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

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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.