Evidence for an alternative nitrogen fixation system in Azotobacter vinelandii.

Abstract

Two A. vinelandii strains capable of growing on N2 (Nif+) were isolated from 2 different mutant strains that lacked dinitrogenase activity (Nif-). Extracts of N2-grown cells of the 2 Nif+ strains lacked significant amounts of the conventional dinitrogenase protein subunits, as determined by 2-dimensional gel electrophoresis. Instead, the extracts contained at least 4 new proteins that appeared to be ammonia-repressible (i.e., they were not detected in extracts of ammonia-grown cells). Based on the results of genetic backcrosses, the 2 Nif+ strains were pseudorevertants. Both Nif+ strains were able to grow in N-free media lacking molybdenum but containing tungsten (conditions that prevented growth of the wild-type strain). The 4 new proteins were observed in extracts of N2-fixing cells of the Nif+ pseudorevertants regardless of whether the cells were grown in the presence of molybdenum or tungsten. The new proteins were also found in molybdenum-starved wild-type A. vinelandii cells grown under N2-fixing conditions. Under conditions of molybdenum deprivation, Nif- mutant strains of several different phenotypic classes underwent phenotypic reversal to Nif+, as shown by their ability to incorporate 15N2 and to grow in N-free media. These results provide evidence that A. vinelandii possesses an alternative N2-fixation system that is expressed during conditions of molybdenum deficiency.