A Rhodobacter capsulatus mutant strain deficient in all pigment-binding peptides and hence incapable of photosynthetic growth was genetically complemented with a plasmid-borne copy of the Rhodobacter sphaeroides puf operon. Hybrid reaction centers composed of R. sphaeroides L and M and R. capsulatus H subunits assembled in vivo, and host cells were photosynthetically competent. Light-harvesting complex B875, also encoded by the R. sphaeroides puf operon, was present along with the hybrid reaction center. These cells emitted fluorescence, however, indicating an impairment in energy transduction.