Repression and induction of the nag regulon of Escherichia coli K-12: the roles of nagC and nagA in maintenance of the uninduced state.

The nag regulon located at 15.5 min on the Escherichia coli chromosome consists of two divergent operons, nagE and nagBACD, encoding genes involved in the uptake and metabolism of N-acetylglucosamine. Null mutations have been created in each of the genes by insertion of antibiotic resistance cartridges. The phenotypes of the strains carrying the insertions in nagE, B and A were consistent with the previous identification of gene products: nagE, EII(Nag), the N-acetylglucosamine specific transporter of the phosphotransferase system and nagB and nagA, the two enzymes necessary for the degradation of N-acetylglucosamine. Insertions in the nagC result in derepression of the nag genes, which is consistent with earlier observations that the nagC gene encodes the repressor of the regulon. Insertions in nagA also provoke a derepression, implying that nagA has a role in the regulation of the expression of the nag regulon as well as in the degradation of the amino-sugars. N-acetylglucosamine-6-phosphate, the intracellular product of N-acetylglucosamine transport and the substrate of the nagA gene product, is shown to be an inducer of the regulon and this suggests how nagA mutations result in derepression: the absence of N-acetylglucosamine-6-phosphate deacetylase allows N-acetylglucosamine-6-phosphate to accumulate and induce the regulon.