Expression of an alternative nitrogen fixation system in Azotobacter vinelandii

Nitrogenase activities were determined from maximum acetylene reduction rates for mutant strains of Azotobacter vinelandii which are unable to fix N2 in the presence of molybdenum (Nif-) but undergo phenotypic reversal to Nif+ under conditions of Mo deficiency. The system responsible for N2 fixation under these conditions is thought to be an alternative N2 fixation system (Bishop et al., Proc. Natl. Acad. Sci. U.S.A. 77:7342-7346, 1980). Phenotypic reversal of Nif- strains to Nif+ strains was also observed in N-free medium without Mo but with either V or Re. Two protein patterns were found on two-dimensional gels of proteins from the extracts of wild-type cells cultured in N-free medium without Mo and with or without V or Re. The expression of each protein pattern in the wild-type strain of A. vinelandii seemed to depend upon the physiological state of the N2-fixing culture. Electron paramagnetic resonance experiments were conducted on whole cells of A. vinelandii grown under conditions of Mo deprivation in the absence of fixed N. No g = 3.65 signal (an electron paramagnetic resonance signal characteristic of the Mo-containing component of nitrogenase) was detectable in these cells, regardless of whether V or Re was present during growth of these cells, These results are discussed from the perspective that the well-known effect of V on N2 fixation by A. vinelandii may involve an alternative N2 fixation system.