The effects of several organic and inorganic nitrogen compounds on nitrogenase mRNA and enzyme activity levels were examined in anaerobic cultures of Anabaena variabilis 29413. Even low concentrations of exogenous ammonia (20 microM) prevented nitrogenase gene expression. Nitrate, in contrast, had little effect, even at very high concentrations. Neither compound had a significant direct effect on existing enzyme activity. The amino acids glutamine and glutamate did not repress nif gene expression. Methionine sulfoximine, but not 7-azatryptophan, was shown to eliminate the repressive effect of ammonia, and this action occurred at the mRNA level. Low concentrations of carbamyl phosphate caused a rapid decrease in nitrogenase mRNA levels. These results are consistent with the ideas that nif gene regulation in Anabaena spp. occurs primarily at the mRNA level and that ammonia, and possibly also glutamine and glutamate, is not the immediate effector of regulation.
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