Regulation of nitrogen fixation by ammonium in diazotrophic species of proteobacteria

Nitrogenase enzyme is not synthesized in diazotrophs growing with a sufficient or excess supply of fixed nitrogen in their environment. In addition, in some bacteria the nitrogenase enzyme is inactivated by ammonium. The mechanisms by which cells respond to ammonium have been studied in several members of the Proteobacteria, including species from three of the four subgroups of this major taxon. In all of them, NifA is a transcriptional activator of expression of all other nif genes, the products of which are required for synthesis of active nitrogenase. NifA is either inactivated or not synthesized in ammonium-grown cultures of the Proteobacterial diazotrophs. The mechanisms by which NifA synthesis and activity are controlled by fixed nitrogen vary widely, as does the function of the PII protein, a major component of the nitrogen regulatory cascade, with respect to nitrogen fixation.