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

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Abstract
External glucose stimulates transcription of several genes including ptsG encoding IICBGlc, 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 IICBGlc‐FLAG interacts physically with Mlc in crude extracts prepared from cells in which IICBGlc is supposed to exist as the non‐phosphorylated form. The IICBGlc–Mlc interaction is no longer observed when IICBGlc is phosphorylated. Exogenously added purified Mlc binds to purified IICBGlc‐FLAG. We also demonstrate that Mlc is associated with membrane when IICBGlc is dephosphorylated while it is in the cytoplasm when IICBGlc is phosphorylated or absent. We conclude that IICBGlc 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 IICBGlc.

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