J. Simon et al., A NapC/NirT-type cytochrome c (NrfH) is the mediator between the quinone pool and the cytochrome c nitrite reductase of Wolinella succinogenes, MOL MICROB, 35(3), 2000, pp. 686-696
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.