Anammox organism KSU-1 expresses a NirK-type copper-containing nitrite reductase instead of a NirS-type with cytochrome cd1

► A nirK-homologous gene of an anammox bacterium strain KSU-1 was expressed in E. coli. ► The recombinant NirK protein was biochemically characterized. ► We confirmed that the protein is an ortholog of Cu-containing nitrite reductase. ► NirK is likely to catalyze the proposed initial step of the anammox process.