The role of high-potential iron protein and cytochrome c(8) as alternative electron donors to the reaction center of Chromatium vinosum.

Abstract

Under anaerobic conditions, intact cells of the purple sulfur bacterium Chromatium vinosum exhibit rapid photooxidation of the two low-potential hemes of the c-type cytochrome associated with the reaction center, after exposure to two short light flashes separated by a dark interval. Reduction of the photooxidized low-potential hemes is very slow under these conditions. On subsequent flashes, rapid photooxidation of a high-potential reaction center heme occurs and is followed by its rereduction on the millisecond time scale. Cells maintained under aerobic conditions exhibit the millisecond time scale reduction of the photooxidized high-potential heme after each flash. Cells grown autotrophically in the presence of Na(2)S and Na(2)S(2)O(3) appear to use the soluble [4Fe-4S]-containing protein, HiPIP, as the only direct electron donor to the reaction center heme under aerobic conditions. In contrast, cells grown in the presence of organic compounds, but in the absence of Na(2)S and Na(2)S(2)O(3), appear to use a soluble c-type cytochrome (most likely cytochrome c(8)) as the only electron donor to the reaction center heme under aerobic conditions. Cells grown autotrophically, in the presence of Na(2)S and Na(2)S(2)O(3), have a slightly higher ratio of HiPIP to cytochrome c(8) and a ratio of Rieske iron-sulfur protein to reaction center that is approximately one-half that of cells grown in the absence of Na(2)S and Na(2)S(2)O(3) but in the presence of organic compounds.