Mobile cytochrome c2 and membrane-anchored cytochrome cy are both efficient electron donors to the cbb3- and aa3-type cytochrome c oxidases during respiratory growth of Rhodobacter sphaeroides

We have recently established that the facultative phototrophic bacterium Rhodobacter sphaeroides, like the closely related Rhodobacter capsulatus species, contains both the previously characterized mobile electron carrier cytochrome 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, is unable to function as an efficient electron carrier between the photochemical 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), we examined whether R. sphaeroides cyt c(y) can act as an electron carrier to either or both of these respiratory terminal oxidases, R. sphaeroides mutants 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 respiratory electron transport pathways between the cyt bc(1) complex and the cyt c oxidases. They were characterized with respect to growth phenotypes, contents of a-, b-, and c-type cytochromes, cyt c oxidase activities, and kinetics of electron transfer mediated by cyt c(2) or cyt c(y). The findings demonstrated that both cyt c(2) and cyt c(y) are able to carry electrons efficiently 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 is 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 subbranches. The presence of multiple electron carriers and cyt c oxidases with different properties that can operate concurrently reveals that the respiratory electron transport pathways of R. sphaeroides are more complex than those of R. capsulatus.