Accumulation of the D2 protein is a key regulatory step for assembly of the photosystem II reaction center complex in Synechocystis PCC 6803

Chloroplast-encoded PsbT is required for efficient biogenesis of photosystem II complex in the green alga Chlamydomonas reinhardtii

The small hydrophobic polypeptide PsbT is associated with the photosystem II (PSII) reaction center (D1/D2 heterodimer). Here, we report the effect of the deletion of PsbT on the biogenesis of PSII complex during light-induced greening of y-1 mutants of the green alga Chlamydomonas reinhardtii. The y-1 is unable to synthesize chlorophylls in the dark but do so in the light. The dark-grown y-1 cells accumulated no major PSII proteins but a small amount of PsbT. Upon illumination, PsbT was immediately synthesized while chlorophylls, major PSII proteins, and O(2)-evolving activity increased after a 1-h lag. The y-1 cells without PsbT accumulated chlorophylls and PSI protein at a similar rate, whereas the accumulation of PSII complex was specifically retarded during greening. The absence of PsbT did not affect the synthesis of PSII proteins. These results indicate that PsbT is required for the efficient biogenesis of PSII complex.

Spectral properties of the CP43-deletion mutant of Synechocystis sp. PCC 6803

Spectral properties, particularly fluorescence spectra and their time-dependent behavior, were investigated for a mutant of the cyanobacterium Synechocystis sp. PCC 6803 lacking the 43 kDa chlorophyll-protein (CP43, PsbC). Lack of CP43 was confirmed by a size shift of the corresponding gene and by Western blotting. The CP43-deletion mutant grown under heterotrophic conditions accumulated a small amount of photosystem (PS) II, but virtually no PS II fluorescence was observed. A 686-nm fluorescence band was clearly observed by phycocyanin excitation, coming from the terminal pigments of phycobilisomes. In contrast, no PS I fluorescence was detected by phycocyanin excitation when accumulation of PS II components was not proved by a fluorescence excitation spectrum, indicating that energy transfer to PS I chlorophyll a was mediated by PS II chlorophyll a. Direct connection of phycobilisomes with PS I was not suggested. Based on these fluorescence properties, the energy flow in the CP43-deletion mutant cells is discussed.

Identification of differentially expressed transcripts from leaves of the boron tolerant plant Gypsophila perfoliata L

Very recently some of the species of Gypsophila genus collected from the boron rich soils in Turkey were shown to be remarkably tolerant to high levels of boron. A limited amount of boron is necessary for the normal development of plants; however, a high level of boron in soil is generally toxic. Nevertheless, the adaptability of plant species allows them to withstand the presence of extreme amounts of metal ion by various strategies. This study is conducted on highly boron tolerant Gypsophila perfoliata L. collected from a location in the boron mining area. The plant samples were transferred into plant nutritional medium in the presence high; approximately 500 (35 mg/kg), 1,000, and 30 microM (considered normal) boron concentrations. We compared the transcriptome of the plant sample treated with the excess levels of boron to that of the samples grown under normal concentration using differential display PCR (DDRT-PCR) method. Thirty bands showing differential expression levels (presence or absence of bands or varying intensities) in either of approximately 500 or 30 microM B concentrations at varying time points were excised, cloned, and sequenced. Among which, 18 of them were confirmed via quantitative reverse transcription real time PCR (qRT-PCR). We are reporting the first preliminary molecular level study of boron tolerance on this organism by attempting to identify putative genes related in the tolerance mechanism. The gene fragments are consistent with the literature data obtained from a proteomics study and a metabolomics study performed in barley under varying boron concentrations.

Importance of the cyanobacterial Gun4 protein for chlorophyll metabolism and assembly of photosynthetic complexes

Gun4 is a porphyrin-binding protein that activates magnesium chelatase, a multimeric enzyme catalyzing the first committed step in chlorophyll biosynthesis. In plants, GUN4 has been implicated in plastid-to-nucleus retrograde signaling processes that coordinate both photosystem II and photosystem I nuclear gene expression with chloroplast function. In this work we present the functional analysis of Gun4 from the cyanobacterium Synechocystis sp. PCC 6803. Affinity co-purification of the FLAG-tagged Gun4 with the ChlH subunit of the magnesium chelatase confirmed the association of Gun4 with the enzyme in cyanobacteria. Inactivation of the gun4 gene abolished photoautotrophic growth of the resulting gun4 mutant strain that exhibited a decreased activity of magnesium chelatase. Consequently, the cellular content of chlorophyll-binding proteins was highly inadequate, especially that of proteins of photosystem II. Immunoblot analyses, blue native polyacrylamide gel electrophoresis, and radiolabeling of the membrane protein complexes suggested that the availability of the photosystem II antenna protein CP47 is a limiting factor for the photosystem II assembly in the gun4 mutant.

The cyanobacterial homologue of HCF136/YCF48 is a component of an early photosystem II assembly complex and is important for both the efficient assembly and repair of photosystem II in Synechocystis sp. PCC 6803

The role of the slr2034 (ycf48) gene product in the assembly and repair of photosystem II (PSII) has been studied in the cyanobacterium Synechocystis PCC 6803. YCF48 (HCF136) is involved in the assembly of Arabidopsis thaliana PSII reaction center (RC) complexes but its mode of action is unclear. We show here that YCF48 is a component of two cyanobacterial PSII RC-like complexes in vivo and is absent in larger PSII core complexes. Interruption of ycf48 slowed the formation of PSII complexes in wild type, as judged from pulse-labeling experiments, and caused a decrease in the final level of PSII core complexes in wild type and a marked reduction in the levels of PSII assembly complexes in strains lacking either CP43 or CP47. Absence of YCF48 also led to a dramatic decrease in the levels of the COOH-terminal precursor (pD1) and the partially processed form, iD1, in a variety of PSII mutants and only low levels of unassembled mature D1 were observed. Yeast two-hybrid analyses using the split ubiquitin system showed an interaction of YCF48 with unassembled pD1 and, to a lesser extent, unassembled iD1, but not with unassembled mature D1 or D2. Overall our results indicate a role for YCF48 in the stabilization of newly synthesized pD1 and in its subsequent binding to a D2-cytochrome b559 pre-complex, also identified in this study. Besides a role in assembly, we show for the first time that YCF48 also functions in the selective replacement of photodamaged D1 during PSII repair.

Isolation of membrane protein complexes by blue native electrophoresis

Blue native PAGE is a discontinuous electrophoretic system that allows the separation of membrane protein complexes in a native, enzymatically active state with high resolution. Membrane protein complexes are solubilized by neutral, nondenaturing detergents like n-dodecyl-beta-D-maltoside. After addition of Coomassie G250 that binds to the surface of the proteins, separation of the negatively charged complexes according to molecular mass is possible. After electrophoresis the structure and function of the isolated protein complexes can be investigated.

The C-terminal extension of ferrochelatase is critical for enzyme activity and for functioning of the tetrapyrrole pathway in Synechocystis strain PCC 6803

Heme and chlorophyll (Chl) share a common biosynthetic pathway up to the branch point where magnesium chelatase and ferrochelatase (FeCH) insert either magnesium for Chl biosynthesis or ferrous iron for heme biosynthesis. A distinctive feature of FeCHs in cyanobacteria is their C-terminal extension, which forms a putative transmembrane segment containing a Chl-binding motif. We analyzed the deltaH324 strain of Synechocystis sp. strain PCC 6803, which contains a truncated FeCH enzyme lacking this C-terminal domain. Truncated FeCH was localized to the membrane fraction, suggesting that the C-terminal domain is not necessary for membrane association of the enzyme. Measurements of enzyme activity and complementation experiments revealed that the deltaH324 mutation dramatically reduced activity of the FeCH, which resulted in highly upregulated 5-aminolevulinic acid synthesis in the deltaH324 mutant, implying a direct role for heme in the regulation of flux through the pathway. Moreover, the deltaH324 mutant accumulated a large amount of protoporphyrin IX, and levels of Chl precursors were also significantly increased, suggesting that some, but not all, of the "extra" flux can be diverted down the Chl branch. Analysis of the recombinant full-length and truncated FeCHs demonstrated that the C-terminal extension is critical for activity of the FeCH and that it is strictly required for oligomerization of this enzyme. The observed changes in tetrapyrrole trafficking and the role of the C terminus in the functioning of FeCH are discussed.

Role of the PsbI protein in photosystem II assembly and repair in the cyanobacterium Synechocystis sp. PCC 6803

The involvement of the PsbI protein in the assembly and repair of the photosystem II (PSII) complex has been studied in the cyanobacterium Synechocystis sp. PCC 6803. Analysis of PSII complexes in the wild-type strain showed that the PsbI protein was present in dimeric and monomeric core complexes, core complexes lacking CP43, and in reaction center complexes containing D1, D2, and cytochrome b-559. In addition, immunoprecipitation experiments and the use of a histidine-tagged derivative of PsbI have revealed the presence in the thylakoid membrane of assembly complexes containing PsbI and either the precursor or mature forms of D1. Analysis of PSII assembly in the psbI deletion mutant and in strains lacking PsbI together with other PSII subunits showed that PsbI was not required for formation of PSII reaction center complexes or core complexes, although levels of unassembled D1 were reduced in its absence. However, loss of PsbI led to a dramatic destabilization of CP43 binding within monomeric and dimeric PSII core complexes. Despite the close structural relationship between D1 and PsbI in the PSII complex, PsbI turned over much slower than D1, whereas high light-induced turnover of D1 was accelerated in the absence of PsbI. Overall, our results suggest that PsbI is an early assembly partner for D1 and that it plays a functional role in stabilizing the binding of CP43 in the PSII holoenzyme.

Small Cab-like proteins retard degradation of photosystem II-associated chlorophyll in Synechocystis sp. PCC 6803: kinetic analysis of pigment labeling with 15N and 13C

Isotope (Na(15)NO(3), ((15)NH(4))SO(4) or [(13)C]glucose) labeling was used to analyze chlorophyll synthesis and degradation rates in a set of Synechocystis mutants that lacked single or multiple small Cab-like proteins (SCPs), as well as photosystem I or II. When all five small Cab-like proteins were inactivated in the wild-type background, chlorophyll stability was not affected unless the scpABCDE(-) strain was grown at a moderately high light intensity of 100-300 micromol photons m(-2) s(-1). However, the half-life time of chlorophyll was 5-fold shorter in the photosystem I-less/scpABCDE(-) strain than in the photosystem I-less strain even when grown at low light intensity (~3 micromol photons m(-2) s(-1)) (32 +/- 5 and 161 +/- 25 h, respectively). In other photosystem I-less mutants that lacked one to four of the scp genes the chlorophyll lifetime was in between these two values, with the chlorophyll lifetime generally decreasing with an increasing number of inactivated scps. In contrast, the chlorophyll biosynthesis rate was only marginally affected by inactivation of scps except when all five scp genes were deleted. Small Cab-like protein deficiency did not significantly affect photoinhibition or turnover of photosystem II-associated beta-carotene. It is concluded that SCPs do not alter the stability of functional photosystem II complexes but retard the degradation of photosystem II-associated chlorophyll, consistent with the proposed involvement of SCPs in photosystem II re-assembly or/and repair processes by temporarily binding chlorophyll while photosystem II protein components are being replaced.

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.

Late assembly steps and dynamics of the cyanobacterial photosystem I

The dynamics of photosystem I assembly in cyanobacteria have been addressed using in vivo pulse-chase labeling of Synechocystis sp. PCC 6803 proteins in combination with blue native polyacrylamide gel electrophoresis. The analyses indicate the existence of three different monomeric photosystem I complexes and also the high stability of photosystem I trimers. We show that in addition to a complete photosystem I monomer, containing all 11 subunits, we detected a PsaK-less monomer and a short-lived PsaL/PsaK-less complex. The latter two monomers were missing in the ycf37 mutant of Synechocystis sp. PCC 6803 that accumulates also less trimers. Pulse-chase experiments suggest that the three monomeric complexes have different functions in the biogenesis of the trimer. Based on these findings we propose a model where PsaK is incorporated in the latest step of photosystem I assembly. The PsaK-less photosystem I monomer may represent an intermediate complex that is important for the exchange of the two PsaK variants during high light acclimation. Implications of the presented data with respect to Ycf37 function are discussed.

Cleavage after residue Ala352 in the C-terminal extension is an early step in the maturation of the D1 subunit of Photosystem II in Synechocystis PCC 6803

We have investigated the pathway by which the 16 amino-acid C-terminal extension of the D1 subunit of photosystem two is removed in the cyanobacterium Synechocystis sp. PCC 6803 to leave Ala344 as the C-terminal residue. Previous work has suggested a two-step process involving formation of a processing intermediate of D1, termed iD1, of uncertain origin. Here we show by mass spectrometry that a synthetic peptide mimicking the C- terminus of the D1 precursor is cleaved by cellular extracts or purified CtpA processing protease after residue Ala352, making this a likely site for formation of iD1. Characteristics of D1 site-directed mutants with either the Leu353 residue replaced by Pro or with a truncation after Ala352 are in agreement with this assignment. Interestingly, analysis of various CtpA and CtpB null mutants further indicate that the CtpA protease plays a crucial role in forming iD1 but that, surprisingly, low levels of C-terminal processing occur in vivo in the absence of CtpA and CtpB, possibly catalysed by other related proteases. A possible role for two-step maturation of D1 in the assembly of PSII is discussed.

Localization of the small CAB-like proteins in photosystem II

The cyanobacterial small CAB-like proteins (SCPs) consist of one-helix proteins that resemble transmembrane regions of the light-harvesting proteins of plants. To determine whether these proteins are associated with protein complexes in the thylakoid membrane, an abundant member of the SCP family, ScpD, was marked with a His tag, and proteins co-isolating with His-tagged ScpD were identified. These proteins included the major Photosystem (PS) II components as well as FtsH, which is involved in degradation of the PSII complex. To ascertain specific interaction between ScpD and the PSII complex, the His-tagged protein fraction was subjected to two-dimensional blue native/SDS-PAGE. Again, PSII components were co-isolated with ScpD-His, and ScpD-His was found to interact most strongly with CP47. ScpD association was most prominent with the monomeric form of PSII, suggesting ScpD association with PSII that is repaired. Using antibodies that recognize both ScpC and ScpD, we found the ScpC protein, which is very similar in primary structure to ScpD, to also co-isolate with the PSII complex. In contrast, ScpE did not co-isolate with a major protein complex in thylakoids. A fourth member of the SCP family, ScpB, could not be immunodetected, but was found by mass spectrometry in samples co-isolating with ScpD-His. Therefore, ScpB may be associated with ScpD as well. No association between SCPs and PSI could be demonstrated. On the basis of these and other data presented, we suggest that members of the SCP family can associate with damaged PSII and can serve as a temporary pigment reservoir while PSII components are being replaced.

Agarose isoelectric focusing can improve resolution of membrane proteins in the two-dimensional electrophoresis of bacterial proteins

2-D separation of bacterial membrane proteins is still difficult despite using high-resolution IPG-IEF/SDS-PAGE. We were searching for alternative methods to avoid typical problems such as precipitation, low solubility, and aggregation of membrane proteins in the 1-D separation with IPG-IEF. Blue native electrophoresis (BNE) and agarose IEF (A-IEF) were tested for their separation capacity and their capability of replacing IPG-IEF in the first dimension. SDS-PAGE was chosen for the second dimension on account of its outstanding resolution. We could confirm that only A-IEF was a useful replacement for the IPG-IEF in the first dimension resulting in 2-D protein distributions with additional membrane protein spots not being found after IPG-IEF/SDS-PAGE. A second interesting result was that the agarose IEF mediates the possibility of separation of membrane proteins in a partially native state in the first dimension. This native A-IEF resulted in drastically changed spot patterns with an acidic shift of nearly all spots and divergent distribution of proteins compared to non-native A-IEF and IPG-IEF. We found out that native and non-native A-IEF are powerful tools to supplement IPG-IEF/SDS-PAGE.

The synechocystis sp PCC 6803 oxa1 homolog is essential for membrane integration of reaction center precursor protein pD1

Synechocystis sp PCC 6803 Slr1471p, an Oxa1p/Alb3/YidC homolog, is an essential protein for cell viability for which functions in thylakoid membrane biogenesis and cell division have been proposed. Using a fusion of green fluorescent protein to the C terminus of Slr1471p, we found that the mutant slr1471-gfp is photochemically inhibited when light intensities increase to 80 micromol x m(-2) x s(-1). We show that photoinhibition correlates with an increased redox potential of the reaction center quinone Q(A)(-) and a decreased redox potential of Q(B)(-). Analysis reveals that membrane integration of the D1 precursor protein is affected, leading to the accumulation of pD1 in the membrane phase. We show that Slr1471p interacts directly with the D1 protein and discuss why the accumulation of pD1 in two reaction center assembly intermediates is dependent on Slr1471p.

Cyanobacterial small chlorophyll-binding protein ScpD (HliB) is located on the periphery of photosystem II in the vicinity of PsbH and CP47 subunits

Cyanobacteria contain several genes coding for small one-helix proteins called SCPs or HLIPs with significant sequence similarity to chlorophyll a/b-binding proteins. To localize one of these proteins, ScpD, in the cells of the cyanobacterium Synechocystis sp. PCC 6803, we constructed several mutants in which ScpD was expressed as a His-tagged protein (ScpDHis). Using two-dimensional native-SDS electrophoresis of thylakoid membranes or isolated Photosystem II (PSII), we determined that after high-light treatment most of the ScpDHis protein in a cell is associated with PSII. The ScpDHis protein was present in both monomeric and dimeric PSII core complexes and also in the core subcomplex lacking CP43. However, the association with PSII was abolished in the mutant lacking the PSII subunit PsbH. In a PSII mutant lacking cytochrome b(559), which does not accumulate PSII, ScpDHis is associated with CP47. The interaction of ScpDHis with PsbH and CP47 was further confirmed by electron microscopy of PSII labeled with Ni-NTA Nanogold. Single particle image analysis identified the location of the labeled ScpDHis at the periphery of the PSII core complex in the vicinity of the PsbH and CP47. Because of the fact that ScpDHis did not form any large structures bound to PSII and because of its accumulation in PSII subcomplexes containing CP47 and PsbH we suggest that ScpD is involved in a process of PSII assembly/repair during the turnover of pigment-binding proteins, particularly CP47.

Detection and analysis of protein–protein interactions in organellar and prokaryotic proteomes by native gel electrophoresis: (Membrane) protein complexes and supercomplexes

It is an essential and challenging task to unravel protein-protein interactions in their actual in vivo context. Native gel systems provide a separation platform allowing the analysis of protein complexes on a rather proteome-wide scale in a single experiment. This review focus on blue-native (BN)-PAGE as the most versatile and successful gel-based approach to separate soluble and membrane protein complexes of intricate protein mixtures derived from all biological sources. BN-PAGE is a charge-shift method with a running pH of 7.5 relying on the gentle binding of anionic CBB dye to all membrane and many soluble protein complexes, leading to separation of protein species essentially according to their size and superior resolution than other fractionation techniques can offer. The closely related colorless-native (CN)-PAGE, whose applicability is restricted to protein species with intrinsic negative net charge, proved to provide an especially mild separation capable of preserving weak protein-protein interactions better than BN-PAGE. The essential conditions determining the success of detecting protein-protein interactions are the sample preparations, e.g. the efficiency/mildness of the detergent solubilization of membrane protein complexes. A broad overview about the achievements of BN- and CN-PAGE studies to elucidate protein-protein interactions in organelles and prokaryotes is presented, e.g. the mitochondrial protein import machinery and oxidative phosphorylation supercomplexes. In many cases, solubilization with digitonin was demonstrated to facilitate an efficient and particularly gentle extraction of membrane protein complexes prone to dissociation by treatment with other detergents. In general, analyses of protein interactomes should be carried out by both BN- and CN-PAGE.

Effects of salinity levels on proteome of Suaeda aegyptiaca leaves

Saline soils are the major problem of cultivated lands of Iran. Suaeda aegyptiaca is a salt-tolerant plant (halophytes) that grow naturally in salt-affected areas of Iran. We have employed proteomics to identify the mechanisms of salt responsiveness in leaves of S. aegyptiaca grown under different salt concentrations. Ten-day-old plants were treated with 0, 150, 300, 450, and 600 mM NaCl. After 30 days of treatment, leaf samples were collected and analyzed using 2-D-PAGE. Out of 700 protein spots reproducible detected within replications, 102 spots showed significant response to salt treatment compared to 0 mM NaCl. We analyzed expression pattern of salt-responsive proteins using a hierarchical and two nonhierarchical (Fuzzy ART and SOM) statistical methods and concluded that Fuzzy ART is the superior method. Forty proteins of 12 different expression groups were analyzed using LC/MS/MS. Of these, 27 protein spots were identified including proteins involved in oxidative stress tolerance, glycinebetain synthesis, cytoskeleton remodeling, photosynthesis, ATP production, protein degradation, cyanide detoxification, and chaperone activities. The expression pattern of these proteins and their possible roles in the adaptation of S. aegyptiaca to salinity is discussed.

Translation of chloroplast psbD mRNA in Chlamydomonas is controlled by a secondary RNA structure blocking the AUG start codon

Translation initiation represents a key step during regulation of gene expression in chloroplasts. Here, we report on the identification and characterization of three suppressor point mutations which overcome a translational defect caused by the deletion of a U-rich element in the 5'-untranslated region (5'-UTR) of the psbD mRNA in the green alga Chlamydomonas reinhardtii. All three suppressors affect a secondary RNA structure encompassing the psbD AUG initiation codon within a double-stranded region as judged by the analysis of site-directed chloroplast mutants as well as in vitro RNA mapping experiments using RNase H. In conclusion, the data suggest that these new element serves as a negative regulator which mediates a rapid shut-down of D2 synthesis.

The FtsH protease slr0228 is important for quality control of photosystem II in the thylakoid membrane of Synechocystis sp. PCC 6803

The cyanobacterium Synechocystis sp. PCC 6803 contains four members of the FtsH protease family. One of these, FtsH (slr0228), has been implicated recently in the repair of photodamaged photosystem II (PSII) complexes. We have demonstrated here, using a combination of blue native PAGE, radiolabeling, and immunoblotting, that FtsH (slr0228) is required for selective replacement of the D1 reaction center subunit in both wild type PSII complexes and in PSII subcomplexes lacking the PSII chlorophyll a-binding subunit CP43. To test whether FtsH (slr0228) has a more general role in protein quality control in vivo, we have studied the synthesis and degradation of PSII subunits in wild type and in defined insertion and missense mutants incapable of proper assembly of the PSII holoenzyme. We discovered that, when the gene encoding FtsH (slr0228) was disrupted in these strains, the overall level of assembly intermediates and unassembled PSII proteins markedly increased. Pulse-chase experiments showed that this was due to reduced rates of degradation in vivo. Importantly, analysis of epitope-tagged and green fluorescent protein-tagged strains revealed that slr0228 was present in the thylakoid and not the cytoplasmic membrane. Overall, our results show that FtsH (slr0228) plays an important role in controlling the removal of PSII subunits from the thylakoid membrane and is not restricted to selective D1 turnover.

Localization of the PsbH subunit in photosystem II from the Synechocystis 6803 using the His-tagged Ni–NTA Nanogold labeling

The PsbH protein belongs to a group of small protein subunits of photosystem II (PSII) complex. This protein is predicted to have a single transmembrane helix and it is important for the assembly of the PSII complex as well as for the proper function at the acceptor side of PSII. To identify the location of the PsbH subunit, the PSII complex with His-tagged PsbH protein was isolated from the cyanobacterium Synechocystis sp. PCC 6803 and labeled by Ni(2+)-nitrilo triacetic acid Nanogold. Electron microscopy followed by single particle image analysis identified the location of the labeled His-tagged PsbH protein at the periphery of the dimeric PSII complex. These results indicate that the N terminus of the PsbH protein is located at the stromal surface of the PSII complex and close to the CP47 protein.

Photosystem II Assembly in CP47 Mutant of Synechocystis sp. PCC 6803 Is Dependent on the Level of Chlorophyll Precursors Regulated by Ferrochelatase

Accumulation of chlorophyll and expression of the chlorophyll (Chl)-binding CP47 protein that serves as the core antenna of photosystem II are indispensable for the assembly of a functional photosystem II. We have characterized the CP47 mutant with an impaired photosystem II assembly and its two spontaneous pseudorevertants with their much improved photoautotrophic growth. The complementing mutations in these pseudorevertants were previously mapped to the ferrochelatase gene (1). We demonstrated that complementing mutations dramatically decrease ferrochelatase activity in pseudorevertants and that this decrease is responsible for their improved photoautotrophic growth. Photoautotrophic growth of the CP47 mutant was also restored by in vivo inhibition of ferrochelatase by a specific inhibitor. The decrease in ferrochelatase activity in pseudorevertants was followed by increased steady-state levels of Chl precursors and Chl, leading to CP47 accumulation and photosystem II assembly. Similarly, supplementation of the CP47 mutant with the Chl precursor Mg-protoporphyrin IX increased the number of active photosystem-II centers, suggesting that synthesis of the mutated CP47 protein is enhanced by an increased Chl availability in the cell. The probable role of ferrochelatase in the regulation of Chl biosynthesis is discussed.

The PsbH protein is associated with the inner antenna CP47 and facilitates D1 processing and incorporation into PSII in the cyanobacterium Synechocystis PCC 6803

Analysis of a number of PSII complexes detectable in the wild-type and mutant cells of the cyanobacterium Synechocystis sp. PCC 6803 showed that the PsbH protein is present in the complexes containing CP47, including unassembled CP47. In a mutant lacking CP47, in which the PSII assembly is stopped at the level of the D1-D2-cytochrome b-559 reaction centre complex, a negligible amount of the PsbH protein was not bound to this complex but was detected in the free form. The results indicate that the PsbH protein has a high affinity for CP47 and during PSII assembly most probably first associates with CP47 and this pair is subsequently attached to the reaction centre complex. Similarly to CP47, the PsbH protein exhibits a slow light-induced degradation in the presence of protein synthesis inhibitor. The absence of the PsbH protein leads to a greatly increased D1 pool that is not associated with other PSII proteins or it is present as a part of the reaction centre complex. We conclude that PsbH is important for the prompt incorporation of the newly synthesized D1 protein into PSII complexes and for the fast D1 maturation.

Autotrophic cells of the Synechocystis psbH deletion mutant are deficient in synthesis of CP47 and accumulate inactive PS II core complexes

Cells of the psbH deletion mutant IC7 of the cyanobacterium Synechocystis PCC 6803 grown in the absence of glucose contain strongly reduced levels of chlorophyll when compared with cells grown in the presence of glucose, or compared with wild-type (WT) cells. Low-temperature fluorescence emission spectra revealed decreased content of both active PS II (Photosystem II) and PS I (Photosystem I) complexes. Analysis of thylakoid membrane complexes of IC7 by native electrophoresis showed a similar set of chlorophyll-proteins, namely a PS II core complex and trimeric and monomeric PS II complexes, as in WT. However, in contrast to WT, the (35)S-methionine protein labeling pattern of the mutant exhibited no preferential labeling of the D1 protein in the PS II core complexes, and the labeled D1 and D2 proteins accumulated predominantly in the PS II reaction center lacking CP47. The results show that in autotrophically grown cells of the psbH deletion mutant, selective D1 turnover is inhibited and synthesis of CP47 becomes a limiting step in the PS II assembly.

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.