The main mechanism causing catabolite repression by glucose and other carbon sources transported by the phosphotransferase system (PTS) in Escherichia coli involves dephosphorylation of enzyme IIA(Glc) as a result of transport and phosphorylation of PTS carbohydrates. Dephosphorylation of enzyme IIA(Glc) leads to 'inducer exclusion': inhibition of transport of a number of non-PTS carbon sources (e.g. lactose, glycerol), and reduced adenylate cyclase activity. In this paper, we show that the non-PTS carbon source glucose 6-phosphate can also cause inducer exclusion. Glucose 6- phosphate was shown to cause inhibition of transport of lactose and the non- metabolizable lactose analogue methyl-β-D-thiogalactoside (TMG). Inhibition was absent in mutants that lacked enzyme IIA(Glc) or were insensitive to inducer exclusion because enzyme IIA(Glc) could not bind to the lactose carrier. Furthermore, we showed that glucose 6-phosphate caused dephosphorylation of enzyme IIA(Glc). In a mutant insensitive to enzyme IIA(Glc)-mediated inducer exclusion, catabolite repression by glucose 6- phosphate in lactose-induced cells was much weaker than that in the wild- type strain, showing that inducer exclusion is the most important mechanism contributing to catabolite repression in lactose-induced cells. We discuss an expanded model of enzyme IIA(Glc)-mediated catabolite repression which embodies repression by non- PTS carbon sources.
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