Model of the molecular basis for hydroxylamine oxidation and nitrous oxide production in methanotrophic bacteria

Many methane-oxidizing bacteria (MOB) have been shown to aerobically oxidize ammonia and hydroxylamine (NH2OH) to produce nitrite and nitrous oxide (N2O). Genome sequences of alphaproteobacterial, gammaproteobacterial, and verrucomicrobial methanotrophs revealed the presence of haoAB, cytL, cytS, nirS or nirK, and norCB genes that may be responsible for N2O production, and additional haoAB genes were sequenced from two strains of Methylomicrobium album. The haoAB genes of M. album ATCC 33003 were inducible by ammonia and NH2OH, similar to haoAB induction by ammonia in Methylococcus capsulatus Bath. Increased expression of genes encoding nitric oxide reductase (cNOR; norCB) was measured upon exposure of M. capsulatus Bath to NaNO2 and NO-releasing sodium nitroprusside. Only incubations of M. capsulatus Bath with methane, ammonia, and nitrite produced N2O. The data suggest a possible pathway of nitrite reduction to NO by reversely operating NH2OH oxidoreductase and NO reduction to N2O by cNOR independently or in conjunction with ammonia-induced enzymes (i.e. HAO or cytochrome c'-beta-). Results of this study show that MOB likely have diverse mechanisms for nitrogen oxide metabolism and detoxification of NH2OH that involve conventional and unconventional enzymes.