SUMMARY: The effect of cultural conditions on the growth and nitrate reductase activity of a bacterium, now identified as , were examined. The bacterium grew well with ammonium chloride, ammonium nitrate or potassium nitrate as sole nitrogen source. Nitrate reductase activity was markedly decreased in organisms deficient in molybdenum or iron and in organisms grown at high oxygen pressures. The enzyme is thus similar to nitrate reductases from other bacteria. The enzyme is constitutive; it is present in organisms grown with ammonium chloride in the absence of nitrate; the ammonium radical did not depress enzyme activity. Under certain conditions (NHNO medium, 10%, v/v, O in N) molybdenum (10 μg./l.) depressed the nitrate reductase activity, but this effect was annulled by repeated subcultivation in molybdenum-deficient medium. Vanadium and especially tungsten, also inhibited the enzyme. The uptake of nitric oxide was decreased in organisms depleted of iron but was increased in organisms deficient in molybdenum. The behaviour of nitrate reductase preparations during fractionation whether from grown with KNO or with NHCl suggests that the enzyme is the same from both sources.


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