PHOTOSYNTHESIS AT DAWN

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The origin and evolution of photosynthesis have long remained enigmatic. To probe early evolutionary history of photosynthesis, we have investigated (B)Chl synthesis, biogenesis of photosynthetic apparatus, and electron transport of purple bacteria, green sulfur bacteria, heliobacteria and cyanobacteria. The summary of this study are as follows (Only the main results are shown).(1)Chlorophyll synthesis : A number of genes involved in (B)Chl synthesis were isolated from various phototrophic prokaryote. Homologues of bchF and bchG from green-sulfur bacterium Chlorobium tepidum were introduced and expressed in non-sulfur bacterium Rhodobacter capsulatus and cyanobacteria Synechocystis sp.PCC6803. Novel pigments accumulated in the cells of R.capsulatus and Synechocystis.(2)Biogenesis of photosynthetic apparatus : Assembly factors of Iron-sulfur cluster, IscR and IscS, from C.tepidum were overexpressed and characterized. Ferredoxin and iron-sulfur type RC were isolated from heliobacteria and they supported NADP^+ photoreduction in vitro. Nitrogenase-like enzyme, Dark POR, was isolated from R.capsulatus and characterized its biochemical properties.(3)Electron transport : Bound cytochrome of Rubrivivax gelatinosus belongs tri-heme type, while that of Blastochloris viridis is tetra-heme type. Chimeric RC of R.gelatinosus which contained B.viridis cytochrome subunit was constructed and investigated their electron transport.