Glucose transporter mutants of Escherichia coli K-12 with changes in substrate recognition of IICBGlc and induction behavior of the ptsG gene

In Escherichia coli K-12, the major glucose transporter with a central role in carbon catabolite repression and in inducer exclusion is the phosphoeno lpyruvate-dependent glucose:phosphotransferase system (PTS). Its membrane-b ound subunit, IICBGlc, is encoded by the gene ptsG; its soluble domain, IIA (Glc), is encoded by crr, which is a member of the pts operon. The system i s inducible by D-glucose and, to a lesser degree, by L-sorbose, The regulat ion of ptsG transcription was analyzed by testing the induction of IICBGlc transporter activity and of a single-copy Phi(ptsGop-lacZ) fusion. Among mu tations found to affect directly ptsG expression were those altering the ac tivity of adenylate cyclase (cyaA), the repressor DgsA (dgsA; also called M lc), the general PTS proteins enzyme I (ptsI) and histidine carrier protein HPr (ptsH), and the IIA(Glc) and IIBGlc domains, as well as several authen tic and newly isolated UmgC mutations. The latter, originally thought to ma p in the repressor gene umgC outside the ptsG locus, were found to represen t ptsG alleles. These affected invariably the substrate specificity of the IICBGlc domain, thus allowing efficient transport and phosphorylation of su bstrates normally transported very poorly or not at all by this PTS. Simult aneously, all of these substrates became inducers for ptsG. From the analys is of the mutants, from cis-trans dominance tests, and from the identificat ion of the amino acid residues mutated in the UmgC mutants, a new regulator y mechanism involved in ptsG induction is postulated. According to this mod el, the phosphorylation state of IIBGlc modulates IICGlc which, directly or indirectly, controls the repressor DgsA and hence ptsG expression. By the same mechanism, glucose uptake and phosphorylation also control the express ion of the pts operon and probably of all operons controlled by the repress or DgsA.