The membrane-attached electron carrier cytochrome c_y from Rhodobacter sphaeroides is functional in respiratory but not in photosynthetic electron transfer

Rhodobacter species are useful model organisms for studying the structure and function of c type cytochromes (Cyt c), which are ubiquitous electron carriers with essential functions in cellular energy and signal transduction. Among these species, Rhodobacter capsulatus has a periplasmic Cyt C₂^Rc and a membrane-bound bipartite Cyt c_y^Rc. These electron carriers participate in both respiratory and photosynthetic electron-transfer chains. On the other hand, until recently, Rhodobacter sphaeroides was thought to have only one of these two cytochromes, the soluble Cyt c₂^Rs. Recent work indicated that this species has a gene, cycY^Rs, that is highly homologous to cycY^Rc, and in the work presented here, functional properties of its gene product (Cyt c_y^Rs) are defined. It was found that Cyt c_y^Rs is unable to participate in photosynthetic electron transfer, although it is active in respiratory electron transfer, unlike its R. capsulatus counterpart, Cyt c_y^Rc. Chimeric constructs have shown that the photosynthetic incapability of Cyt c_y^Rs is caused, at least in part, by its redox active subdomain, which carries the covalently bound heme. It, therefore, seems that this domain interacts differently with distinct redox partners, like the photochemical reaction center and the Cyt c oxidase, and allows the bacteria to funnel electrons efficiently to various destinations under different growth conditions. These findings raise an intriguing evolutionary issue in regard to cellular apoptosis: why do the mitochondria of higher organisms, unlike their bacterial ancestors, use only one soluble electron carrier in their respiratory electron-transport chains?