PARENT SESSION
Posters P7B Evolution of photosynthesis. Abstracts (579-591)

Interaction of acidic lipids with the Photosystem complexes and its evolution. Norihiro Sato^*,1, Kouichi Sugimoto¹, Ayano Meguro¹, Kunihiro Suda¹, Mikio Tsuzuki¹, ¹ School of Life Science, Tokyo University of Pharmacy and Life Science, Hachioji, Tokyo, Japan

ABSTRACT- Chloroplasts and their postulated ancestors, cyanobacteria, contain sulfoquinovosyl diacylglycerol (SQDG) and phosphatidylglycerol (PG) as acidic membrane lipids. SQDG, despite its prevalent distribution among photosynthetic organisms, has been proved to play roles that are not necessarily common, but rather depend on species (refer to the poster by M. Aoki et al. in this congress). We cloned a cDNA for UDP-sulfoquinovose synthase from a green alga, Chlamydomonas reinhardtii, for molecular phylogenetic analysis as to the enzyme to find grouping of C. reinhardtii together with species that require SQDG for the functioning of the PSII complex, but not with those that do not utilize the lipid for photosynthesis. The results implied that the roles of SQDG have evolved in harmony with the system of the membrane lipid synthesis such as the UDP-sulfoquinovose synthase gene. On the other hand, PG has been shown to play roles quite distinct from those of SQDG as was represented by the chlorophyll-less phenotype of cyanobacterial mutants defective in PG synthesis. With the use of the PG-mutant of Synechocystis sp. PCC6803, we performed Western analysis concerning PSI and PSII and non-denaturing dodecylmaltoside-PAGE to show that the decrease in the chlorophyll content concomitantly with in vivo deprivation of PG was due predominantly to a quantitative reduction in the PSI complex. Besides, the mutant failed to accumulate the PSI trimer, which usually exists to function in cyanobacteria, with an increase in the level of its monomer, but maintained the PSI activity measured with diaminodurene and methylviologen as the electron donor and acceptor, respectively. Similar results, i.e. dissociation of the PSI trimer into the monomer with little deleterious effects on the PSI activity, were observed also with wild-type thylakoid membranes after in vitro deprivation of PG by the action of a phospholipase. These results obtained through in vivo and in vitro deprivation of PG demonstrated that PG contributes to quantitative and conformational maintenance of the PSI complex. Since the PSI complex in plants exists as the monomeric form, it was considered that the role of PG at least in the conformational regulation to construct the trimeric form has been lost during evolution from cyanobacteria to chloroplasts of plants.

KEY WORDS: Synechocystis, Chlamydomonas, sulfoquinovosyl diacylglycerol, phosphatidylglycerol