Autoregulation of nitrogenase expression in Klebsiella pneumoniae

An investigation into the influence of N2 on the expression of Klebsiella pneumoniae nitrogenase has led to a reassessment of the role of the nitrogenase MoFe protein in autoregulation. Anaerobic derepression of nitrogenase (C2H2-reducing) activity, of NifD and K polypeptides, and of nifH-lac expression, following the removal of excess NH+4, were greater under N2 than Ar. This enhancement occurred in Nif+ but not in Nif- strains, and in Nif+ strains was prevented by C2H2, an inhibitor of N2 fixation. Thus N2 fixation is important for maintaining derepression. Derepression of nifH-lac under Ar in various Nif+ and Nif- strains (including NifH-, NifD-, NifB- and NifL- mutants) and of wild-type lac under N2 or Ar in a Nif+ strain were measured to investigate the regulation. The mechanism regulating the enhancement under N2 neither involved the MoFe protein of nitrogenase, as proposed by Dixon et al. (1980, Nature 286, 128-132), nor the nifL 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 nifH-lac expression in the Nif+ and Nif- strains suggested that the nifH gene product (or Fe protein) may have a positive autoregulatory function.