Expression of ptsG, the gene for the major glucose PTS transporter in Escherichia coli, is repressed by Mlc and induced by growth on glucose.

The gene for the glucose-specific transporter of the phosphotransferase system, ptsG, is expressed from two promoters separated by 141 bp. The expression of the major, shorter transcript is very strongly dependent upon cAMP/CAP. However, unlike other CAP-activated genes, the expression of ptsG is higher in glucose media than in glycerol, implying that ptsG is controlled by a glucose-inducible regulator. A mutation in the mlc gene greatly enhances ptsG expression in a glycerol-grown culture but has no effect on ptsG expression during growth on glucose. The mlc gene encodes a transcriptional regulator that has been shown to affect the expression of manXYZ and malT. ptsG mRNA levels are lower in the mlc strain grown on glucose than in the same strain grown on glycerol. This is presumably because of the greater catabolite repression in the glucose culture than in glycerol. The final level of expression of ptsG in a mlc+ strain in glucose is a compromise between specific induction by glucose and generalized catabolite repression. The result is that ptsG expression is very similar in glucose-grown cultures of wild-type and mlc strains. The Mlc protein binds to two sites centred at -6 and -175 upstream of the major ptsG transcript. CAP binds at -40.5 compared with this site, typical of class II CAP-regulated promoters, and the binding of CAP and Mlc is co-operative.