INDUCER EXCLUSION IN ESCHERICHIA-COLI BY NON-PTS SUBSTRATES - THE ROLE OF THE PEP TO PYRUVATE RATIO IN DETERMINING THE PHOSPHORYLATION STATE OF ENZYME IIA(GLC)

The main mechanism causing catabolite repression in Escherichia coli i s the dephosphorylation of enzyme IIA(Glc), one of the enzymes of the phosphoenolpyruvate:carbohydrate phosphotransferase system (PTS). The PTS is involved in the uptake of a large number of carbohydrates that are phosphorylated during transport, phosphoenolpyruvate (PEP) being t he phosphoryl donor. Dephosphorylation of enzyme IIA(Glc) causes inhib ition of uptake of a number of non-PTS carbon sources, a process calle d inducer exclusion. In this paper, we show that dephosphorylation of enzyme IIA(Glc) is not only caused by the transport of PTS carbohydrat es, as has always been thought, and that an additional mechanism causi ng dephosphorylation exists. Direct monitoring of the phosphorylation state of enzyme IIA(Glc) also showed that many carbohydrates that are not transported by the PTS caused dephosphorylation during growth. In the case of glucose 6-phosphate, it was shown that transport and the f irst metabolic step are not involved in the dephosphorylation of enzym e IIA(Glc), but that later steps in the glycolysis are essential. Evid ence is provided that the [PEP]-[pyruvate] ratio, the driving force fo r the phosphorylation of the PTS proteins, determines the phosphorylat ion state of enzyme IIA(Glc). The implications of these new findings f or our view on catabolite repression and inducer exclusion are discuss ed.