Histidine residue 252 of the Photosystem II D1 polypeptide is involved in a light-induced cross-linking of the polypeptide with the α subunit of cytochrome b-559: study of a site-directed mutant of Synechocystis PCC 6803

Properties of the Photosystem II (PSII) complex were examined in the wild-type (control) strain of the cyanobacterium Synechocystis PCC 6803 and its site-directed mutant D1-His252Leu in which the histidine residue 252 of the D1 polypeptide was replaced by leucine. This mutation caused a severe blockage of electron transfer between the PSII electron acceptors Q_A and Q_B and largely inhibited PSII oxygen evolving activity. Strong illumination induced formation of a D1-cytochrome b-559 adduct in isolated, detergent-solubilized thylakoid membranes from the control but not the mutant strain. The light-induced generation of the adduct was suppressed after prior modification of thylakoid proteins either with the histidine modifier platinum-terpyridine-chloride or with primary amino group modifiers. Anaerobic conditions and the presence of radical scavengers also inhibited the appearance of the adduct. The data suggest that the D1-cytochrome adduct is the product of a reaction between the oxidized residue His²⁵² of the D1 polypeptide and the N-terminal amino group of the cytochrome α subunit. As the rate of the D1 degradation in the control and mutant strains is similar, formation of the adduct does not seem to represent a required intermediary step in the D1 degradation pathway.     

Role of the carboxy terminus of polypeptide D1 in the assembly of a functional water-oxidizing manganese cluster in photosystem II of the cyanobacterium Synechocystis sp. PCC 6803: assembly requires a free carboxyl group at C-terminal position 344.

The D1 polypeptide of the photosystem II (PSII) reaction center is synthesized as a precursor polypeptide which is posttranslationally processed at the carboxy terminus. It has been shown in spinach that such processing removes nine amino acids, leaving Ala344 as the C-terminal residue [Takahashi, M., Shiraishi, T., & Asada, K. (1988) FEBS Lett. 240, 6-8; Takahashi, Y., Nakane, H., Kojima, H., & Satoh, K. (1990) Plant Cell Physiol. 31, 273-280]. We show here that processing on the carboxy side of Ala344 also occurs in the cyanobacterium Synechocystis 6803, resulting in the removal of 16 amino acids. By constructing a deletion strain of Synechocystis 6803 that lacks the three copies of the psbA gene encoding D1, we have developed a system for generating psbA mutants. Using this system, we have constructed mutants of Synechocystis 6803 that are modified in the region of the C-terminus of the D1 polypeptide. Characterization of these mutants has revealed that (1) processing of the D1 polypeptide is blocked when the residue after the cleavage site is changed from serine to proline (mutant Ser345Pro) with the result that the manganese cluster is unable to assemble correctly; (2) the C-terminal extension of 16 amino acid residues can be deleted with little consequence either for insertion of D1 into the thylakoid membrane or for assembly of D1 into a fully active PSII complex; (3) removal of only one more residue (mutant Ala344stop) results in a loss of assembly of the manganese cluster; and (4) the ability of detergent-solubilized PSII core complexes (lacking the manganese cluster) to bind and oxidize exogenous Mn2+ by the secondary donor, Z+, is largely unaffected in the processing mutants (the Ser345Pro mutant of Synechocystis 6803 and the LF-1 mutant of Scenedesmus obliquus) and the truncation mutant Ala344stop. Our results are consistent with a role for processing in regulating the assembly of the photosynthetic manganese cluster and a role for the free carboxy terminus of the mature D1 polypeptide in the ligation of one or more manganese ions of the cluster.     

Chloroplast-encoded polypeptide PsbT is involved in the repair of primary electron acceptor QA of photosystem II during photoinhibition in Chlamydomonas reinhardtii

PsbT is a small chloroplast-encoded hydrophobic polypeptide associated with the D1/D2 heterodimer of the photosystem II (PSII) reaction center and is required for the efficient post-translational repair of photodamaged PSII. Here we addressed that role in detail in Chlamydomonas reinhardtii wild type and DeltapsbT cells by analyzing the activities of PSII, the assembly of PSII proteins, and the redox components of PSII during photoinhibition and repair. Strong illumination of cells for 15 min decreased the activities of electron transfer through PSII and Q(A) photoreduction by 50%, and it reduced the amount of atomic manganese by 20%, but it did not affect the steady-state level of PSII proteins, photoreduction of pheophytin (pheo(D1)), and the amount of bound plastoquinone (Q(A)), indicating that the decrease in PSII activity resulted mainly from inhibition of the electron transfer from pheo(D1) to Q(A). In wild type cells, we observed parallel recovery of electron transfer activity through PSII and Q(A) photoreduction, suggesting that the recovery of Q(A) activity is one of the rate-limiting steps of PSII repair. In DeltapsbT cells, the repairs of electron transfer activity through PSII and of Q(A) photoreduction activity were both impaired, but PSII protein turnover was unaffected. Moreover, about half the Q(A) was lost from the PSII core complex during purification. Since PsbT is intimately associated with the Q(A)-binding region on D2, we propose that this polypeptide enhances the efficient recovery of Q(A) photoreduction by stabilizing the structure of the Q(A)-binding region.     

Molecular and biophysical analysis of herbicide-resistant mutants of Chlamydomonas reinhardtii: structure-function relationship of the photosystem II D1 polypeptide.

Plants and green algae can develop resistance to herbicides that block photosynthesis by competing with quinones in binding to the chloroplast photosystem II (PSII) D1 polypeptide. Because numerous herbicide-resistant mutants of Chlamydomonas reinhardtii with different patterns of resistance to such herbicides are readily isolated, this system provides a powerful tool for examining the interactions of herbicides and endogenous quinones with the photosynthetic membrane, and for studying the structure-function relationship of the D1 protein with respect to PSII electron transfer. Here we report the results of DNA sequence analysis of the D1 gene from four mutants not previously characterized at the molecular level, the correlation of changes in specific amino acid residues of the D1 protein with levels of resistance to the herbicides atrizine, diuron, and bromacil, and the kinetics of fluorescence decay for each mutant, which show that changes at two different amino acid residues dramatically slow PSII electron transfer. Our analyses, which identify a region of 57 amino acids of the D1 polypeptide involved in herbicide binding and which define a D1 binding niche for the second quinone acceptor, QB of PSII, provide a strong basis of support for structural and functional models of the PSII reaction center.     

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.     

PsbT polypeptide is required for efficient repair of photodamaged photosystem II reaction center.

PsbT is a small chloroplast-encoded hydrophobic polypeptide associated with the photosystem II (PSII) core complex. A psbT-deficient mutant (Delta psbT) of the green alga Chlamydomonas reinhardtii grows photoautotrophically, whereas its growth is significantly impaired in strong light. To understand the photosensitivity of Delta psbT, we have studied the effect of strong illumination on PSII activity and proteins. It is shown that the level of PSII activity and proteins is reduced in the Delta psbT more significantly than in wild type under strong light. When recovery of the photodamaged PSII is inhibited by a chloroplast protein synthesis inhibitor, the light-induced inactivation and degradation of PSII occur similarly in wild-type and mutant cells. On the contrary, the recovery of PSII activity after partial photoinactivation is remarkably delayed in the Delta psbT cells, suggesting that PsbT is required for efficient recovery of the photodamaged PSII complex. These results therefore present the first evidence for involvement of this small PSII polypeptide in the recovery process. Partial disintegration of the purified PSII core complex and localization of PSII proteins in the resulting PSII subcore complexes have revealed that PsbT is associated with D1/D2 heterodimer. A possible role of PsbT in the recovery process is discussed.     

Hg2+对菠菜离体类囊体膜光化学活性和多肽组分的影响

重金属Ｈｇ＾２＋对菠菜（Ｓｐｉｎａｃｉａ ｏｌｅｒａｃｅａ Ｌ．）离体类囊体膜的光合电子传递活性、室温吸收光谱、室温荧光发射光谱以及多肽组分影响的研究结果表明：Ｈｇ＾２＋对两个光系统的电子传递活性都有抑制作用,且Ｈｇ＾２＋对ＰＳＩ的抑制作用较ＰＳⅡ大;Ｈｇ＾２＋处理使类囊体膜的室温吸收光谱峰及室温荧光发射峰降低,但未使类囊体膜的多肽组分发生改变。     

Changes of amino acid sequence in PEST-like area and QEEET motif affect degradation rate of D1 polypeptide in photosystem II

The degradation rate of the D1 polypeptide was measured in threeSynechocystis PCC 6803 mutantsin vivo. Mutations were introduced into a putative cleavage area of the D1 polypeptide (QEEET motif) and into the PEST-like area. PEST sequences are often found in proteins with a high turnover rate. The QEEET-motif mutants are CA1 [(E242-E244);Q241H] and E243K, and the third mutation, E229D, was directed to the PEST-like area. During high-light illumination (1500 mol photons m^-2s^-1) that induced photoinhibition of photosystem II (PSII), the half-life time of the D1 polypeptide in mutant E229D (t _1/2=35 min) was about twice as long as in AR (control strain) cells (t _1/2=19 min). In growth light (40 mol photons m^-2s^-1), the degradation rate of the D1 polypeptide in E229D and AR strains was the same (t _1/25 h). In growth light the D1 polypeptide was degraded faster in both QEEET-motif mutants than in the AR strain, but in photoinhibitory light the degradation rates were similar. According to these results, the highly conservative QEEET motif as such is not required for the proteolytic cut of the D1 polypeptide, but it does affect the rate of degradation. No simple correlation existed between the degradation rate of the D1 polypeptide and the susceptibility of PSII to photoinhibition in mutant and AR cells under our experimental conditions.     

Participation of glutamate-354 of the CP43 polypeptide in the ligation of manganese and the binding of substrate water in photosystem II

In the current X-ray crystallographic structural models of photosystem II, Glu354 of the CP43 polypeptide is the only amino acid ligand of the oxygen-evolving Mn(4)Ca cluster that is not provided by the D1 polypeptide. To further explore the influence of this structurally unique residue on the properties of the Mn(4)Ca cluster, the CP43-E354Q mutant of the cyanobacterium Synechocystis sp. PCC 6803 was characterized with a variety of biophysical and spectroscopic methods, including polarography, EPR, X-ray absorption, FTIR, and mass spectrometry. The kinetics of oxygen release in the mutant were essentially unchanged from those in wild type. In addition, the oxygen flash yields exhibited normal period four oscillations having normal S state parameters, although the yields were lower, correlating with the mutant's lower steady-state rate (approximately 20% compared to wild type). Experiments conducted with H(2)(18)O showed that the fast and slow phases of substrate water exchange in CP43-E354Q thylakoid membranes were accelerated 8.5- and 1.8-fold, respectively, in the S(3) state compared to wild type. Purified oxygen-evolving CP43-E354Q PSII core complexes exhibited a slightly altered S(1) state Mn-EXAFS spectrum, a slightly altered S(2) state multiline EPR signal, a substantially altered S(2)-minus-S(1) FTIR difference spectrum, and an unusually long lifetime for the S(2) state (>10 h) in a substantial fraction of reaction centers. In contrast, the S(2) state Mn-EXAFS spectrum was nearly indistinguishable from that of wild type. The S(2)-minus-S(1) FTIR difference spectrum showed alterations throughout the amide and carboxylate stretching regions. Global labeling with (15)N and specific labeling with l-[1-(13)C]alanine revealed that the mutation perturbs both amide II and carboxylate stretching modes and shifts the symmetric carboxylate stretching modes of the α-COO(-) group of D1-Ala344 (the C-terminus of the D1 polypeptide) to higher frequencies by 3-4 cm(-1) in both the S(1) and S(2) states. The EPR and FTIR data implied that 76-82% of CP43-E354Q PSII centers can achieve the S(2) state and that most of these can achieve the S(3) state, but no evidence for advancement beyond the S(3) state was observed in the FTIR data, at least not in a majority of PSII centers. Although the X-ray absorption and EPR data showed that the CP43-E354Q mutation only subtly perturbs the structure and spin state of the Mn(4)Ca cluster in the S(2) state, the FTIR and H(2)(18)O exchange data show that the mutation strongly influences other properties of the Mn(4)Ca cluster, altering the response of numerous carboxylate and amide groups to the increased positive charge that develops on the cluster during the S(1) to S(2) transition and weakening the binding of both substrate water molecules (or water-derived ligands), especially the one that exchanges rapidly in the S(3) state. The FTIR data provide evidence that CP43-Glu354 coordinates to the Mn(4)Ca cluster in the S(1) state as a bridging ligand between two metal ions but provide no compelling evidence that this residue changes its coordination mode during the S(1) to S(2) transition. The H(2)(18)O exchange data provide evidence that CP43-Glu354 interacts with the Mn ion that ligates the substrate water molecule (or water-derived ligand) that is in rapid exchange in the S(3) state.     

The Arabidopsis PsbO2 protein regulates dephosphorylation and turnover of the photosystem II reaction centre D1 protein

The extrinsic photosystem II (PSII) protein of 33 kDa (PsbO), which stabilizes the water-oxidizing complex, is represented in Arabidopsis thaliana (Arabidopsis) by two isoforms. Two T-DNA insertion mutant lines deficient in either the PsbO1 or the PsbO2 protein were retarded in growth in comparison with the wild type, while differing from each other phenotypically. Both PsbO proteins were able to support the oxygen evolution activity of PSII, although PsbO2 was less efficient than PsbO1 under photoinhibitory conditions. Prolonged high light stress led to reduced growth and fitness of the mutant lacking PsbO2 as compared with the wild type and the mutant lacking PsbO1. During a short period of treatment of detached leaves or isolated thylakoids at high light levels, inactivation of PSII electron transport in the PsbO2-deficient mutant was slowed down, and the subsequent degradation of the D1 protein was totally inhibited. The steady-state levels of in vivo phosphorylation of the PSII reaction centre proteins D1 and D2 were specifically reduced in the mutant containing only PsbO2, in comparison with the mutant containing only PsbO1 or with wild-type plants. Phosphorylation of PSII proteins in vitro proceeded similarly in thylakoid membranes from both mutants and wild-type plants. However, dephosphorylation of the D1 protein occurred much faster in the thylakoids containing only PsbO2. We conclude that the function of PsbO1 in Arabidopsis is mostly in support of PSII activity, whereas the interaction of PsbO2 with PSII regulates the turnover of the D1 protein, increasing its accessibility to the phosphatases and proteases involved in its degradation.     

Dimeric and monomeric organization of photosystem II. Distribution of five distinct complexes in the different domains of the thylakoid membrane

The supramolecular organization of photosystem II (PSII) was characterized in distinct domains of the thylakoid membrane, the grana core, the grana margins, the stroma lamellae, and the so-called Y100 fraction. PSII supercomplexes, PSII core dimers, PSII core monomers, PSII core monomers lacking the CP43 subunit, and PSII reaction centers were resolved and quantified by blue native PAGE, SDS-PAGE for the second dimension, and immunoanalysis of the D1 protein. Dimeric PSII (PSII supercomplexes and PSII core dimers) dominate in the core part of the thylakoid granum, whereas the monomeric PSII prevails in the stroma lamellae. Considerable amounts of PSII monomers lacking the CP43 protein and PSII reaction centers (D1-D2-cytochrome b559 complex) were found in the stroma lamellae. Our quantitative picture of the supramolecular composition of PSII, which is totally different between different domains of the thylakoid membrane, is discussed with respect to the function of PSII in each fraction. Steady state electron transfer, flash-induced fluorescence decay, and EPR analysis revealed that nearly all of the dimeric forms represent oxygen-evolving PSII centers. PSII core monomers were heterogeneous, and a large fraction did not evolve oxygen. PSII monomers without the CP43 protein and PSII reaction centers showed no oxygen-evolving activity.     

Modulating the redox potential of the stable electron acceptor, Q(B), in mutagenized photosystem II reaction centers

One of the unique features of electron transfer processes in photosystem II (PSII) reaction centers (RC) is the exclusive transfer of electrons down only one of the two parallel cofactor branches. In contrast to the RC core polypeptides (psaA and psaB) of photosystem I (PSI), where electron transfer occurs down both parallel redox-active cofactor branches, there is greater protein-cofactor asymmetry between the PSII RC core polypeptides (D1 and D2). We have focused on the identification of protein-cofactor relationships that determine the branch along which primary charge separation occurs (P(680)(+)/pheophytin(-)(Pheo)). We have previously shown that mutagenesis of the strong hydrogen-bonding residue, D1-E130, to less polar residues (D1-E130Q,H,L) shifted the midpoint potential of the Pheo(D1)/Pheo(D1)(-) couple to more negative values, reducing the quantum yield of primary charge separation. We did not observe, however, electron transfer down the inactive branch in D1-E130 mutants. The protein residue corresponding to D1-E130 on the inactive branch is D2-Q129 which presumably has a reduced hydrogen-bonding interaction with Pheo(D2) relative to the D1-E130 residue with Pheo(D1). Analysis of the recent 2.9 ? cyanobacterial PSII crystal structure indicated, however, that the D2-Q129 residue was too distant from the Pheo(D2) headgroup to serve as a possible hydrogen bond donor and directly impact its midpoint potential as well as potentially determine the directionality of electron transfer. Our objective was to characterize the function of this highly conserved inactive branch residue by replacing it with a nonconservative leucine or a conservative histidine residue. Measurements of Chl fluorescence decay kinetics and thermoluminescence studies indicate that the mutagenesis of D2-Q129 decreases the redox gap between Q(A) and Q(B) due to a lowering of the redox potential of Q(B). The resulting increased yield of S(2)Q(B)(-) charge recombination in the D2-Q129 mutants leads to an increased susceptibility to photoinhibitory light presumably due to (3)P(680)-mediated oxidative damage. The results indicate that the D2-Q129 residue plays a critical role in stabilizing the charge-separated state in PSII and further documents the structural and functional asymmetry between the two cofactor branches in PSII.     

Carboxyl-terminal processing of the D1 protein and photoactivation of water-splitting in photosystem II. Partial purification and characterization of the processing enzyme from Scenedesmus obliquus and Pisum sativum.

The D1 polypeptide of photosystem II (PSII) is synthesized as a precursor that is processed by cleavage at the carboxyl terminus during assembly of the active PSII complex. A mutant of the green alga Scenedesmus obliquus, LF-1, inactive in water-splitting, lacks the D1 processing activity but assembles otherwise normal PSII complexes containing the precursor D1 molecule. We have isolated and partially purified a soluble protease from sonicated thylakoids of both wild-type S. obliquus and Pisum sativum which will process the precursor D1 molecule in PSII-enriched membranes from the LF-1 mutant to the mature size. After processing (but not before), photoactivation of these PSII membranes in the presence of manganese restores water-splitting to levels seen after photoactivation of PSII membranes from dark-grown, wild-type, cells. The protease is unable to process D1 in intact thylakoids from the LF-1 mutant but processes D1 if present during sonication of the thylakoids, indicating that processing of the carboxyl-terminal extension of D1 occurs in the lumen of the thylakoid. The processing protease from both S. obliquus and P. sativum is a single subunit enzyme of native molecular mass 33-35 kDa. Processing rate is optimal at pH 6.5. Processing in vitro is evident within 5 min and is markedly inhibited by millimolar concentrations of divalent cations (Cu, Zn greater than Mn greater than Ca, Mg) but not by any known inhibitors of the major classes of proteases. The protease is inactive against the precursors of other thylakoidal proteins and is thus distinct from the thylakoidal amino-terminal processing enzyme involved in the removal of transit peptides from cytoplasmically-synthesised proteins imported into the thylakoid lumen.     

Substitution of Ala-251 of the D1 reaction centre polypeptide with a charged residue results in impaired function of photosystem II

Ala-251 in the membrane-parallel helix in the D-E loop of the D1 polypeptide close to the QB pocket of photosystem II (PS II), was mutated to aspartate (D), lysine (K), leucine (L) or serine (S) in Synechocystis 6803. O2 evolution rates (H2ODCBQ; 2,6-dichloro-p-benzoquinone) of A251D, A251L and A251S were lower, being 38, 16, 62 and 70%, respectively, of that of the control, and there was an even more drastic impairment of O2 evolution when measured from H2O to DMBQ (2,5-dimethyl-p-benzoquinone), demonstrating modifications in the QB pocket. However, in all other mutants but A251K, the QB function could sustain O2 evolution at a level high enough to support photosynthetic growth. The mutant A251S, carrying a substitution of alanine for a chemically quite similar residue serine, was less severely affected. Substitution by a positively charged residue drastically delayed chlorophyll a fluorescence relaxation in the non-photosynthetic strain A251K, implying strong impairment of QA-to-QB electron transfer. Delay of fluorescence relaxation was clear in A251D as well, carrying a substitution of alanine for a negatively charged residue. The effects of the substitutions of A251 demonstrate the importance of this residue of the D1 polypeptide in the conformation of the acceptor side of PS II and, accordingly, the effect on the acceptor-side function of PS II was very clear. Nevertheless, the tolerance of PS II activity to high-light-induced photoinhibition in vivo and the subsequent D1 degradation were not much impaired in any of the photosynthetic mutant strains as compared to the control.     

Aspartate 170 of the photosystem II reaction center polypeptide D1 is involved in the assembly of the oxygen-evolving manganese cluster.

Eleven site-directed mutations were constructed at aspartate 170 of the D1 polypeptide of the photosystem II (PSII) reaction center of the cyanobacterium Synechocystis sp. PCC 6803. The light-saturated rates of O2 evolution (VO2) measured in whole cells range from close to that of wild-type for Asp170Glu to zero for Asp170Ser and Ala. Those mutant strains that are best able to evolve O2 are also those that show the lowest Km in PSII core complexes for the oxidation of Mn2+ by oxidized Tyr161, the normal oxidant of the Mn cluster responsible for O2 evolution. To a first approximation, the lower the pKa of the residue at position 170, the higher the VO2 and the lower the Km. D1-Asp170 appears to participate in the early steps associated with the assembly of the Mn cluster. It is also the first reported example of an amino acid residue critical to the function and assembly of the oxygen-evolving complex.     

Two functionally distinct manganese clusters formed by introducing a mutation in the carboxyl terminus of a photosystem II reaction center polypeptide, D1, of the green alga Chlamydomonas reinhardtii

To study the function of the carboxyl-terminal domain of a photosystem II (PSII) reaction center polypeptide, D1, chloroplast mutants of the green alga Chlamydomonas reinhardtii have been generated in which Leu-343 and Ala-344 have been simultaneously or individually replaced by Phe and Ser, respectively. The mutants carrying these replacements individually, L343F and A344S, showed a wild-type phenotype. In contrast, the double mutant, L343FA344S, evolved O2 at only 20-30% of the wild-type rate and was unable to grow photosynthetically. In this mutant, PSII accumulated to 60% of the wild-type level, indicating that the O2-evolving activity per PSII was reduced to approximately half that of the wild-type. However, the amount of Mn atom detected in the thylakoids suggested that a normal amount of Mn cluster was assembled. An investigation of the kinetics of flash-induced fluorescence yield decay revealed that the electron transfer from Q(-)(A) to Q(B) was not affected. When a back electron transfer from Q(-)(A) to a donor component was measured in the presence of 3-(3,4-dichlorophenol)-1,1-dimethylurea, a significantly slower component of the Q(-)(A) oxidation was detected in addition to the normal component that corresponds to the back electron transfer from the Q(-)(A) to the S(2)-state of the Mn cluster. Thermoluminescence measurements revealed that L343FA344S cells contained two functionally distinct Mn clusters. One was equivalent to that of the wild-type, while the other was incapable of water oxidation and was able to advance the transition from the S(1)-state to the S(2)-state. These results suggested that a fraction of the Mn cluster had been impaired by the L343FA344S mutation, leading to decreased O2 evolution. We concluded that the structure of the C-terminus of D1 is critical for the formation of the Mn cluster that is capable of water oxidation, in particular, transition to higher S-states.     

Small CAB-like proteins prevent formation of singlet oxygen in the damaged photosystem II complex of the cyanobacterium Synechocystis sp. PCC 6803

The cyanobacterial small CAB-like proteins (SCPs) are single-helix membrane proteins mostly associated with the photosystem II (PSII) complex that accumulate under stress conditions. Their function is still ambiguous although they are assumed to regulate chlorophyll (Chl) biosynthesis and/or to protect PSII against oxidative damage. In this study, the effect of SCPs on the PSII-specific light-induced damage and generation of singlet oxygen ((1)O(2)) was assessed in the strains of the cyanobacterium Synechocystis sp. PCC 6803 lacking PSI (PSI-less strain) or lacking PSI together with all SCPs (PSI-less/scpABCDE(-) strain). The light-induced oxidative modifications of the PSII D1 protein reflected by a mobility shift of the D1 protein and by generation of a D1-cytochrome b-559 adduct were more pronounced in the PSI-less/scpABCDE(-) strain. This increased protein oxidation correlated with a faster formation of (1)O(2) as detected by the green fluorescence of Singlet Oxygen Sensor Green assessed by a laser confocal scanning microscopy and by electron paramagnetic resonance spin-trapping technique using 2, 2, 6, 6-tetramethyl-4-piperidone (TEMPD) as a spin trap. In contrast, the formation of hydroxyl radicals was similar in both strains. Our results show that SCPs prevent (1)O(2) formation during PSII damage, most probably by the binding of free Chl released from the damaged PSII complexes.     

Decrease in the efficiency of the electron donation to tyrosine Z of photosystem II in an SQDG-deficient mutant of Chlamydomonas

Photosystem (PS) II activity of a sulfoquinovosyl diacylglycerol (SQDG)-deficient mutant (hf-2) of Chlamydomonas was partially decreased compared with that of wild-type. The susceptibility to 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) was also modified in the mutant. Photometric measurements in the isolated thylakoid membranes of hf-2 revealed that the lowered activity in the mutant was derived from a decrease in the efficiency of the electron donation from water to tyrosine Z, not from the efficiency of the electron transport from Q(A) to Q(B). This result was confirmed by the decay kinetics of chlorophyll fluorescence determined in vivo. We conclude that SQDG contributes to maintaining the conformation of PSII complexes, particularly that of D1 polypeptides, which are necessary for maximum activities in Chlamydomonas.     

Site-directed mutagenesis on the heme axial-ligands of cytochrome b559 in photosystem II by using cyanobacteria Synechocystis PCC 6803

Cytochrome (cyt) b559 has been proposed to play an important role in the cyclic electron flow processes that protect photosystem II (PSII) from light-induced damage during photoinhibitory conditions. However, the exact role(s) of cyt b559 in the cyclic electron transfer pathway(s) in PSII remains unclear. To study the exact role(s) of cyt b559, we have constructed a series of site-directed mutants, each carrying a single amino acid substitution of one of the heme axial-ligands, in the cyanobacterium Synechocystis sp. PCC6803. In these mutants, His-22 of the alpha or the beta subunit of cyt b559 was replaced with either Met, Glu, Tyr, Lys, Arg, Cys or Gln. On the basis of oxygen-evolution and chlorophyll a fluorescence measurements, we found that, among all mutants that were constructed, only the H22Kalpha mutant grew photoautotrophically, and accumulated stable PSII reaction centers ( approximately 81% compared to wild-type cells). In addition, we isolated one pseudorevertant of the H22Ybeta mutant that regained the ability to grow photoautotrophically and to assemble stable PSII reaction centers ( approximately 79% compared to wild-type cells). On the basis of 77 K fluorescence emission measurements, we found that energy transfer from the phycobilisomes to PSII reaction centers was uncoupled in those cyt b559 mutants that assembled little or no stable PSII. Furthermore, on the basis of immunoblot analyses, we found that in thylakoid membranes of cyt b559 mutants that assembled little or no PSII, the amounts of the D1, D2, cyt b559alpha and beta polypeptides were very low or undetectable but their CP47 and PsaC polypeptides were accumulated to the wild-type level. We also found that the amounts of cyt b559beta polypeptide were significantly increased (larger than two folds) in thylakoid membranes of cyt b559 H22YbetaPS+ mutant cells. We suspected that the increase in the amounts of cyt b559 H22YbetaPS+ mutant polypeptides in thylakoid membranes might facilitate the assembly of functional PSII in cyt b559 H22YbetaPS+ mutant cells. Moreover, we found that isolated His-tagged PSII particles from H22Kalpha mutant cells gave rise to redox-induced optical absorption difference spectra of cyt b559. Therefore, our results concluded that significant fractions of H22Kalpha mutant PSII particles retained the heme of cyt b559. Finally, this work is the first report of cyt b559 mutants having substitutions of an axial heme-ligands that retain the ability to grow photoautotrophically and to assemble stable PSII reaction centers. These two cyt b559 mutants (H22Kalpha and H22YbetaPS+) and their PSII reaction centers will be very suitable for further biophysical and biochemical studies of the functional role(s) of cyt b559 in PSII.     

A role of the C-terminal extension of the photosystem II D1 protein in sensitivity of the cyanobacterium Synechocystis PCC 6803 to photoinhibition.

The D1 protein, a key protein subunit of Photosystem II complex (PSII), is synthesised as a precursor (pD1) with a carboxyl-terminal extension. In the cyanobacterium Synechocystis sp. PCC 6803, this extension consists of 16 amino acid residues and it is cleaved by a specific protease in two putative steps with the final cleavage after the residue Ala344. In order to define the importance of the extension for the functioning of PSII, we constructed and characterized several site-directed mutants of Synechocystis that differ in the length and amino acid sequence of this extension. The mutant lacking the entire C-terminal extension exhibited slightly increased sensitivity to photoinhibition. Analysis of the PSII assembly in the mutant by the blue-native electrophoresis in combination with radioactive labelling revealed an increased level of the unassembled D1 protein in this strain. Replacement of the amino acid residue Asn359 by His or Asp also led to the higher vulnerability to photoinhibition of both mutants. In the Asn359His mutant, this vulnerability was accompanied by an increased level of the PSII core lacking CP43 indicating limitation of the repair cycle in the CP43 reassembly step.