TRANSCRIPTIONAL REPRESSION OF GDHA IN ESCHERICHIA-COLI IS MEDIATED BYTHE NAC PROTEIN

In this work we show that the nac gene from Escherichia coli is transc riptionally active, and that its expression is dependent on NRI (NtrC) and sigma-54. Northern blot experiments show a monocistronic nac-spec ific mRNA that is detected when wild-type cells are grown in nitrogen- limiting conditions. Our data also show that in nitrogen-limiting cond itions Nar is involved in the transcriptional repression of the gdhA g ene (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 conditio ns, but not in L-glutamine or N-excess. These results suggest the exis tence of an additional mechanism responsible for overcoming repression by Nac. (C) 1998 Federation of European Microbiological Societies. Pu blished by Elsevier Science B.V. All rights reserved.