An investigation into the influence of N on the expression of nitrogenase has led to a reassessment of the role of the nitrogenase MoFe protein in autoregulation. Anaerobic derepression of nitrogenase (CH-reducing) activity, of NifD and K polypeptides, and of expression, following the removal of excess NH , were greater under N than Ar. This enhancement occurred in Nif but not in Nif strains, and in Nif strains was prevented by CH, an inhibitor of N fixation. Thus N fixation is important for maintaining derepression. Derepression of under Ar in various Nif and Nif strains (including NifH, NifD, NifB and NifL mutants) and of wild-type under N or Ar in a Nif strain were measured to investigate the regulation. The mechanism regulating the enhancement under N neither involved the MoFe protein of nitrogenase, as proposed by Dixon (1980, 286, 128-132), nor the product, but was probably due to a general upgrading of the N status. Moreover, during batch growth limited by a non-repressing fixed N source, the levels of expression in the Nif and Nif strains suggested that the gene product (or Fe protein) may have a positive autoregulatory function.


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