Role of phosphatidylglycerol in the function and assembly of Photosystem II reaction center, studied in a cdsA-inactivated PAL mutant strain of Synechocystis sp. PCC6803 that lacks phycobilisomes

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

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

Biosynthesis of phosphatidylglycerol in photosynthetic organisms

Phosphatidylglycerol (PG) is a unique phospholipid class with its indispensable role in photosynthesis and growth in land plants, algae, and cyanobacteria. PG is the only major phospholipid in the thylakoid membrane of cyanobacteria and plant chloroplasts and a main lipid component in photosynthetic protein-cofactor complexes such as photosystem I and photosystem II. In plants and algae, PG is also essential as a substrate for the biosynthesis of cardiolipin, which is a unique lipid present only in mitochondrial membranes and crucial for the functions of mitochondria. PG biosynthesis pathways in plants include three membranous organelles, plastids, mitochondria, and the endoplasmic reticulum in a complex manner. While the molecular biology underlying the role of PG in photosynthetic functions is well established, many enzymes responsible for the PG biosynthesis are only recently cloned and functionally characterized in the model plant species including Arabidopsis thaliana and Chlamydomonas reinhardtii and cyanobacteria such as Synechocystis sp. PCC 6803. The characterization of those enzymes helps understand not only the metabolic flow for PG production but also the crosstalk of biosynthesis pathways between PG and other lipids. This review aims to summarize recent advances in the understanding of the PG biosynthesis pathway and functions of involved enzymes.

Deacylation of galactolipids decomposes photosystem II dimers to enhance degradation of damaged D1 protein

Photosystem II (PSII) contains many lipid molecules that are essential for the function and maintenance of PSII. Under strong light conditions, PSII complexes are dynamically modified during the repair process; however, the molecular mechanism of the dynamic changes in the PSII structure is still unclear. In the present study, we investigated the role of a lipase in the repair of PSII in Synechocystis sp. PCC 6803. We identified a protein encoded by the sll1969 gene, previously named lipase A (lipA), in the Synechocystis sp. PCC 6803 genome as a candidate for the lipase involved in PSII repair. Recombinant protein expressed in Escherichia coli cells hydrolyzed fatty acids at the sn-1 position of monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol as well as triacylglycerol esterified with stearic acids. PSII repair in a disrupted mutant of the lipA gene was suppressed by the slow degradation of damaged D1 protein under strong light. The level of the PSII dimer remained higher in lipA mutant cells than wild-type (WT) cells under strong light. LipA protein was associated with the PSII dimer in vivo, and recombinant LipA protein decomposed PSII dimers purified from WT cells to monomers by reducing MGDG content in the PSII complex. These results indicate that LipA reacts with PSII dimers, dissociates them into monomers by digesting MGDG, and enhances D1 degradation during PSII repair.

Protection and Damage Repair Mechanisms Contributed To the Survival of Chroococcidiopsis sp. Exposed To a Mars-Like Near Space Environment

Chroococcidiopsis spp. can withstand extremely harsh environments, including a Mars-like environment. However, studies are lacking on the molecular mechanisms of Chroococcidiopsis sp. surviving in Mars-like environments. In the HH-21-5 mission, the desert cyanobacterium Chroococcidiopsis sp. was exposed to a Mars-like environment (near space; 35 km altitude) for 4 h, and a single-factor environment of near space was simulated on the ground. We investigated the survival and endurance mechanisms of Chroococcidiopsis sp. ASB-02 after exposing it to near space by studying its physiological and transcriptional properties. After the exposure, Chroococcidiopsis sp. ASB-02 exhibited high cell viability, although photosystem II activity decreased and the levels of reactive oxygen species increased. The single-factor simulation experiments revealed that for the survival of Chroococcidiopsis sp. ASB-02 in near space, UV radiation was the most important limiting factor, and it was followed by temperature. The near space environment triggered multiple metabolic pathway responses in Chroococcidiopsis sp. ASB-02. The upregulation of extracellular polysaccharides as well as carotenoid and scytonemin biosynthesis genes in response to UV radiation attenuated the extent of radiation reaching the cells. At the same time, genes related to protein synthesis were upregulated in response to the low temperature, overcoming the decrease in metabolic activity that was caused by the low temperature. In near space and after rehydration, the genes involved in various DNA and photosystem II repair pathways were upregulated. This reflected the damage to the DNA and photosystem II protein subunits in cells during the flight and suggested that repair mechanisms play an important role in the recovery of Chroococcidiopsis sp. ASB-02. IMPORTANCE This study reported that the protective and repair mechanisms of Chroococcidiopsis sp. ASB-02 contributed to its endurance ability in a Mars-like near space environment. In Chroococcidiopsis sp. ASB-02, a Mars-like near space environment activated the expression of genes involved in extracellular polysaccharides (EPS), carotenoid, scytonemin, and protein syntheses, which provided additional protection. Additionally, the cell damage repair process enhanced the recovery rate of Chroococcidiopsis sp. ASB-02 after the flight. This study will help to enhance the understanding of the tolerance mechanism of Chroococcidiopsis sp. and to provide important guidance as to the survival requirements for microbial life in a Mars-like environment.

Photosystem II antenna modules CP43 and CP47 do not form a stable 'no reaction centre complex' in the cyanobacterium Synechocystis sp. PCC 6803

The repair of photosystem II is a key mechanism that keeps the light reactions of oxygenic photosynthesis functional. During this process, the PSII central subunit D1 is replaced with a newly synthesized copy while the neighbouring CP43 antenna with adjacent small subunits (CP43 module) is transiently detached. When the D2 protein is also damaged, it is degraded together with D1 leaving both the CP43 module and the second PSII antenna module CP47 unassembled. In the cyanobacterium Synechocystis sp. PCC 6803, the released CP43 and CP47 modules have been recently suggested to form a so-called no reaction centre complex (NRC). However, the data supporting the presence of NRC can also be interpreted as a co-migration of CP43 and CP47 modules during electrophoresis and ultracentrifugation without forming a mutual complex. To address the existence of NRC, we analysed Synechocystis PSII mutants accumulating one or both unassembled antenna modules as well as Synechocystis wild-type cells stressed with high light. The obtained results were not compatible with the existence of a stable NRC since each unassembled module was present as a separate protein complex with a mutually similar electrophoretic mobility regardless of the presence of the second module. The non-existence of NRC was further supported by isolation of the His-tagged CP43 and CP47 modules from strains lacking either D1 or D2 and their migration patterns on native gels.

Phycobilisome integrity and functionality in lipid unsaturation and xanthophyll mutants in Synechocystis

The major light-harvesting system in cyanobacteria, the phycobilisome, is an essential component of the photosynthetic apparatus that regulates the utilization of the natural light source-the Sun. Earlier works revealed that the thylakoid membrane composition and its physical properties might have an important role in antennas docking. Polyunsaturated lipids and xanthophylls are among the most significant modulators of the physical properties of thylakoid membranes. In the nature, the action of these molecules is orchestrated in response to environmental stimuli among which the growth temperature is the most influential. In order to further clarify the significance of thylakoid membrane physical properties for the phycobilisomes assembly (i.e. structural integrity) and their ability to efficiently direct the excitation energy towards the photosynthetic complexes, in this work, we utilize cyanobacterial Synechocystis sp. PCC 6803 mutants deficient in polyunsaturated lipids (AD mutant) and xanthophylls (RO mutant), as well as a strain depleted of both xanthophylls and polyunsaturated lipids (ROAD multiple mutant). For the first time, we discuss the effect of those mutations on the phycobilisomes assembly, integrity and functionality at optimal (30 °C) and moderate low (25 °C) and high (35 °C) temperatures. Our results show that xanthophyll depletion exerts a much stronger effect on both phycobilisome's integrity and the response of cells to growth at suboptimal temperatures than lipid unsaturation level. The strongest effects were observed for the combined ROAD mutant, which exhibited thermally destabilized phycobilisomes and a population of energetically uncoupled phycocyanin units.

Phosphatidylglycerol is implicated in divisome formation and metabolic processes of cyanobacteria

Phosphatidylglycerol is an essential phospholipid for photosynthesis and other cellular processes. We investigated the role of phosphatidylglycerol in cell division and metabolism in a phophatidylglycerol-auxotrophic strain of Synechococcus PCC7942. Here we show that phosphatidylglycerol is essential for the photosynthetic electron transfer and for the oligomerisation of the photosynthetic complexes, notably, we revealed that this lipid is important for non-linear electron transport. Furthermore, we demonstrate that phosphatidylglycerol starvation elevated the expressions of proteins of nitrogen and carbon metabolism. Moreover, we show that phosphatidylglycerol-deficient cells changed the morphology, became elongated, the FtsZ ring did not assemble correctly, and subsequently the division was hindered. However, supplementation with phosphatidylglycerol restored the ring-like structure at the mid-cell region and the normal cell size, demonstrating the phosphatidylglycerol is needed for normal septum formation. Taken together, central roles of phosphatidylglycerol were revealed; it is implicated in the photosynthetic activity, the metabolism and the fission of bacteria.

Time-series lipidomic analysis of the oleaginous green microalga species Ettlia oleoabundans under nutrient stress

BACKGROUND

Microalgae are uniquely advantageous organisms cultured and harvested for several value-added biochemicals. A majority of these compounds are lipid-based, such as triacylglycerols (TAGs), which can be used for biofuel production, and their accumulation is most affected under nutrient stress conditions. As such, the balance between cellular homeostasis and lipid metabolism becomes more intricate to achieve efficiency in bioproduct synthesis. Lipidomics studies in microalgae are of great importance as biochemical diversity also plays a major role in lipid regulation among oleaginous species.

METHODS

The aim of this study was to analyze time-series changes in lipid families produced by microalga under different nutrient conditions and growth phases to gain comprehensive information at the cellular level. For this purpose, we worked with a highly adaptable, oleaginous, non-model green microalga species, Ettlia oleoabundans (a.k.a. Neochloris oleoabundans). Using a mass spectrometry-based untargeted and targeted metabolomics' approach, we analyzed the changes in major lipid families under both replete and deplete nitrogen and phosphorus conditions at four different time points covering exponential and stationary growth phases.

RESULTS

Comprehensive analysis of the lipid metabolism highlighted the accumulation of TAGs, which can be utilized for the production of biodiesel via transesterification, and depletion of chlorophylls and certain structural lipids required for photosynthesis, under nutrient deprived conditions. We also found a correlation between the depletion of digalactosyldiacylglycerols (DGDGs) and sulfoquinovosyldiacylglycerols (SQDGs) under nutrient deprivation.

CONCLUSIONS

High accumulation of TAGs under nutrient limitation as well as a depletion of other lipids of interest such as phosphatidylglycerols (PGs), DGDGs, SQDGs, and chlorophylls seem to be interconnected and related to the microalgal photosynthetic efficiency. Overall, our results provided key biochemical information on the lipid regulation and physiology of a non-model green microalga, along with optimization potential for biodiesel and other value-added product synthesis.

Lipid and carotenoid cooperation-driven adaptation to light and temperature stress in Synechocystis sp. PCC6803

Polyunsaturated lipids are important components of photosynthetic membranes. Xanthophylls are the main photoprotective agents, can assist in protection against light stress, and are crucial in the recovery from photoinhibition. We generated the xanthophyll- and polyunsaturated lipid-deficient ROAD mutant of Synechocystis sp. PCC6803 (Synechocystis) in order to study the little-known cooperative effects of lipids and carotenoids (Cars). Electron microscopic investigations confirmed that in the absence of xanthophylls the S-layer of the cellular envelope is missing. In wild-type (WT) cells, as well as the xanthophyll-less (RO), polyunsaturated lipid-less (AD), and the newly constructed ROAD mutants the lipid and Car compositions were determined by MS and HPLC, respectively. We found that, relative to the WT, the lipid composition of the mutants was remodeled and the Car content changed accordingly. In the mutants the ratio of non-bilayer-forming (NBL) to bilayer-forming (BL) lipids was found considerably lower. Xanthophyll to β-carotene ratio increased in the AD mutant. In vitro and in vivo methods demonstrated that saturated, monounsaturated lipids and xanthophylls may stabilize the trimerization of Photosystem I (PSI). Fluorescence induction and oxygen-evolving activity measurements revealed increased light sensitivity of RO cells compared to those of the WT. ROAD showed a robust increase in light susceptibility and reduced recovery capability, especially at moderate low (ML) and moderate high (MH) temperatures, indicating a cooperative effect of xanthophylls and polyunsaturated lipids. We suggest that both lipid unsaturation and xanthophylls are required for providing the proper structure and functioning of the membrane environment that protects against light and temperature stress.

Role of MGDG and Non-bilayer Lipid Phases in the Structure and Dynamics of Chloroplast Thylakoid Membranes

In this chapter we focus our attention on the enigmatic structural and functional roles of the major, non-bilayer lipid monogalactosyl-diacylglycerol (MGDG) in the thylakoid membrane. We give an overview on the state of the art on the role of MGDG and non-bilayer lipid phases in the xanthophyll cycles in different organisms. We also discuss data on the roles of MGDG and other lipid molecules found in crystal structures of different photosynthetic protein complexes and in lipid-protein assemblies, as well as in the self-assembly of the multilamellar membrane system. Comparison and critical evaluation of different membrane models--that take into account and capitalize on the special properties of non-bilayer lipids and/or non-bilayer lipid phases, and thus to smaller or larger extents deviate from the 'standard' Singer-Nicolson model--will conclude this review. With this chapter the authors hope to further stimulate the discussion about, what we think, is perhaps the most exciting question of membrane biophysics: the why and wherefore of non-bilayer lipids and lipid phases in, or in association with, bilayer biological membranes.

Comparison of the Photosynthetic Yield of Cyanobacteria and Green Algae: Different Methods Give Different Answers

The societal importance of renewable carbon-based commodities and energy carriers has elicited a particular interest for high performance phototrophic microorganisms. Selection of optimal strains is often based on direct comparison under laboratory conditions of maximal growth rate or additional valued features such as lipid content. Instead of reporting growth rate in culture, estimation of photosynthetic efficiency (quantum yield of PSII) by pulse-amplitude modulated (PAM) fluorimetry is an often applied alternative method. Here we compared the quantum yield of PSII and the photonic yield on biomass for the green alga Chlorella sorokiniana 211-8K and the cyanobacterium Synechocystis sp. PCC 6803. Our data demonstrate that the PAM technique inherently underestimates the photosynthetic efficiency of cyanobacteria by rendering a high F0 and a low FM, specifically after the commonly practiced dark pre-incubation before a yield measurement. Yet when comparing the calculated biomass yield on light in continuous culture experiments, we obtained nearly equal values for both species. Using mutants of Synechocystis sp. PCC 6803, we analyzed the factors that compromise its PAM-based quantum yield measurements. We will discuss the role of dark respiratory activity, fluorescence emission from the phycobilisomes, and the Mehler-like reaction. Based on the above observations we recommend that PAM measurements in cyanobacteria are interpreted only qualitatively.

The proteome and lipidome of Synechocystis sp. PCC 6803 cells grown under light-activated heterotrophic conditions

Cyanobacteria are photoautotrophic prokaryotes with a plant-like photosynthetic machinery. Because of their short generation times, the ease of their genetic manipulation, and the limited size of their genome and proteome, cyanobacteria are popular model organisms for photosynthetic research. Although the principal mechanisms of photosynthesis are well-known, much less is known about the biogenesis of the thylakoid membrane, hosting the components of the photosynthetic, and respiratory electron transport chain in cyanobacteria. Here we present a detailed proteome analysis of the important model and host organism Synechocystis sp. PCC 6803 under light-activated heterotrophic growth conditions. Because of the mechanistic importance and severe changes in thylakoid membrane morphology under light-activated heterotrophic growth conditions, a focus was put on the analysis of the membrane proteome, which was supported by a targeted lipidome analysis. In total, 1528 proteins (24.5% membrane integral) were identified in our analysis. For 641 of these proteins quantitative information was obtained by spectral counting. Prominent changes were observed for proteins associated with oxidative stress response and protein folding. Because of the heterotrophic growth conditions, also proteins involved in carbon metabolism and C/N-balance were severely affected. Although intracellular thylakoid membranes were significantly reduced, only minor changes were observed in their protein composition. The increased proportion of the membrane-stabilizing sulfoqinovosyl diacyl lipids found in the lipidome analysis, as well as the increased content of lipids with more saturated acyl chains, are clear indications for a coordinated synthesis of proteins and lipids, resulting in stabilization of intracellular thylakoid membranes under stress conditions.

Influence of thylakoid membrane lipids on the structure and function of the plant photosystem II core complex

MGDG leads to a dimerization of isolated, monomeric PSII core complexes. SQDG and PG induce a detachment of CP43 from the PSII core, thereby disturbing the intrinsic PSII electron transport. The influence of the four thylakoid membrane lipids monogalactosyldiacylglycerol (MGDG), digalactosyldiacylglycerol (DGDG), sulfoquinovosyldiacylglycerol (SQDG) and phosphatidylglycerol (PG) on the structure and function of isolated monomeric photosystem (PS) II core complexes was investigated. Incubation with the negatively charged lipids SQDG and PG led to a loss of the long-wavelength 77 K fluorescence emission at 693 nm that is associated with the inner antenna proteins. The neutral galactolipids DGDG and MGDG had no or only minor effects on the fluorescence emission spectra of the PSII core complexes, respectively. Pigment analysis, absorption and 77 K fluorescence excitation spectroscopy showed that incubation with SQDG and PG led to an exposure of chlorophyll molecules to the surrounding medium followed by conversion to pheophytin under acidic conditions. Size-exclusion chromatography and polypeptide analysis corroborated the findings of the spectroscopic measurements and pigment analysis. They showed that the negatively charged lipid SQDG led to a dissociation of the inner antenna protein CP43 and the 27- and 25-kDa apoproteins of the light-harvesting complex II, that were also associated with a part of the PSII core complexes used in the present study. Incubation of PSII core complexes with MGDG, on the other hand, induced an almost complete dimerization of the monomeric PSII. Measurements of the fast PSII fluorescence induction demonstrated that MGDG and DGDG only had a minor influence on the reduction kinetics of plastoquinone QA and the artificial PSII electron acceptor 2,5-dimethyl-p-benzoquinone (DMBQ). SQDG and, to a lesser extent, PG perturbed the intrinsic PSII electron transport significantly.

Temperature dependence of photosynthesis and thylakoid lipid composition in the red snow alga Chlamydomonas cf. nivalis (Chlorophyceae)

Here, we report an effect of short acclimation to a wide span of temperatures on photosynthetic electron transfer, lipid and fatty acid composition in the snow alga Chlamydomonas cf. nivalis. The growth and oxygen evolution capacity were low at 2 °C yet progressively enhanced at 10 °C and were significantly higher at temperatures from 5 to 15 °C in comparison with the mesophilic control Chlamydomonas reinhardtii. In search of the molecular mechanisms responsible for the adaptation of photosynthesis to low temperatures, we have found unprecedented high rates of QA to QB electron transfer. The thermodynamics of the process revealed the existence of an increased structural flexibility that we explain with the amino acid changes in the D1 protein combined with the physico-chemical characteristics of the thylakoid membrane composed of > 80% negatively charged phosphatidylglycerol.

Crosstalk between DnaA protein, the initiator of Escherichia coli chromosomal replication, and acidic phospholipids present in bacterial membranes

Anionic (i.e., acidic) phospholipids such as phosphotidylglycerol (PG) and cardiolipin (CL), participate in several cellular functions. Here we review intriguing in vitro and in vivo evidence that suggest emergent roles for acidic phospholipids in regulating DnaA protein-mediated initiation of Escherichia coli chromosomal replication. In vitro acidic phospholipids in a fluid bilayer promote the conversion of inactive ADP-DnaA to replicatively proficient ATP-DnaA, yet both PG and CL also can inhibit the DNA-binding activity of DnaA protein. We discuss how cellular acidic phospholipids may positively and negatively influence the initiation activity of DnaA protein to help assure chromosomal replication occurs once, but only once, per cell-cycle. Fluorescence microscopy has revealed that PG and CL exist in domains located at the cell poles and mid-cell, and several studies link membrane curvature with sub-cellular localization of various integral and peripheral membrane proteins. E. coli DnaA itself is found at the cell membrane and forms helical structures along the longitudinal axis of the cell. We propose that there is cross-talk between acidic phospholipids in the bacterial membrane and DnaA protein as a means to help control the spatial and temporal regulation of chromosomal replication in bacteria.

Effect of partial or complete elimination of light-harvesting complexes on the surface electric properties and the functions of cyanobacterial photosynthetic membranes

Influence of the modification of the cyanobacterial light-harvesting complex [i.e. phycobilisomes (PBS)] on the surface electric properties and the functions of photosynthetic membranes was investigated. We used four PBS mutant strains of Synechocystis sp. PCC6803 as follows: PAL (PBS-less), CK (phycocyanin-less), BE (PSII-PBS-less) and PSI-less/apcE(-) (PSI-less with detached PBS). Modifications of the PBS content lead to changes in the cell morphology and surface electric properties of the thylakoid membranes as well as in their functions, such as photosynthetic oxygen-evolving activity, P700 kinetics and energy transfer between the pigment-protein complexes. Data reveal that the complete elimination of PBS in the PAL mutant causes a slight decrease in the electric dipole moments of the thylakoid membranes, whereas significant perturbations of the surface charges were registered in the membranes without assembled PBS-PSII macrocomplex (BE mutant) or PSI complex (PSI-less mutant). These observations correlate with the detected alterations in the membrane structural organization. Using a polarographic oxygen rate electrode, we showed that the ratio of the fast to the slow oxygen-evolving PSII centers depends on the partial or complete elimination of light-harvesting complexes, as the slow operating PSII centers dominate in the PBS-less mutant and in the mutant with detached PBS.

Cadmium exerts its toxic effects on photosynthesis via a cascade mechanism in the cyanobacterium, Synechocystis PCC 6803

Despite intense research, the mechanism of Cd(2+) toxicity on photosynthesis is still elusive because of the multiplicity of the inhibitory effects and different barriers in plants. The quick Cd(2+) uptake in Synechocystis PCC 6803 permits the direct interaction of cadmium with the photosynthetic machinery and allows the distinction between primary and secondary effects. We show that the CO(2) -dependent electron transport is rapidly inhibited upon exposing the cells to 40 µm Cd(2+) (50% inhibition in ∼15 min). However, during this time we observe only symptoms of photosystem I acceptor side limitation and a build of an excitation pressure on the reaction centres, as indicated by light-induced P700 redox transients, O(2) polarography and changes in chlorophyll a fluorescence parameters. Inhibitory effects on photosystem II electron transport and the degradation of the reaction centre protein D1 can only be observed after several hours, and only in the light, as revealed by chlorophyll a fluorescence transients, thermoluminescence and immunoblotting. Despite the marked differences in the manifestations of these short- and long-term effects, they exhibit virtually the same Cd(2+) concentration dependence. These data strongly suggest a cascade mechanism of the toxic effect, with a primary effect in the dark reactions.

Recent advances in understanding the assembly and repair of photosystem II

BACKGROUND

Photosystem II (PSII) is the light-driven water:plastoquinone oxidoreductase of oxygenic photosynthesis and is found in the thylakoid membrane of chloroplasts and cyanobacteria. Considerable attention is focused on how PSII is assembled in vivo and how it is repaired following irreversible damage by visible light (so-called photoinhibition). Understanding these processes might lead to the development of plants with improved growth characteristics especially under conditions of abiotic stress.

SCOPE

Here we summarize recent results on the assembly and repair of PSII in cyanobacteria, which are excellent model organisms to study higher plant photosynthesis.

CONCLUSIONS

Assembly of PSII is highly co-ordinated and proceeds through a number of distinct assembly intermediates. Associated with these assembly complexes are proteins that are not found in the final functional PSII complex. Structural information and possible functions are beginning to emerge for several of these 'assembly' factors, notably Ycf48/Hcf136, Psb27 and Psb28. A number of other auxiliary proteins have been identified that appear to have evolved since the divergence of chloroplasts and cyanobacteria. The repair of PSII involves partial disassembly of the damaged complex, the selective replacement of the damaged sub-unit (predominantly the D1 sub-unit) by a newly synthesized copy, and reassembly. It is likely that chlorophyll released during the repair process is temporarily stored by small CAB-like proteins (SCPs). A model is proposed in which damaged D1 is removed in Synechocystis sp. PCC 6803 by a hetero-oligomeric complex composed of two different types of FtsH sub-unit (FtsH2 and FtsH3), with degradation proceeding from the N-terminus of D1 in a highly processive reaction. It is postulated that a similar mechanism of D1 degradation also operates in chloroplasts. Deg proteases are not required for D1 degradation in Synechocystis 6803 but members of this protease family might play a supplementary role in D1 degradation in chloroplasts under extreme conditions.

Involvement of carotenoids in the synthesis and assembly of protein subunits of photosynthetic reaction centers of Synechocystis sp. PCC 6803

The crtB gene of Synechocystis sp. PCC 6803, encoding phytoene synthase, was inactivated in the Delta crtH mutant to generate a carotenoidless Delta crtH/B double mutant. Delta crtH mutant cells were used because they had better transformability than wild-type cells, most probably due to their adaptation to partial carotenoid deficiency. Cells of the Delta crtH/B mutant were light sensitive and could grow only under light-activated heterotrophic growth conditions in the presence of glucose. Carotenoid deficiency did not significantly affect the cellular content of phycobiliproteins while the chlorophyll content of the mutant cells decreased. The mutant cells exhibited no oxygen-evolving activity, suggesting the absence of photochemically active PSII complexes. This was confirmed by 2D electrophoresis of photosynthetic membrane complexes. Analyses identified only a small amount of a non-functional PSII core complex lacking CP43, while the monomeric and dimeric PSII core complexes were absent. On the other hand, carotenoid deficiency did not prevent formation of the cytochrome b(6)f complex and PSI, which predominantly accumulated in the monomeric form. Radioactive labeling revealed very limited synthesis of inner PSII antennae, CP47 and especially CP43. Thus, carotenoids are indispensable constituents of the photosynthetic apparatus, being essential not only for antioxidative protection but also for the efficient synthesis and accumulation of photosynthetic proteins and especially that of PSII antenna subunits.

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.

Cyanobacterial photosystem II at 2.9-A resolution and the role of quinones, lipids, channels and chloride

Photosystem II (PSII) is a large homodimeric protein-cofactor complex located in the photosynthetic thylakoid membrane that acts as light-driven water:plastoquinone oxidoreductase. The crystal structure of PSII from Thermosynechococcus elongatus at 2.9-A resolution allowed the unambiguous assignment of all 20 protein subunits and complete modeling of all 35 chlorophyll a molecules and 12 carotenoid molecules, 25 integral lipids and 1 chloride ion per monomer. The presence of a third plastoquinone Q(C) and a second plastoquinone-transfer channel, which were not observed before, suggests mechanisms for plastoquinol-plastoquinone exchange, and we calculated other possible water or dioxygen and proton channels. Putative oxygen positions obtained from a Xenon derivative indicate a role for lipids in oxygen diffusion to the cytoplasmic side of PSII. The chloride position suggests a role in proton-transfer reactions because it is bound through a putative water molecule to the Mn(4)Ca cluster at a distance of 6.5 A and is close to two possible proton channels.