Catabolite control of Escherichia coli regulatory protein BglG activity by antagonistically acting phosphorylations.

In bacteria various sugars are taken up and concomitantly phosphorylated by sugar-specific enzymes II (EII) of the phosphoenolpyruvate:sugar phosphotransferase system (PTS). The phosphoryl groups are donated by the phosphocarrier protein HPr. BglG, the positively acting regulatory protein of the Escherichia coli bgl (beta-glucoside utilization) operon, is known to be negatively regulated by reversible phosphorylation catalyzed by the membrane spanning beta-glucoside-specific EIIBgl. Here we present evidence that in addition BglG must be phosphorylated by HPr at a distinct site to gain activity. Our data suggest that this second, shortcut route of phosphorylation is used to monitor the state of the various PTS sugar availabilities in order to hierarchically tune expression of the bgl operon in a physiologically meaningful way. Thus, the PTS may represent a highly integrated signal transduction network in carbon catabolite control.     

TheEscherichia coli antiterminator protein BglG stabilizes the 5’ region of thebgl mRNA

Theβ-glucoside utilization (bgl) genes ofEscherichia coli are positively regulated by the product of thebglG gene, which functions as an antiterminator by binding to specific sequences present within thebgl mRNA. BglG is inactivated by phosphorylation in the absence of β-glucosides by BglF, thebgl-specific component of the phosphotransferase system (PTS). Here, we present evidence for an additional function for BglG, namely the stabilization of the 5’ end of thebgl mRNA. Half-life measurements of the promoter-proximal region of thebgl mRNA indicate a five fold enhancement of stability in the presence of active (unphosphorylated) BglG. This enhancement is lost when the binding of BglG to mRNA is prevented by deletion of the binding site. Interestingly, stabilization by BglG does not extend to downstream sequences. The enhanced stability of the upstream sequences suggest that BglG remains bound to its target on the mRNA even after the downstream sequences have been degraded. Implications of these observations for the mechanism of positive regulation of the operon by BglG are discussed.