Nitrite Reductase of Nitrosomonas europaea Is Not Essential for Production of Gaseous Nitrogen Oxides and Confers Tolerance to Nitrite

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Abstract
A gene that encodes a periplasmic copper-type nitrite reductase (NirK) was identified in Nitrosomonas europaea . Disruption of this gene resulted in the disappearance of Nir activity in cell extracts. The nitrite tolerance of NirK-deficient cells was lower than that of wild-type cells. Unexpectedly, NirK-deficient cells still produced nitric oxide (NO) and nitrous oxide (N 2 O), the latter in greater amounts than that of wild-type cells. This demonstrates that NirK is not essential for the production of NO and N 2 O by N. europaea . Inactivation of the putative fnr gene showed that Fnr is not essential for the expression of nirK .