The electron transport system of the facultative phototroph Rhodoferax fermentans. II. Flash-induced oxidation of membrane-bound cytochromes c

Abstract

Flash-induced oxidation of the membrane-bound c-type haems was studied in light-grown cells of Rhodoferax fermentans, a new taxon of the purple nonsulfur photosynthetic bacteria. At least three c-type cytochromes were found to rapidly (<20 μs) re-reduce the photo-oxidized primary donor of the reaction centre: a first haem peaking at 556 nm with E m = 354 mV, a second haem peaking at 560 nm with E m = 294 mV, and a third haem peaking at 551 nm with E m = 79 mV. The photo-oxidized minus reduced spectrum of the primary donor was found to be very similar to those of other purple bacteria. The primary donor midpoint potential was determined (471 mV) and o-phenanthroline was found to inhibit electron transfer to the secondary acceptor, thus indicating the presence of a Q-type reaction centre. The midpoint potential of the primary quinone acceptor was also determined (13 mV). A model accounting for the post-flash redox equilibria within the RC-cytochromes c chain is presented. A tetrahaem cytochrome c is proposed to operate in Rhodoferax fermentans and its spectral and thermodynamic features are discussed in relation with other species of purple photosynthetic bacteria.