The cytochrome bc1 complex of Rhodobacter sphaeroides can restore cytochrome c2-independent photosynthetic growth to a Rhodobacter capsulatus mutant lacking cytochrome bc1.

Plasmids encoding the structural genes for the Rhodobacter capsulatus and Rhodobacter sphaeroides cytochrome (cyt) bc1 complexes were introduced into strains of R. capsulatus lacking the cyt bc1 complex, with and without cyt c2. The R. capsulatus merodiploids contained higher than wild-type levels of cyt bc1 complex, as evidenced by immunological and spectroscopic analyses. On the other hand, the R. sphaeroides-R. capsulatus hybrid merodiploids produced only barely detectable amounts of R. sphaeroides cyt bc1 complex in R. capsulatus. Nonetheless, when they contained cyt c2, they were capable of photosynthetic growth, as judged by the sensitivity of this growth to specific inhibitors of the photochemical reaction center and the cyt bc1 complex, such as atrazine, myxothiazol, and stigmatellin. Interestingly, in the absence of cyt c2, although the R. sphaeroides cyt bc1 complex was able to support the photosynthetic growth of a cyt bc1-less mutant of R. capsulatus in rich medium, it was unable to do so when C4 dicarboxylic acids, such as malate and succinate, were used as the sole carbon source. Even this conditional ability of R. sphaeroides cyt bc1 complex to replace that of R. capsulatus for photosynthetic growth suggests that in the latter species the cyt c2-independent rereduction of the reaction center is not due to a structural property unique to the R. capsulatus cyt bc1 complex. Similarly, the inability of R. sphaeroides to exhibit a similar pathway is not due to some inherent property of its cyt bc1 complex.