EXPRESSION OF PTSG, THE GENE FOR THE MAJOR GLUCOSE PTS TRANSPORTER INESCHERICHIA-COLI, IS REPRESSED BY MIC AND INDUCED BY GROWTH ON GLUCOSE

The gene for the glucose-specific transporter of the phosphotransferas e system, ptsG, is expressed from two promoters separated by 141 bp. T he expression of the major, shorter transcript is very strongly depend ent upon cAMP/CAP. However, unlike other CAP-activated genes, the expr ession of ptsG is higher in glucose media than in glycerol, implying t hat ptsG is controlled by a glucose-inducible regulator. A mutation in the mle gene greatly enhances ptsG expression in a glycerol-grown cul ture but has no effect on ptsG expression during growth on glucose. Th e mle gene encodes a transcriptional regulator that has been shown to affect the expression of manXYZ and malT. ptsG mRNA levels are lower i n the mle strain grown on glucose than in the same strain grown on gly cerol. This is presumably because of the greater catabolite repression in the glucose culture than in glycerol. The final level of expressio n of ptsG in a mlc(+) strain in glucose is a compromise between specif ic induction by glucose and generalized catabolite repression. The res ult is that pfsG expression is very similar in glucose-grown cultures of wild-type and mle strains. The Mlc protein binds to two sites centr ed at -6 and -175 upstream of the major pfsG transcript. CAP binds at -40.5 compared with this site, typical of class II CAP-regulated promo ters, and the binding of CAP and Mlc is cc-operative.