Clade II nitrous oxide respiration of Wolinella succinogenes depends on the NosG, -C1, -C2, -H electron transport module, NosB and a Rieske/cytochrome bc complex.

Microbial reduction of nitrous oxide (N O) is an environmentally significant process in the biogeochemical nitrogen cycle. However, it has been recognized only recently that the gene encoding N O reductase (nosZ) is organized in varying genetic contexts, thereby defining clade I (or 'typical') and clade II (or 'atypical') N O reductases and nos gene clusters. This study addresses the enzymology of the clade II Nos system from Wolinella succinogenes, a nitrate-ammonifying and N O-respiring Epsilonproteobacterium that contains a cytochrome c N O reductase (cNosZ). The characterization of single non-polar nos gene deletion mutants demonstrated that the NosG, -C1, -C2, -H and -B proteins were essential for N O respiration. Moreover, cells of a W. succinogenes mutant lacking a putative menaquinol-oxidizing Rieske/cytochrome bc complex (QcrABC) were found to be incapable of N O (and also nitrate) respiration. Proton motive menaquinol oxidation by N O is suggested, supported by the finding that the molar yield for W. succinogenes cells grown by N O respiration using formate as electron donor exceeded that of fumarate respiration by about 30%. The results demand revision of the electron transport chain model of clade II N O respiration and challenge the assumption that NosGH(NapGH)-type iron-sulfur proteins are menaquinol-reactive.