Signal transduction to the Azotobacter vinelandii NIFL-NIFA regulatory system is influenced directly by interaction with 2-oxoglutarate and the PII regulatory protein

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Abstract
PII‐like signal transduction proteins, which respond to the nitrogen status via covalent modification and signal the carbon status through the binding of 2‐oxoglutarate, have been implicated in the regulation of nitrogen fixation in several diazotrophs. The NIFL–NIFA two‐component regulatory system, which integrates metabolic signals to fine‐tune regulation of nitrogenase synthesis in Azotobacter vinelandii, is a potential target for PII‐mediated signal transduction. Here we demonstrate that the inhibitory activity of the A.vinelandii NIFL protein is stimulated by interaction with the non‐uridylylated form of PII‐like regulatory proteins. We also observe that the NIFL–NIFA system is directly responsive to 2‐oxoglutarate. We propose that the PII protein signals the nitrogen status by interaction with the NIFL–NIFA system under conditions of nitrogen excess, and that the inhibitory activity of NIFL is relieved by elevated levels of 2‐oxoglutarate when PII is uridylylated under conditions of nitrogen limitation. Our observations suggest a model for signal transduction to the NIFL–NIFA system in response to carbon and nitrogen status which is clearly distinct from that suggested from studies on other diazotrophs.