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Role of nitrite reductase in the ammonia-oxidizing pathway of Nitrosomonas europaea

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Metabolism of ammonia (NH3) and hydroxylamine (NH2OH) by wild-type and a nitrite reductase (nirK) deficient mutant of Nitrosomonas europaea was investigated to clarify the role of NirK in the NH3 oxidation pathway. NirK-deficient N. europaea grew more slowly, consumed less NH3, had a lower rate of nitrite (NO2 ) production, and a significantly higher rate of nitrous oxide (N2O) production than the wild-type when incubated with NH3 under high O2 tension. In incubations with NH3 under low O2 tension, NirK-deficient N. europaea grew more slowly, but had only modest differences in NH3 oxidation and product formation rates relative to the wild-type. In contrast, the nirK mutant oxidized NH2OH to NO2 at consistently slower rates than the wild-type, especially under low O2 tension, and lost a significant pool of NH2OH–N to products other than NO2 and N2O. The rate of N2O production by the nirK mutant was ca. three times higher than the wild-type during hydrazine-dependent NO2 reduction under both high and low O2 tension. Together, the results indicate that NirK activity supports growth of N. europaea by supporting the oxidation of NH3 to NO2 via NH2OH, and stimulation of hydrazine-dependent NO2 reduction by NirK-deficient N. europaea indicated the presence of an alternative, enzymatic pathway for N2O production.

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Hydroxylamine oxidoreductase


Ammonia monooxygenase


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The authors gratefully acknowledge Dr. Hubertus J. E. Beaumont for providing the nirK strain of N. europaea. We thank Miyuki Yamamoto and Stuart Le for assistance with the physiology assays and Norman Hommes and the reviewers for critical reading of the manuscript. This work was supported with funding from the Agricultural Experiment Station and Academic Senate of the University of California, Riverside.

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Correspondence to Lisa Y. Stein.

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Communicated by Jack Meeks.

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Cantera, J.J.L., Stein, L.Y. Role of nitrite reductase in the ammonia-oxidizing pathway of Nitrosomonas europaea . Arch Microbiol 188, 349–354 (2007).

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