THE ROLE OF THE NAC PROTEIN IN THE NITROGEN REGULATION OF KLEBSIELLA-AEROGENES

The NAC (nitrogen assimilation control) protein from Klebsiella aerogenes is a LysR-like regulator for transcription of several operons involved in nitrogen metabolism, and couples the transcription of these sigma-70-dependent operons to regulation by the sigma-54-dependent NTR system. NAC activates expression of operons (e.g. histidine utilization, hut), allowing use of poor nitrogen sources, and represses expression of operons (e.g. glutamate dehydrogenase, gdh) allowing assimilation of the preferred nitrogen source, ammonium. NAC is both necessary and sufficient to activate transcription, but the expression of the nac gene is totally dependent on the central nitrogen regulatory system (NTR) and RNA polymerase carrying the sigma-54 sigma factor (RNAP-sigma-54). Nitrogen starvation signals the NTR system to transcribe nac, and NAC activates the transcription of hut, put (proline utilization), and urease. NAC does not affect the transcription of RNAP-sigma-54-dependent operons like glnA or nifLA, which respond directly to the NTR system, but activates transcription of RNAP-sigma-70-dependent operons. Thus NAC acts as a bridge between RNAP-sigma-70-dependent operons like hut and the RNAP-sigma-54-dependent NTR system. The activation of operons like hut by NAC in response to nitrogen starvation is at least superficially similar to their activation by CAP-cAMP in response to carbon and energy starvation.