Abstract
Previous studies indicated that the ability of phosphoenolpyruvate:sugar phosphotransferase system (PTS) substrates to inhibit the uptake of glycerol or maltose in S. typhimurium is dependent on the relative cellular content of the PTS-sensitive uptake system and of the PTS protein glucose-specific factor III (IIIGlc). The present study confirms and extends those observations. The maltose and glycerol uptake systems were rendered (wholly or partially) insensitive to PTS inhibition by the presence of a 2nd PTS-sensitive uptake system (respectively, that for glycerol or maltose) and its substrate. The 2nd PTS-sensitive uptake system and its substrate were needed for this protective effect. Galactose and the galactose permease (a PTS-insensitive transport system) did not have any effect on PTS-mediated inhibition of the maltose uptake system. The protective effect of the 2nd PTS-sensitive uptake system and its substrate was counteracted by increasing the cellular levels of IIIGlc. Overproduction of IIIGlc in crr-plasmid-containing strains rendered the glycerol and maltose uptake systems hypersensitive to inhibition by PTS substrates. The results were interpreted on the basis of stoichiometric interaction between IIIGlc and a PTS-sensitive uptake system, in which the IIIGlc-transport-system complex is inactive. Competition between 2 PTS-sensitive transport system for formation of inactive complex with IIIGlc lowers the free intracellular concentration of IIIGlc resulting in a mutual protective effect against inhibition by IIIGlc.

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