Participation of Nitrite Reductase in Conversion of NO2- to NO3 - in a Heterotrophic Nitrifier, Burkholderia cepacia NH-17, with Denitrification Activity

The metabolic characteristics of the NO2 - transforming activities of Burkholderia cepacia NH-17, which was isolated as a heterotrophic nitrifying bacterium with O2 tolerant denitrification activity, were characterized. The conversion of NO2- to N2O and NO3- occurred concomitantly with a decrease in NO2- under aerobic conditions in growing cultures. In an in vivo assay, production of N2O and NO3- was induced by NO2- as an inducer for denitrification, and nitrite reductase activity in sonicated fraction (NiR) assay indicated that in vitro nitrite reductase activity was also induced by NO2-. These results suggested that nitrification and denitrification in Burkholderia cepacia NH-17 might be closely related. Therefore, we constructed a nirS knockout mutant of Burkholderia cepacia NH-17. The mutant had no in vitro nitrite reductase activity and did not convert NO2 - to N2O and NO3-. These properties were restored by introducing the intact nirS gene into the mutant strain, indicating that reduction of NO2 - to NO is necessary for the conversion of NO2 - to NO3- in Burkholderia cepacia NH-17.