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.