Nitrogen Fixation by Azotobacter vinelandii Strains Having Deletions in Structural Genes for Nitrogenase.

Phenotypic reversal of Nif(-) mutant strains to Nif(+) under molybdenum-deficient conditions has been cited as evidence that Azotobacter vinelandii possesses two nitrogen fixation systems: the conventional molybdenum-enzyme system and an alternative nitrogen-fixation system. Since explanations other than the existence of an alternative system were possible, deletion strains of A. vinelandii lacking the structural genes for conventional nitrogenase (nifHDK) were constructed. These strains were found to grow in molybdenum-deficient nitrogen-free media, reduce acetylene (at low rates), and incorporate molecular nitrogen labeled with nitrogen-15. Thus it can be concluded that the phenotypic reversal phenomenon cannot be due to altered phenotypic expression of nif mutations under molybdenum-deficient conditions, but is due to the existence of an alternative nitrogen-fixation system in A. vinelandii as originally proposed.