Studies on the Escherichia coli glucose-specific permease, PtsG, with a point mutation in its N-terminal amphipathic leader sequence

Previous work has resulted in the isolation of several mutant glucose permeases (IIGlc or PtsG) of the Escherichia coli phosphotransferase system (PTS) with altered N-terminal amphipathic leader sequences. The mutations were reported to (1) broaden permease substrate specificity, (2) promote facilitated diffusion of some sugars and (3) increase ptsG gene transcription. Detailed biochemical analyses were conducted, showing that one such mutant (V12F-IIGlc) (1) contains dramatically increased amounts of IIGlc, (2) displays correspondingly increased in vitro phosphorylation and in vivo transport activities, (3) shows increased utilization of several metabolizable sugars and (4) shows decreased susceptibility to detergent activation. These results are interpreted as suggesting that the V12F substitution in the N-terminal amphipathic leader sequence of IIGlc alters the facility with which the permease is integrated into the membrane. Consequent changes in conformation alter its catalytic properties and increase its affinity for the pleiotropic transcriptional repressor, Mlc. These changes together are proposed to promote transcription of the ptsG gene and account for the observed phenotypic changes.