The role of the NAC protein in the nitrogen regulation of Klebsiella aerogenes

Abstract

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 σ70‐dependent operons to regulation by the σ54‐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 σ54 sigma factor (RNAPσ54). Nitrogen starvation signals the NTR system to transcribe nac, and NAC activates the transcription of hut, put (pro‐line utilization), and urease. NAC does not affect the transcription of RNAP σ54‐dependent operons like gInA or nifLA, which respond directly to the NTR system, but activates transcription of RNAPσ70‐dependent operons. Thus NAC acts as a bridge between RNAPσ70‐dependent operons like hut and the RNAPσ54‐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.