Three Nitrogen Fixation Systems in Azotobacter vinelandii

Abstract

In the early 1980s we presented evidence indicating that the N2-fixing bacterium, Azotobacter vinelandii, contained at least two nitrogenase systems: the conventional Mo-containing nitrogenase (nitrogenase-1) system; and an alternative nitrogenase system expressed in the absence of Mo. This evidence primarily centered on the observation that Nif- (unable to fix N2) mutant strains underwent phenotypic reversal (i.e., Nif- to Nif+) under conditions of Mo deprivation. These reports (Bishop et al. 1980; Bishop et al. 1982; Page and Collinson 1982; Premakumar et al. 1984) were received with skepticism because they challenged the long-held belief that Mo was absolutely required for N2 fixation and that nitrogenases were essentially the same regardless of their source. The latter notion was further supported by the results of Southern blot experiments by Ruvkun and Ausubel (1980) which indicated that some of the structural genes encoding nitrogenases from diverse diazotrophic organisms were highly conserved at the nucleotide sequence level.