Noncoupled NADH : ubiquinone oxidoreductase of Azotobacter vinelandii is required for diazotrophic growth at high oxygen concentrations

The gene encoding the noncoupled NADH:ubiquinone oxidoreductase (NDH II) fr om Azotobacter vinelandii was cloned, sequenced, and used to construct an N DH II-deficient mutant strain. Compared to the wild type, this strain showe d a marked decreased in respiratory activity. It was unable to grow diazotr ophically at high aeration, while it was fully capable of growth at low aer ation or in the presence of NH4+. 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.