Regulatory mutation that controls nif expression and histidine transport in Azospirillum brasilense

Mutagenesis of Azospirillum brasilense with nitrosoguanidine and selection on ethylenediamine yielded prototrophs which fixed nitrogen in the presence of ammonia. Nitrogenase activity in mutant strains exceeded that of the wild type three- to sixfold. The same mutants were also constitutive for histidine transport. Enzyme activities involved in ammonia assimilation were not affected by the mutation. The data suggest that the mutation occurred at a site which regulates nif and histidine transport functions.