A functional hybrid between the cytochrome bc₁ complex and its physiological membrane-anchored electron acceptor cytochrome c_y in Rhodobacter capsulatus

The membrane integral ubihydroquinone (QH₂): cytochrome (cyt) c oxidoreductase (or the cyt bc₁ complex) and its physiological electron acceptor, the membrane-anchored cytochrome c_y (cyt c_y), are discrete components of photosynthetic and respiratory electron transport chains of purple non-sulfur, facultative phototrophic bacteria of Rhodobacter species. In Rhodobacter capsulatus, it has been observed previously that, depending on the growth condition, absence of the cyt bc₁ complex is often correlated with a similar lack of cyt c_y (Jenney, F. E., et al. (1994) Biochemistry 33, 2496-2502), as if these two membrane integral components form a non-transient larger structure. To probe whether such a structural super complex can exist in photosynthetic or respiratory membranes, we attempted to genetically fuse cyt c_y to the cyt bc₁ complex. Here, we report successful production, and initial characterization, of a functional cyt bc₁-c_y fusion complex that supports photosynthetic growth of an appropriate R. capsulatus mutant strain. The three-subunit cyt bc₁-c_y fusion complex has an unprecedented bis-heme cyt c₁-c_y subunit instead of the native mono-heme cyt c₁, is efficiently matured and assembled, and can sustain cyclic electron transfer in situ. The remarkable ability of R. capsulatus cells to produce a cyt bc₁-c_y fusion complex supports the notion that structural super complexes between photosynthetic or respiratory components occur to ensure efficient cellular energy production.