The membrane-bound cytochrome c(y) of Rhodobacter capsulatus can serve as an electron donor to the photosynthetic reaction center of Rhodobacter sphaeroides

Rhodobacter capsulatus has two different pathways for reduction of the photo-oxidized reaction center, one using water-soluble cytochrome c(2), the other via membrane-associated cytochrome c(y). Rhodobacter sphaeroides differs in that it lacks a cytochrome c(y) homologue capable of functioning in photosynthetic electron transfer; cytochrome c(2) is thus the sole electron carrier, and is required for photosynthetic (Ps(+)) growth. Genetic evidence indicates that cytochrome c(y) of R. capsulatus can complement a Ps(-) cytochrome-c(2)-deficient mutant of R. sphaeroides (Jenney, F.E. and Daldal, F. (1993) EMBO J. 12, 1283-1292). Here, we show that it transfers electrons from the cytochrome bc(1) complex to the reaction center in R. sphaeroides? albeit at a lower rate than that catalyzed by the endogenous cytochrome c(2). When cytochrome c(y) is expressed in R. sphaeroides in the presence of cytochrome c(2), there is an increase in the amount of photo-oxidizible c-type cytochrome. In the absence of cytochrome c(2), electron transfer via cytochrome c(y) shows significantly different kinetics for reaction center reduction and cytochrome c oxidation. These findings further establish that cytochrome c(y), the electron carrier permitting soluble cytochrome c(2)-independent photosynthetic growth in R. capsulatus, can function in a similar capacity in R. sphaeroides.