<p>NifL from <taxon tax_id="354">Azotobacter vinelandii</taxon> senses both the redox and fixed nitrogen status to regulate nitrogen fixation. NifL acts by modulating the activity of the nitrogen fixation positive regulator protein NifA: NifL inhibits NifA in response to oxygen and low level of fixed nitrogen. NifA and NifL are encoded by adjacent genes. NifL is FAD-containing, and has a domain architecture similar to that of the cytoplasmic histidine protein kinases, containing two N-terminal PAS domains and a C-terminal transmitter region containing a conserved histidine residue (H domain) and a nucleotide binding GHKL domain corresponding to the catalytic core of the histidine kinases [<cite idref="PUB00042901"/>]. However, NifL does not exhibit kinase activity and regulates its partner NifA by direct protein-protein interactions rather than phosphorylation. </p>