Our earlier studies with the pgsA mutant of Synechocystis PCC6803 demonstrated the important role of phosphatidylglycerol (PG) in PSII dimer formation and in electron transport between the primary and secondary electron-accepting plastoquinones of PSII. Using a long-term depletion of PG from pgsA mutant cells, we could induce a decrease not only in PSII but also in PSI activity. Simultaneously with the decrease in PSI activity, dramatic structural changes of the PSI complex were detected. A 21-d PG depletion resulted in the degradation of PSI trimers and concomitant accumulation of monomer PSI. The analyses of PSI particles isolated by MonoQ chromatography showed that, following the 21-d depletion, PSI trimers were no longer detectable in the thylakoid membranes. Immunoblot analyses revealed that the PSI monomers accumulating in the PG-depleted mutant cells do not contain PsaL, the protein subunit thought to be responsible for the trimer formation. Nevertheless, the trimeric structure of PSI reaction center could be restored by readdition of PG, even in the presence of the protein synthesis inhibitor lincomycin, indicating that free PsaL was present in thylakoid membranes following the 21-d PG depletion. Our data suggest an indispensable role for PG in the PsaL-mediated assembly of the PSI reaction center.Phosphatidylglycerol (PG) is an integral component of photosynthetic membranes. PG molecules are important for both the formation and functioning of photosynthetic apparatus. In cyanobacterial cells, PG is the only representative of the phospholipid family (Wada and Murata, 1998), and the majority of PG molecules are localized in the thylakoid membranes that are the site of oxygenic electron transport. Photosynthesis is the basic energy source of cyanobacteria and generally for photosynthetic organisms. The availability of the complete genomic sequence of Synechocystis PCC6803 (Kaneko et al., 1996) opened the way for studying the structural and functional roles of PG via molecular genetic approaches. The pgsA gene encoding PG phosphate synthase was inactivated in Synechocystis PCC6803 cells by inserting a kanamycin resistance gene cassette (Hagio et al., 2000). Maintenance of the recently generated pgsA mutant strain requires exogenously supplied PG. A 40% decrease in photosynthetic oxygen-evolving activity could be detected following a 3-d depletion of PG, which resulted in an approximately 50% decrease in the amount of PG molecules in the cellular membranes. The photosynthetic processes, as measured by fluorescence induction in the PG-depleted cells, slowed down, and a perturbation of the surroundings of the secondary quinone acceptor (Q B ) was observed (Gombos et al., 2002). This perturbation was similar to the effect of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), which can block electron transport between primary quinone acceptor (Q A ) and Q B . Additionally, Fragata et al. (1991) demonstrated that in isolated barley (Hordeum vulgare) PSII particles, oxygen-evolving activity strongly depends on the pr...

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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.