the absence of the AtCYP38 phosphatase inhibitor domain. It is proposed that during PSII assembly AtCYP38 protein guides the proper folding of D1 (and CP43) into PSII, thereby enabling the correct assembly of the water-splitting Mn(4)-Ca cluster even with high turnover of PSII.

Computational insights into the O2-evolving complex of photosystem II

Mechanistic investigations of the water-splitting reaction of the oxygen-evolving complex (OEC) of photosystem II (PSII) are fundamentally informed by structural studies. Many physical techniques have provided important insights into the OEC structure and function, including X-ray diffraction (XRD) and extended X-ray absorption fine structure (EXAFS) spectroscopy as well as mass spectrometry (MS), electron paramagnetic resonance (EPR) spectroscopy, and Fourier transform infrared spectroscopy applied in conjunction with mutagenesis studies. However, experimental studies have yet to yield consensus as to the exact configuration of the catalytic metal cluster and its ligation scheme. Computational modeling studies, including density functional (DFT) theory combined with quantum mechanics/molecular mechanics (QM/MM) hybrid methods for explicitly including the influence of the surrounding protein, have proposed chemically satisfactory models of the fully ligated OEC within PSII that are maximally consistent with experimental results. The inorganic core of these models is similar to the crystallographic model upon which they were based, but comprises important modifications due to structural refinement, hydration, and proteinaceous ligation which improve agreement with a wide range of experimental data. The computational models are useful for rationalizing spectroscopic and crystallographic results and for building a complete structure-based mechanism of water-splitting in PSII as described by the intermediate oxidation states of the OEC. This review summarizes these recent advances in QM/MM modeling of PSII within the context of recent experimental studies.

A mutant small heat shock protein with increased thylakoid association provides an elevated resistance against UV-B damage in synechocystis 6803

Besides acting as molecular chaperones, the amphitropic small heat shock proteins (sHsps) are suggested to play an additional role in membrane quality control. We investigated sHsp membrane function in the model cyanobacterium Synechocystis sp. PPC 6803 using mutants of the single sHsp from this organism, Hsp17. We examined mutants in the N-terminal arm, L9P and Q16R, for altered interaction with thylakoid and lipid membranes and examined the effects of these mutations on thylakoid functions. These mutants are unusual in that they retain their oligomeric state and chaperone activity in vitro but fail to confer thermotolerance in vivo. We found that both mutant proteins had dramatically altered membrane/lipid interaction properties. Whereas L9P showed strongly reduced binding to thylakoid and model membranes, Q16R was almost exclusively membrane-associated, properties that may be the cause of reduced heat tolerance of cells carrying these mutations. Among the lipid classes tested, Q16R displayed the highest interaction with negatively charged SQDG. In Q16R cells a specific alteration of the thylakoid-embedded Photosystem II (PSII) complex was observed. Namely, the binding of plastoquinone and quinone analogue acceptors to the Q(B) site was modified. In addition, the presence of Q16R dramatically reduced UV-B damage of PSII activity because of enhanced PSII repair. We suggest these effects occur at least partly because of increased interaction of Q16R with SQDG in the PSII complex. Our findings further support the model that membrane association is a functional property of sHsps and suggest sHsps as a possible biotechnological tool to enhance UV protection of photosynthetic organisms.

Multifrequency pulsed electron paramagnetic resonance study of the S2 state of the photosystem II manganese cluster

Multifrequency electron spin-echo envelope modulation (ESEEM) spectroscopy is employed to measure the strength of the hyperfine coupling of magnetic nuclei to the paramagnetic (S = 1/2) S2 form of photosystem II (PSII). Previous X-band-frequency ESEEM studies indicated that one or more histidine nitrogens are electronically coupled to the tetranuclear manganese cluster in the S2 state of PSII. However, the spectral resolution was relatively poor at the approximately 9 GHz excitation frequency, precluding any in-depth analysis of the corresponding bonding interaction between the detected histidine and the manganese cluster. Here we report ESEEM experiments using higher X-, P-, and Ka-band microwave frequencies to target PSII membranes isolated from spinach. The X- to P-band ESEEM spectra suffer from the same poor resolution as that observed in previous experiments, while the Ka-band spectra show remarkably well-resolved features that allow for the direct determination of the nuclear quadrupolar couplings for a single I = 1(14)N nucleus. The Ka-band results demonstrate that at an applied field of 1.1 T we are much closer to the exact cancellation limit (alpha iso = 2nu(14)N) that optimizes ESEEM spectra. These results reveal hyperfine (alpha iso = 7.3 +/- 0.20 MHz and alpha dip = 0.50 +/- 0.10 MHz) and nuclear quadrupolar (e(2)qQ = 1.98 +/- 0.05 MHz and eta = 0.84 +/- 0.06) couplings for a single (14)N nucleus magnetically coupled to the manganese cluster in the S 2 state of PSII. These values are compared to the histidine imidazole nitrogen hyperfine and nuclear quadrupolar couplings found in superoxidized manganese catalase as well as (14)N couplings in relevant manganese model complexes.

Multifrequency electron spin-echo envelope modulation studies of nitrogen ligation to the manganese cluster of photosystem II

The CalEPR Center at UC-Davis (http://brittepr.ucdavis.edu) is equipped with five research grade electron paramagnetic resonance (EPR) instruments operating at various excitation frequencies between 8 and 130GHz. Of particular note for this RSC meeting are two pulsed EPR spectrometers working at the intermediate microwave frequencies of 31 and 35GHz. Previous lower frequency electron spin-echo envelope modulation (ESEEM) studies indicated that histidine nitrogen is electronically coupled to the Mn cluster in the S2 state of photosystem II (PSII). However, the amplitude and resolution of the spectra were relatively poor at these low frequencies, precluding any in-depth analysis of the electronic structure properties of this closely associated nitrogen nucleus. With the intermediate frequency instruments, we are much closer to the 'exact cancellation' limit, which optimizes ESEEM spectra for hyperfine-coupled nuclei such as 14N and 15N. Herein, we report the results from ESEEM studies of both 14N- and 15N-labelled PSII at these two frequencies. Spectral simulations were constrained by both isotope datasets at both frequencies, with a focus on high-resolution spectral examination of the histidine ligation to the Mn cluster in the S2 state.

Protein Ligation of the Photosynthetic Oxygen-Evolving Center

Photosynthetic water oxidation is catalyzed by a unique Mn(4)Ca cluster in Photosystem II. The ligation environment of the Mn(4)Ca cluster optimizes the cluster's reactivity at each step in the catalytic cycle and minimizes the release of toxic, partly oxidized intermediates. However, our understanding of the cluster's ligation environment remains incomplete. Although the recent X-ray crystallographic structural models have provided great insight and are consistent with most conclusions of earlier site-directed mutagenesis studies, the ligation environments of the Mn(4)Ca cluster in the two available structural models differ in important respects. Furthermore, while these structural models and the earlier mutagenesis studies agree on the identity of most of the Mn(4)Ca cluster's amino acid ligands, they disagree on the identity of others. This review describes mutant characterizations that have been undertaken to probe the ligation environment of the Mn(4)Ca cluster, some of which have been inspired by the recent X-ray crystallographic structural models. Many of these characterizations have involved Fourier Transform Infrared (FTIR) difference spectroscopy because of the extreme sensitivity of this form of spectroscopy to the dynamic structural changes that occur during an enzyme's catalytic cycle.

Mutation of arginine 357 of the CP43 protein of photosystem II severely impairs the catalytic S-state cycle of the H2O oxidation complex

Basic amino acid side chains situated in active sites may mediate critical proton transfers during an enzymatic catalytic cycle. In the case of photosynthetic water oxidation, a strong base is postulated to facilitate the deprotonation of the active site Mn4-Ca cluster, thereby allowing the otherwise thermodynamically constrained transfer of an electron away from the Mn4-Ca cluster to the oxidized redox active tyrosine radical, YZ*, generated by photosynthetic charge separation. Arginine 357 of the CP43 polypeptide may be located in the second coordination shell of the O2-evolving Mn4-Ca cluster of photosystem II (PSII) according to current structural models. An ostensibly conservative substitution mutation, CP43-357K, was investigated using polarographic and fluorescence techniques in evaluating its potential impact on S-state cycling. Cells containing the CP43-357K mutation lost their capacity for autotrophic growth and exhibited a drastic reduction in O2 evolving activity ( approximately 15% of that of the wild type) despite the fact that mutant cells contained more than 80% of the concentration of charge-separating PSII reaction centers and more than half of these contained photooxidizable Mn. Fluorescence kinetics indicated that acceptor side electron transfer, dominated by the transfer of electrons from QA- to QB, was unaffected, but the fraction of centers containing Mn clusters capable of forming the S2 state was reduced to approximately 40% of that of the wild type. Analysis of O2 yields using a bare platinum electrode indicated a severe defect in the S-state cycling properties of the mutant H2O oxidation complexes. Although O2 evolution was delayed to the third flash during a train of single-turnover saturating flashes, the pattern of O2 emission did not exhibit a discernible periodicity indicating a very high miss factor, which was estimated to be approximately 45% compared to the wild-type value of approximately 10%. On the other hand, the multiflash fluorescence measurements indicate that the yield of formation of the S2 state from S1 is diminished by approximately 20%, although this latter estimate is complicated by the presence of damaged PSII centers. Taken together, the experiments indicate that the high miss factor observed during S-state cycling is likely due to a defect in the higher S-state transitions. These results are discussed in relation to the idea that CP43-R357 may serve as a ligand to bicarbonate or as the catalytic base proposed to mediate proton-coupled electron transfer (PCET) in the higher S states of the catalytic cycle of H2O oxidation.

TLP18.3, a novel thylakoid lumen protein regulating photosystem II repair cycle

A proteome analysis of Arabidopsis thaliana thylakoid-associated polysome nascent chain complexes was performed to find novel proteins involved in the biogenesis, maintenance and turnover of thylakoid protein complexes, in particular the PSII (photosystem II) complex, which exhibits a high turnover rate. Four unknown proteins were identified, of which TLP18.3 (thylakoid lumen protein of 18.3 kDa) was selected for further analysis. The Arabidopsis mutants (SALK_109618 and GABI-Kat 459D12) lacking the TLP18.3 protein showed higher susceptibility of PSII to photoinhibition. The increased susceptibility of DeltaTLP18.3 plants to high light probably originates from an inefficient reassembly of PSII monomers into dimers in the grana stacks, as well as from an impaired turnover of the D1 protein in stroma exposed thylakoids. Such dual function of the TLP18.3 protein is in accordance with its even distribution between the grana and stroma thylakoids. Notably, the lack of the TLP18.3 protein does not lead to a severe collapse of the PSII complexes, suggesting a redundancy of proteins assisting these particular repair steps to assure functional PSII. The DeltaTLP18.3 plants showed no clear visual phenotype under standard growth conditions, but when challenged by fluctuating light during growth, the retarded growth of DeltaTLP18.3 plants was evident.

The exposed N-terminal tail of the D1 subunit is required for rapid D1 degradation during photosystem II repair in Synechocystis sp PCC 6803

The selective replacement of photodamaged D1 protein within the multisubunit photosystem II (PSII) complex is an important photoprotective mechanism in chloroplasts and cyanobacteria. FtsH proteases are involved at an early stage of D1 degradation, but it remains unclear how the damaged D1 subunit is recognized, degraded, and replaced. To test the role of the N-terminal region of D1 in PSII biogenesis and repair, we have constructed mutants of the cyanobacterium Synechocystis sp PCC 6803 that are truncated at the exposed N terminus. Removal of 5 or 10 residues blocked D1 synthesis, as assessed in radiolabeling experiments, whereas removal of 20 residues restored the ability to assemble oxygen-evolving dimeric PSII complexes but inhibited PSII repair at the level of D1 degradation. Overall, our results identify an important physiological role for the exposed N-terminal tail of D1 at an early step in selective D1 degradation. This finding has important implications for the recognition of damaged D1 and its synchronized replacement by a newly synthesized subunit.

The PsbP protein is required for photosystem II complex assembly/stability and photoautotrophy in Arabidopsis thaliana

Interfering RNA was used to suppress the expression of the genes At1g06680 and At2g30790 in Arabidopsis thaliana, which encode the PsbP-1 and PsbP-2 proteins, respectively, of photosystem II (PS II). A phenotypic series of transgenic plants was recovered that expressed intermediate and low amounts of PsbP. Chlorophyll fluorescence induction and Q(A)(-) decay kinetics analyses were performed. Decreasing amounts of expressed PsbP protein led to the progressive loss of variable fluorescence and a marked decrease in the fluorescence quantum yield (F(V)/F(M)). This was primarily due to the loss of the J to I transition. Analysis of the fast fluorescence rise kinetics indicated no significant change in the number of PS II(beta) centers present in the mutants. Analysis of Q(A)(-) decay kinetics in the absence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea indicated a defect in electron transfer from Q(A)(-) to Q(B), whereas experiments performed in the presence of this herbicide indicated that charge recombination between Q(A)(-) and the oxygen-evolving complex was seriously retarded in the plants that expressed low amounts of the PsbP protein. These results demonstrate that the amount of functional PS II reaction centers is compromised in the plants that exhibited intermediate and low amounts of the PsbP protein. Plants that lacked detectable PsbP were unable to survive in the absence of sucrose, indicating that the PsbP protein is required for photoautotrophy. Immunological analysis of the PS II protein complement indicated that significant losses of the CP47 and D2 proteins, and intermediate losses of the CP43 and D1 proteins, occurred in the absence of the PsbP protein. This demonstrates that the extrinsic protein PsbP is required for PS II core assembly/stability.

Functional analysis of photosystem II in a PsbO-1-deficient mutant in Arabidopsis thaliana

The Arabidopsis thaliana mutant psbo1 (formerly the mutant LE18-30), which contains a point mutation in the psbO-1 gene leading to defective expression of the PsbO-1 protein, has recently been described [Murakami, R. et al. (2002) FEBS Lett. 523, 138-142]. This mutant completely lacks the PsbO-1 protein and overexpresses the PsbO-2 protein. To further study the effect of PsbO-1 deficiency on the function of photosystem II, the polyphasic chlorophyll a fluorescence rise and flash fluorescence induction and decay of the relative fluorescence quantum yield were measured in whole leaves from wild type and the psbo1 mutant. Additionally, flash oxygen yield experiments were performed on thylakoid membranes isolated from wild type and the psbo1 mutant. The results obtained indicate that during fluorescence induction the psbo1 gene exhibited an enhanced O to P transition. Additionally, while the J to I transition in wild type accounted for more than 30% of the total fluorescence yield, in the mutant it accounted for less than 2% rise in the total. Analysis of the flash-induced fluorescence rise in the presence of DCMU indicated that in wild type the ratio of PS IIalpha to PS IIbeta reaction centers was approximately 1.2 while in the mutant the ratio was approximately 0.3. Fluorescence decay kinetics in the absence of DCMU indicated that electron transfer to QB was significantly altered in the mutant. Fluorescence decay kinetics in the presence of DCMU indicated that the charge recombination between QA- and the S2 state of the oxygen-evolving complex was retarded. Furthermore, flash oxygen yield analysis indicated that both the S2 and S3 states exhibited significantly longer lifetimes in the psbo1 mutant than in wild type. Our data indicate that while PsbO-1-deficient plants can grow photoautotrophically (although at a reduced growth rate) the photochemistry of PS II is significantly altered.

Insights into the function of PsbR protein in Arabidopsis thaliana

The functional state of the Photosystem (PS) II complex in Arabidopsis psbR T-DNA insertion mutant was studied. The DeltaPsbR thylakoids showed about 34% less oxygen evolution than WT, which correlates with the amounts of PSII estimated from Y(D)(ox) radical EPR signal. The increased time constant of the slow phase of flash fluorescence (FF)-relaxation and upshift in the peak position of the main TL-bands, both in the presence and in the absence of DCMU, confirmed that the S(2)Q(A)(-) and S(2)Q(B)(-) charge recombinations were stabilized in DeltaPsbR thylakoids. Furthermore, the higher amount of dark oxidized Cyt-b559 and the increased proportion of fluorescence, which did not decay during the 100s time span of the measurement thus indicating higher amount of Y(D)(+)Q(A)(-) recombination, pointed to the donor side modifications in DeltaPsbR. EPR measurements revealed that S(1)-to-S(2)-transition and S(2)-state multiline signal were not affected by mutation. The fast phase of the FF-relaxation in the absence of DCMU was significantly slowed down with concomitant decrease in the relative amplitude of this phase, indicating a modification in Q(A) to Q(B) electron transfer in DeltaPsbR thylakoids. It is concluded that the lack of the PsbR protein modifies both the donor and the acceptor side of the PSII complex.

The role of D1-Ala344 in charge stabilization and recombination in Photosystem II

The Ala344 residue of the D1 protein has been identified as a crucial residue of the catalytic cluster of the water-oxidizing complex, however, its function has not been fully clarified. Here we have used thermoluminescence and flash-induced chlorophyll fluorescence measurements to characterize the effect of the D1-Ala344stop mutation on the electron transport of Photosystem II in intact cells of the cyanobacterium Synechocystis 6803. Although the mutant cannot grow photoautotrophically it shows flash-induced thermoluminescence and chlorophyll fluorescence signals reflecting the stabilization of negative and positive charges on the Q(A) and Q(B) quinone electron acceptors, and stable Photosystem II donors, respectively. Decay of flash induced chlorophyll fluorescence yield is multiphasic in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), with 6 ms, 350 ms, and 26 s time constants. When cells are illuminated with repetitive flashes, fired at 1 ms intervals, the 6 ms phase is gradually decreased with the concomitant increase of the 350 ms phase. After 45 min dark adaptation of mutant cells the 6 ms and 350 ms phases were significantly decreased and a very slow decaying component was formed. Flash induced oscillation of the thermoluminescence B band, which reflects the redox cycling of the water-oxidizing complex in the wild-type cells, was completely abolished in the D1-Ala344stop mutant. The results demonstrate that low efficiency photooxidation of Mn occurs in about 60% of the PSII centers. The photooxidizable Mn is unstable in the dark, and formation of higher S states is inhibited. In addition, the Q(A) to Q(B) electron transfer step is slowed down as an indirect consequence of the donor side modification. Our data indicate that the stabilization of a Mn ion by the alpha-carboxylate chain of the D1-Ala344 residue might represent one of the final steps in the assembly of functional catalytic sites for water oxidation.