<p>This entry represents NifA, a DNA-binding regulatory protein for nitrogen fixation. Not included in this group are: the homologue in <taxon tax_id="63363">Aquifex aeolicus</taxon> (which lacks nitrogenase), transcriptional activators of alternative nitrogenases (VFe or FeFe instead of MoFe), and truncated forms.</p><p>In diazotrophic proteobacteria, the sigma54-dependent activator NifA activates transcription of the nif (nitrogen fixation) genes by a conserved mechanism common to members of the enhancer binding protein family. Although NifA proteins have similar domain structures, both transcriptional regulation of nifA expression and posttranslational regulation of NifA activity by oxygen and fixed nitrogen vary significantly from one organism to another. In <taxon tax_id="573">Klebsiella pneumoniae</taxon> and <taxon tax_id="354">Azotobacter vinelandii</taxon>, nifA is co-ordinately transcribed with a second gene, nifL, whose product inhibits NifA activity in response to oxygen and fixed nitrogen [<cite idref="PUB00015127"/>].</p>