Negative regulation of the pts operon by Mlc: Mechanism underlying glucose induction in Escherichia coli

Background: The pts operon of Escherichia coli consists of three genes ptsH, ptsI and crr, each encoding for central components of the phosphoenolpyruvate: carbohydrate phosphotransferase system, HPr, enzyme I and IIA(Glc), respectively. Transcription of the pts operon is stimulated when glucose is present in the culture medium. One of the two major promoters, P0, is responsible for this glucose induction. However, no regulatory protein responsible for the glucose induction of the pts operon has been identified yet and molecular mechanism by which glucose stimulates the pts transcription is not known. Results: We found by Northern blotting that the pts mRNA levels in cells lacking Mlc, a new global repressor of carbohydrate metabolism, were increased without external glucose and that the addition of glucose had no effect on the pts mRNA levels in the mutant cells. Western blotting revealed that the enzyme I level in the mlc^- cells was also elevated without glucose and no further increase in the enzyme I level was observed in the presence of glucose. S1 analysis revealed that transcription of the glucose-sensitive promoter, P0, occurs constitutively in the mlc^- cells independently from the external glucose. In vitro transcription studies indicated that Mlc strongly inhibited P0 transcription. DNase I footprinting experiment revealed that Mlc bound to P0 promoter region to prevent RNA polymerase binding at P0. Conclusion: We conclude that Mlc is a repressor for the pts transcription acting as a major regulatory protein involved in the glucose induction of pts operon. We propose that glucose induces the pts transcription by modulating the Mlc activity. The mechanism by which glucose modulates the Mlc action remains to be studied.