Isolation and characterization of Azotobacter vinelandii mutant strains with potential as bacterial fertilizer

Abstract

Mutant strains of Azotobacter vinelandii which might have potential for use as bacterial fertilizer have been isolated and fall into two categories: constitutive mutants that synthesize nitrogenase in the presence of ammonium and mutants that overproduce nitrogenase when grown in nitrogen-free medium. The constitutive mutants described in this paper were isolated from the wild type as methylalanine-resistant strains and express up to 23% of the fully derepressed nitrogenase level when grown in medium containing excess ammonium. By contrast, ammonium-grown cultures of wild type have less than 0.003% of the fully derepressed level. Strains which fix more N2 than the wild type in nitrogen-free medium were isolated as mefhylammonium-resistant mutants. Although the methylammonium-resistant mutant strains fix more N2 than the wild type, they grow no faster. The excess nitrogen produced by these mutants is excreted into the medium, resulting in up to 60% more nitrogen than in the medium of the wild type. Higher nitrogenase activity in the methylammonium-resistant mutant strains was found to be a result of increased levels of nitrogenase protein, suggesting that regulation of nitrogenase synthesis may be altered.