Identification of nitric oxide reductase activity in Rhodobacter capsulatus: The electron transport pathway can either use or bypass both cytochrome c₂ and the cytochrome bc₁ complex

Several strains of Rhodobacter capsulatus have been shown to possess a nitric oxide reductase activity (reaction product nitrous oxide) after anaerobic phototrophic growth, but not after aerobic growth. The reductase is associated with the cytoplasmic membrane and electrons can reach the enzyme via the cytochrome bc₁ complex. However, use of appropriate strains has shown that neither the latter, cytochrome c₂ nor cytochrome c' is essential for the reduction of nitric oxide. Inhibition by myxothiazol of nitric oxide reduction in a strain that lacks a cytochrome c₂ establishes that in phototrophically grown R. capsulatus the cytochrome bc₁, complex is able to transfer electrons to an acceptor that is alternative to cytochrome c₂. Electron transport to nitric oxide from NADH or succinate generated a membrane potential. When isoascorbate plus 2,3,5,6-tetramethyl-p-phenylenediamine (DAD) was the electron donor a membrane potential was not generated. This observation implies that nitric oxide is reduced at the periplasmic surface of the membrane and that the reductase is not proton translocating.