Transcriptional repression of gdhA in Escherichia coli is mediated by the Nac protein.

In this work we show that the nac gene from Escherichia coli is transcriptionally active, and that its expression is dependent on NRI (NtrC) and sigma-54. Northern blot experiments show a monocistronic nac-specific mRNA that is detected when wild-type cells are grown in nitrogen-limiting conditions. Our data also show that in nitrogen-limiting conditions Nac is involved in the transcriptional repression of the gdhA gene (encoding glutamate dehydrogenase) except when L-glutamine is used as the only nitrogen source. Moreover, the high level of GDH activity observed in a nac mutant strain is reduced when a wild-type nac gene is introduced under control of the lac promoter in N-limiting conditions, but not in L-glutamine or N-excess. These results suggest the existence of an additional mechanism responsible for overcoming repression by Nac.