Induction of denitrifying enzymes in oxygen-limited Achromobacter cycloclastes continuous culture

Denitrifying enzyme synthesis and activity was studied in Achromobacter cycloclastes. Nitrite and nitrous oxide reductases were synthesized in response to anaerobiosis in the absence of nitrogenous oxides, but synthesis was promoted when a nitrogenous oxide was present. Nitrite reductase synthesis was promoted by NO₃^- and NO₂^-, while N₂O reductase synthesis was promoted by NO₂^- and N₂O. In continuous culture at dissolved O₂ concentrations ranging from air saturation to complete anaerobiosis, NO₃^- and N₂O reductase proteins were synthesized at all O₂ concentrations. Full induction occurred when dissolved O₂ concentrations fell below 2 μM. Nitrate reductase activity was present at all O₂ concentrations, but the N₂O reductase protein was not active until O₂ concentrations were decreased to 23 μM. Nitrite reduction was the most O₂-sensitive denitrifying step. Synthesis did not occur until dissolved O₂ was below 2 μM. The same pattern was observed for NO-reducing activity. The synthesis of the suite of denitrifying enzymes was not uniformly regulated by particular nitrogenous oxides as inducers, nor by similar O₂ concentrations. Because of the stringent control by O₂ on NO₂^- reductase synthesis, complete denitrification in this organism could not occur until O₂ concentrations were very low, approximately 2 μM (64 μg O₂ l^-1).