DNA binding sites for the Mlc and NagC proteins: regulation of nagE, encoding the N-acetylglucosamine-specific transporter in Escherichia coli.

The NagC and Mlc proteins are homologous transcriptional regulators that control the expression of several phosphotransferase system (PTS) genes in Escherichia coli. NagC represses nagE, encoding the N:-acetylglucosamine-specific transporter, while Mlc represses three PTS operons, ptsG, manXYZ and ptsHIcrr, involved in the uptake of glucose. NagC and Mlc can bind to each others operator, at least in vitro. A binding site selection procedure was used to try to distinguish NagC and Mlc sites. The major difference was that all selected NagC binding sites had a G or a C at positions +11/-11 from the centre of symmetry. This is also the case for most native NagC sites, but not the nagE operator, which thus looks like a potential Mlc target. The nagE operator does exhibit a higher affinity for Mlc than NagC, but no regulation of nagE by physiological concentrations of Mlc was detected in vivo. Regulation of wild-type nagE by NagC is achieved because of the chelation effect due to a second high affinity NagC operator covering the nagB promoter. Replacing the A/T at +11/-11 with C/G allows repression by NagC in the absence of the nagB operator.