A GLOBAL REPRESSOR (MIC) IS INVOLVED IN GLUCOSE INDUCTION OF THE PTSGGENE ENCODING MAJOR GLUCOSE-TRANSPORTER IN ESCHERICHIA-COLI

Glucose stimulates the expression of ptsG encoding the major glucose t ransporter in Escherichia coli. We isolated Tn 10 insertion mutations that confer constitutive expression of ptsG. The mutated gene was iden tified as mlc, encoding a protein that is known to be a repressor for transcription of several genes involved in carbohydrate utilization. E xpression of ptsG was eliminated in a mle crp double-negative mutant. The Mlc protein was overproduced and purified. In vitro transcription studies demonstrated that transcription of ptsG is stimulated by CRP-c AMP and repressed by Mlc. The action of Mlc is dominant over that of C RP-cAMP. DNase I footprinting experiments revealed that CRP-cAMP binds at two sites centred at -40.5 and -95.5 and that Mlc binds at two reg ions centred around -8 and -175. The binding of CRP-cAMP stimulated th e binding of RNA polymerase to the promoter while Mlc inhibited the bi nding of RNA polymerase but not the binding of CRP-cAMP. Gel-mobility shift assay indicated that glucose does not affect the Mlc binding to the ptsG promoter. Our results suggest that Mlc is responsible for the repression of ptsG transcription and that glucose modulates the Mlc a ctivity by unknown mechanism.