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

摘要

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 c2 (cyt c2) and the more recently discovered membrane-anchored cyt cy. However, R. sphaeroides cyt cy, unlike that of R. capsulatus, is unable to function as an efficient electron carrier between the photochemical reaction center and the cyt bc1 complex during photosynthetic growth. Nonetheless, R. sphaeroides cyt cy can act at least in R. capsulatus as an electron carrier between the cyt bc1 complex and the cbb3-type cyt c oxidase (cbb3-Cox) to support respiratory growth. Since R. sphaeroides harbors both a cbb3-Cox and an aa3-type cyt c oxidase (aa3-Cox), we examined whether R. sphaeroides cyt cy can act as an electron carrier to either or both of these respiratory terminal oxidases. R. sphaeroides mutants which lacked either cyt c₂ or cyt c_y and either the aa₃-C_ox or the cbb₃-C_ox were obtained. These double mutants contained linear respiratory electron transport pathways between the cyt bc₁ 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 c₂ or cyt c_y. The findings demonstrated that both cyt c₂ and cyt c_y are able to carry electrons efficiently from the cyt bc₁ complex to either the cbb₃-C_ox or the aa₃-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 bc₁ complex appears to be mediated via the cyt c₂-to-cbb₃-C_ox and cyt c_y-to-cbb₃-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.