ELECTROGENIC REDUCTION OF THE PRIMARY ELECTRON-DONOR P700(-I BY REDOXDYES() IN PHOTOSYSTEM)

The kinetics of reduction of the photo-oxidized primary electron donor P700(+) by redox dyes N,N,N',N'-tetramethyl-p-phenylendiamine, 2,6-di chlorophenol-indophenol and phenazine methosulfate was studied in prot eoliposomes containing Photosystem I complexes from cyanobacteria Syne -chocystis sp. PCC 6803 using direct electrometrical technique, In the presence of high concentrations of redox dyes, the fast generation of a membrane potential related to electron transfer between P700 and th e terminal iron-sulfur clusters F-A/F-B was followed by a new electrog enic phase in the millisecond time domain, which contributes approxima tely 20% to the overall photoelectric response, This phase is ascribed to the vectorial transfer of an electron from the redox dye to the pr otein-embedded chlorophyll of P700(+), Since the contribution of this electrogenic phase in the presence of artificial redox dyes is approxi mately equal to that of the phase observed earlier in the presence of cytochrome c(6), it is likely that electrogenic reduction of P700(+) i n vivo occurs due to vectorial electron transfer within RC molecule ra ther than within the cytochrome c(6)-P700 complex. (C) 1997 Federation of European Biochemical Societies.