Signal transduction between a membrane-bound transporter, PtsG, and a soluble transcription factor, Mlc, of Escherichia coli

The global regulator Mlc controls several genes implicated in sugar utilization systems, notably the phosphotransferase system (PTS) genes, ptsG, manXYZ and ptsHI, as well as the malT activator. No specific low molecular weight inducer has been identified that can inactivate Mlc, but its activity appeared to be modulated by transport of glucose via Enzyme IICBGlc (PtsG). Here we demonstrate that inactivation of Mlc is achieved by sequestration of Mlc to membranes containing dephosphorylated Enzyme IICBGlc. We show that Mle binds specifically to membrane fractions which carry PtsG and that excess Mlc can inhibit Enzyme IICBGlc phosphorylation by the general PTS proteins and also Enzyme IICBGlc-mediated phosphorylation of alpha-methylglucoside. Binding of Mlc to Enzyme IICBGlc in vitro required the IIB domain and the IIC-B junction region. Moreover, we show that these same regions are sufficient for Mlc regulation in vivo, via cross-dephosphorylation of IIBGlc during transport of other PTS sugars. The control of Mlc activity by sequestration to a transport protein represents a novel form of signal transduction in gene regulation.