A NapC/NirT-type cytochrome c (NrfH) is the mediator between the quinone pool and the cytochrome c nitrite reductase of Wolinella succinogenes

Wolinella succinogenes can grow by anaerobic respiration with nitrate or ni trite using formate as electron donor. Two forms of nitrite reductase were isolated from the membrane fraction of W. succinogenes. One form consisted of a 58 kDa polypeptide (NrfA) that was identical to the periplasmic nitrit e reductase, The other form consisted of NrfA and a 22 kDa polypeptide (Nrf H). Both forms catalysed nitrite reduction by reduced benzyl viologen, but only the dimeric form catalysed nitrite reduction by dimethylnaphthoquinol. Liposomes containing heterodimeric nitrite reductase, formate dehydrogenas e and menaquinone catalysed the electron transport from formate to nitrite; this was coupled to the generation of an electrochemical proton potential (positive outside) across the liposomal membrane. It is concluded that the electron transfer from menaquinol to the catalytic subunit (NrfA) of W. suc cinogenes nitrite reductase is mediated by NrfH, The structural genes nrfA and nrfH were identified in an apparent operon (nrfHAIJ) with two additiona l genes. The gene nrfA encodes the precursor of NrfA carrying an N-terminal signal peptide (22 residues). NrfA (485 residues) is predicted to be a hyd rophilic protein that is similar to the NrfA proteins of Sulfurospirillum d eleyianum and of Escherichia coli, NrfH (177 residues) is predicted to be a membrane-bound tetrahaem cytochrome c belonging to the NapC/NirT family. T he products of nrfl and nrfJ resemble proteins involved in cytochrome c bio genesis, The C-terminal third of Nrfl (902 amino acid residues) is similar to CcsA proteins from Gram-positive bacteria, cyanobacteria and chloroplast s, The residual N-terminal part of Nrfl resembles Ccs1 proteins. The deduce d NrfJ protein resembles the thioredoxin-like proteins (ResA) of Helicobact er pylori and of Bacillus subtilis, but lacks the common motif CxxC of ResA , The properties of three deletion mutants of W, succinogenes (Delta nrfJ, Delta nrflJ and Delta nrfAlJ) were studied, Mutants Delta nrfAlJ and Delta nrflJ did not grow with nitrite as terminal electron acceptor or with nitra te in the absence of NH4+ and lacked nitrite reductase activity, whereas mu tant Delta nrfJ showed wild-type properties, The NrfA protein formed by mut ant Delta nrflJ seemed to lack part of the haem C, suggesting that Nrfl is involved in NrfA maturation.