Anaerobic ammonium oxidation (anammox) and denitrification are two distinct microbial reactions relevant to the global nitrogen cycle. The proposed initial step of the anammox reactions, reduction of nitrite to nitric oxide, has been postulated to be identical to that in denitrification catalyzed by the dissimilatory nitrite reductase of the cytochrome cd 1-type. Here, we characterized the copper-containing nitrite reductase homolog encoded by nirK detected in the genome of an anammox bacterium strain KSU-1. We hypothesize that this NirK-type nitrite reductase, rather than a nitrite reductase of the cytochrome cd 1-type (NirS), is likely to catalyze nitrite reduction in anammox organism KSU-1.
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