title: Evidence for an alternative nitrogen fixation system in Azotobacter vinelandii.
abstract: Two Azotobacter vinelandii strains capable of growing on N2(Nif+) were isolated from two different mutant strains that lacked dinitrogenase activity (Nif-). Extracts of N2-grown cells of the two Nif+ strains lacked significant amounts of the "conventional" dinitrogenase protein subunits, as determined by two-dimensional gel electrophoresis. Instead, the extracts contained at least four 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 two Nif+ strains were shown to be pseudorevertants. Both Nif+ pseudorevertant strains were able to grow in N-free media lacking molybdenum but containing tungsten (conditions that prevented growth of the wild-type strain). The four 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-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.
mesh_terms: D001395:Azotobacter / Q000235:genetics / Q000378:metabolism*; D008982:Molybdenum / Q000378:metabolism; D009154:Mutation; D009586:Nitrogen Fixation; D009591:Nitrogenase / Q000235:genetics / Q000378:metabolism; D014414:Tungsten / Q000494:pharmacology
chemical_list: D008982:Molybdenum; D009591:Nitrogenase; D014414:Tungsten