Unusual high redox potential ferredoxins and soluble cytochromes from the moderately halophilic purple phototrophic bacterium Rhodospirillum salinarum

Abstract

Two soluble cytochromes and two high redox potential ferredoxins (HiPIP) were purified from extracts of the halophilic non-sulfur purple phototrophic bacterium Rhodospirillum salinarum. All four proteins are highly acidic, as are those from other halophilic phototrophs. Cytochrome c′ is otherwise like those of other species. Cytochrome c-551 has a low redox potential (−143 mV), and appears to be monomeric (12 kDa). We did not find a protein similar to the cytochrome c 2 in most species of non-sulfur purple bacteria. The two HiPIP isozymes in R. salinarum differ in native molecular weight (iso-1, 10000 and iso-2, 45000), although iso-2 HiPIP may be a tetramer (subunit size 11 kDa). The redox potential of Iso-1 HiPIP is 265 mV, but iso-2 HiPIP is labile to ferricyanide and other oxidants tested, thus the redox potential was not measured. This is to our knowledge the first report of either an aggregated or a labile HiPIP. Abundant membrane bound c-type cytochromes were observed in R. salinarum by difference spectroscopy. About one-half of the heme (alpha peak maximum 553 nm) could be reduced by ascorbate, whereas the remainder of the heme was reduced by dithionite (552 nm maximum). These observations suggest the presence in R. salinarum of a tetraheme reaction center cytochrome which overshadows the cytochrome bc 1 complex.