Beta-glucoside permease represses the bgl operon of Escherichia coli by phosphorylation of the antiterminator protein and also interacts with glucose-specific enzyme III, the key element in catabolite control.

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Abstract
The .beta.-glucoside (bgl) operon of Escherichia coli is subject to both positive control by transcriptional termination/antitermination and negative control by the .beta.-glucoside-specific transport protein, an integral membrane protein known as enzyme IIBgl. Previous results led us to speculate that enzyme IIBgl exerts its negative control by phosphorylating and thereby inactivating the antiterminator protein, BglG. Specifically, our model postulated that the transport protein enzyme IIBgl exhibits protein-phosphotransferase activity in the absence of .beta.-glucosides. We now present biochemical evidence that the phosphorylation of protein BglG does indeed occur in vivo and that it is accompanied by the loss of antitermiantion activity. BglG persists in the phosphorylated state in the absence of .beta.-glucosides but is rapidly dephosphorylated when .beta.-glucosides become available for transport. Our data also suggest specific interactions between the .beta.-glucoside transport protein and the glucose-specific enzyme III (enzyme IIIGlc), a component of glucose transport and a key element in regulation of catabolite repression. These obsrvations indicate that enzyme IIIGlc may, in conjunction with enzyme IIBgl, modulate the transport of .beta.-glucosides and the phosphorylation of the antiterminator protein. In the absence of both sugars, when the catabolite-controlled promoter of the operon is derepressed, enzyme IIIGlc may mediate tight repression of antitermination.