Intermonomer Electron Transfer between the Low-Potential b Hemes of Cytochrome bc1

Cytochrome (cyt) bc1 is a structural dimer with its monomers consisting of the Fe-S protein, cyt b, and cyt c1 subunits. Its three-dimensional architecture depicts it as a symmetrical homodimer, but the mobility of the head domain of the Fe-S protein indicates that the functional enzyme exists in asymmetrical heterodimeric conformations. Here, we report a new genetic system for studying intra- and intermonomer interactions within the cyt bc1 using the facultative phototrophic bacterium Rhodobacter capsulatus. The system involves two different sets of independently expressed cyt bc1 structural genes carried by two plasmids that are coharbored by a cell without its endogenous enzyme. Our results indicate that coexpressed cyt bc1 subunits were matured, assorted, and assembled in vivo into homo- and heterodimeric enzymes that can bear different mutations in each monomer. Using the system, the occurrence of intermonomer electron transfer between the low-potential b hemes of cyt bc1 was probed by choosing mutat...