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

Abstract

The membrane integral ubihydroquinone (QH 2): cytochrome (cyt) c oxidoreductase (or the cyt bc 1 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 bc 1 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 bc 1 complex. Here, we report successful production, and initial characterization, of a functional cyt bc 1- c y fusion complex that supports photosynthetic growth of an appropriate R. capsulatus mutant strain. The three-subunit cyt bc 1- c y fusion complex has an unprecedented bis-heme cyt c 1- c y subunit instead of the native mono-heme cyt c 1, is efficiently matured and assembled, and can sustain cyclic electron transfer in situ. The remarkable ability of R. capsulatus cells to produce a cyt bc 1- c y fusion complex supports the notion that structural super complexes between photosynthetic or respiratory components occur to ensure efficient cellular energy production.