Noncoupled NADH:Ubiquinone Oxidoreductase of Azotobacter vinelandii Is Required for Diazotrophic Growth at High Oxygen Concentrations

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Abstract
The gene encoding the noncoupled NADH:ubiquinone oxidoreductase (NDH II) from Azotobacter vinelandii was cloned, sequenced, and used to construct an NDH II-deficient mutant strain. Compared to the wild type, this strain showed a marked decrease in respiratory activity. It was unable to grow diazotrophically at high aeration, while it was fully capable of growth at low aeration or in the presence of NH 4 + . This result suggests that the role of NDH II is as a vital component of the respiratory protection mechanism of the nitrogenase complex in A. vinelandii . It was also found that the oxidation of NADPH in the A. vinelandii respiratory chain is catalyzed solely by NDH II.