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

Abstract

ABSTRACT 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 cyt 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 .