Mobile cytochrome c(2) and membrane-anchored cytochrome c(y) are both efficient electron donors to the cbb(3)- and aa(3)-type cytochrome c oxidases during respiratory growth of Rhodobacter sphaeroides

We have recently established that the facultative phototrophic bacterium Rh odobacter sphaeroides, like the closely related Rhodobacter capsulatus spec ies, contains both the previously characterized mobile electron carrier cyt ochrome c(2) (cyt c(2)) and the more recently discovered membrane-anchored cyt c(y). However, R. sphaeroides cyt c(y), unlike that of R. capsulatus, i s unable to function as an efficient electron carrier between the photochem ical reaction center and the cyt bc(1) complex during photosynthetic growth . Nonetheless, R. sphaeroides cyt c(y) can act at least in R. capsulatus as an electron carrier between the cyt bc(1) complex and the cbb(3)-type cyt c oxidase (cbb(3)-C-ox) to support respiratory growth. Since R. sphaeroides harbors both a cbb(3)-C-ox and an aa(3)-type cyt c oxidase (aa(3)-C-ox), w e examined whether R. sphaeroides cyt c(y) can act as an electron carrier t o either or both of these respiratory terminal oxidases, R. sphaeroides mut ants which lacked either cyt c(2) or cyt c(y) and either the aa(3)-C-ox or the cbb(3)-C-ox were obtained. These double mutants contained linear respir atory electron transport pathways between the cyt bc(1) complex and the cyt c oxidases. They were characterized with respect to growth phenotypes, con tents of a-, b-, and c-type cytochromes, cyt c oxidase activities, and kine tics of electron transfer mediated by cyt c(2) or cyt c(y). The findings de monstrated that both cyt c(2) and cyt c(y) are able to carry electrons effi ciently from the cyt bc(1) complex to either the cbb(3)-C-ox or the aa(3)-C -ox. Thus, no dedicated electron carrier for either of the cyt c oxidases i s present in R. sphaeroides. However, under semiaerobic growth conditions, a larger portion of the electron flow out of the cyt bc(1) complex appears to be mediated via the cyt c(2)-to-cbb(3)-C-ox and c(y)-to-cbb(3)-C-ox subb ranches. The presence of multiple electron carriers and cyt c oxidases with different properties that can operate concurrently reveals that the respir atory electron transport pathways of R. sphaeroides are more complex than t hose of R. capsulatus.