Redundant roles of Bradyrhizobium oligotrophicum Cu-type (NirK) and cd1-type (NirS) nitrite reductase genes under denitrifying conditions

Reduction of nitrite to nitric oxide gas by respiratory nitrite reductases (NiRs) is the key step of denitrification. Denitrifiers are strictly divided into two functional groups based on whether they possess the copper-containing nitrite reductase (CuNiR) encoded by nirK or the cytochrome cd1 nitrite reductase (cdNiR) encoded by nirS. Recently, some organisms carrying both nirK and nirS genes have been found. Bradyrhizobium oligotrophicum S58 is a nitrogen-fixing oligotrophic bacterium that carries a set of genes for complete denitrification of nitrate to dinitrogen, including nirK and nirS genes. We show that denitrification in S58 is functional under low-oxygen conditions (anaerobiosis and microaerobiosis), but not under aerobiosis. Under denitrifying conditions, the ΔnirK and ΔnirS single S58 mutants grew normally and their NiR activity was not affected. However, the ΔnirKS double mutant grew more slowly, presumably because the impaired NiR activity resulted in nitrite accumulation in the medium. These results suggest a redundant role for nirK and nirS genes in B. oligotrophicum S58 denitrification. In addition, we found that the nirS gene product, but not that of nirK, maintains swimming motility of S58 under aerobic and low-oxygen conditions in the presence of nitrate.