A novel regulatory role of glucose transporter of Escherichia coli: membrane sequestration of a global repressor Mlc

Abstract

External glucose stimulates transcription of several genes including ptsG encoding IICB(Glc), a membrane component of the phosphotransferase system (PTS), by relieving the negative regulation of a global repressor Mlc in Escherichia coli. We investigate here how glucose modulates Mlc action. The Mlc-mediated repression is eliminated by a ptsI mutation, while Mlc is constitutively active in a ptsG mutant. We show that IICB(Glc)-FLAG interacts physically with Mlc in crude extracts prepared from cells in which IICB(Glc) is supposed to exist as the non-phosphorylated form. The IICB(Glc)-Mlc interaction is no longer observed when IICB(Glc) is phosphorylated. Exogenously added purified Mlc binds to purified IICB(Glc)-FLAG. We also demonstrate that Mlc is associated with membrane when IICB(Glc) is dephosphorylated while it is in the cytoplasm when IICB(Glc) is phosphorylated or absent. We conclude that IICB(Glc) regulates the cellular localization of Mlc, depending on its phosphorylation state, which is determined by the availability of external glucose. Thus, glucose induces the transcription of Mlc-regulated promoters by sequestering Mlc to the membrane through dephosphorylation of IICB(Glc).