Direct interaction of the NifL regulatory protein with the GlnK signal transducer enables the Azotobacter vinelandii NifL-NifA regulatory system to respond to conditions replete for nitrogen.

The Azotobacter vinelandii NifL-NifA regulatory system integrates metabolic signals for redox, energy, and nitrogen status to fine tune regulation of the synthesis of molybdenum nitrogenase. The NifL protein utilizes discrete mechanisms to perceive these signals leading to the formation of a protein-protein complex, which inhibits NifA activity. Whereas redox signaling is mediated via a flavin-containing PAS domain in the N-terminal region of NifL, the nitrogen status is sensed via interaction with PII-like signal transduction proteins and small molecular weight effectors. The nonuridylylated form of the PII-like protein encoded by A. vinelandii glnK (Av GlnK) stimulates NifL to inhibit transcriptional activation by NifA in vitro. Here we demonstrate that the nonmodified form of Av GlnK directly interacts with A. vinelandii NifL and that this interaction is dependent on Mg(2+), ATP, and 2-oxoglutarate. Differences were observed in the regulation of the Av GlnK-NifL interaction by 2-oxoglutarate compared with the role of this effector in modulating the interaction of enteric PII-like proteins with their receptors. We also report that the interaction between Av GlnK and NifL is abolished by site-directed substitution of a single amino acid in the T-loop region of Av GlnK and that uridylylation of the conserved tyrosine residue in the T-loop inhibits the interaction. No association was detected between Av GlnK and the N-terminal region of NifL and our results demonstrate that Av GlnK directly interacts with the C-terminal histidine protein kinase-like domain.