ELECTROGENICITY ACCOMPANIES PHOTOREDUCTION OF THE IRON-SULFUR CLUSTERS F-A AND F-B IN PHOTOSYSTEM-I

Photovoltage responses accompanying electron transfer on the acceptor side of photosystem I (PS I) were investigated in proteoliposomes cont aining PS I complexes from the cyanobacterium Synechococcus sp, PCC 63 01 using a direct electrometrical technique. The relative contribution s of the F-X-->F-B and the F-X-->F-A electron transfer reactions to th e overall electrogenicity were elucidated by comparing the sodium dith ionite-induced decrease in the magnitude of the total photoelectric re sponses in control and in F-B-less (HgCl2-treated) PS I complexes. The results obtained suggest that the electrogenesis on the acceptor side of PS I is related to electron transfers between both F-X and F-A and F-A and F-B. Based on the electrogenic nature of the latter reaction in PS I complexes, we conclude that F-A rather than F-B is the accepto r proximal to F-X. (C) 1998 Federation of European Biochemical Societi es.