II-BGlc, a glucose receptor of the bacterial phosphotransferase system: molecular cloning of ptsG and purification of the receptor from an overproducing strain of Escherichia coli.

The bacterial phosphoenolpyruvate:glycose phosphotransferase system (PTS) consists of interacting cytoplasmic and membrane proteins that catalyze the phosphorylation and translocation of sugar substrates across the cell membrane. One PTS protein, II-BGlc, is the membrane receptor specific for glucose and methyl D-glucopyranosides; the protein has been purified to homogeneity from Salmonella typhimurium [Erni, B., Trachsel, H., Postma, P. & Rosenbusch, J. (1982) J. Biol. Chem. 257, 13726-13730]. In the present experiments, the Escherichia coli ptsG locus, which encodes II-BGlc, was isolated from a transducing phage library and subcloned into plasmid vectors. The resulting plasmids complement the following phenotypic defects of ptsG mutants: growth on glucose, uptake and phosphorylation of methyl alpha-D-glucoside, and repression of the utilization of non-PTS sugars by methyl alpha-glucoside. The transformed cells overproduce II-BGlc 4- to 10-fold, and a Mr 43,000 polypeptide was synthesized from the plasmids in an in vitro transcription/translation system. The E. coli and S. typhimurium II-BGlc proteins differ in their physical properties, and a modified, three-step purification procedure was developed for isolating the E. coli protein.