Membrane-associated cytochrome c(y) of Rhodobacter capsulatus is an electron carrier from the cytochrome bc1 complex to the cytochrome c oxidase during respiration

We have recently established that the facultative phototrophic bacterium Rhodobacter capsulatus has two different pathways for reduction of the photooxidized reaction center during photosynthesis (F. E. Jenney and F. Daldal, EMBO J. 12:1283-1292, 1993; F. E. Jenney, R. C. Prince, and F. Daldal, Biochemistry 33:2496-2502, 1994). One pathway is via the well- characterized, water-soluble cytochrome c2 (cyt c2), and the other is via a novel membrane-associated c-type cytochrome named cyt c(y). In this work, we probed the role of cyt c(y) in respiratory electron transport by isolating a set of R. capsulatus mutants lacking either cyt c2 or cyt c(y), in the presence or in the absence of a functional quinol oxidase-dependent alternate respiratory pathway. The growth and inhibitor sensitivity patterns of these mutants, their respiratory rates in the presence of specific inhibitors, and the oxidation-reduction kinetics of c-type cytochromes monitored under appropriate conditions demonstrated that cyt c(y), like cyt c2, connects the Bc1 complex and the cyt c oxidase during respiratory electron transport. Whether cyt c2 and cyt c(y) are the only electron carriers between these two energy-transducing membrane complexes of R. capsulatus is unknown.