Mechanism of catabolite repression of tryptophanase synthesis in Escherichia coli

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Abstract
Repression of tryptophanase (tryptophan indole-lyase) by glucose and its non-metabolizable analogue methyl α-glucoside has been studied employing a series of isogenic strains of Escherichia coli lacking cyclic AMP phosphodiesterase and altered for two of the proteins of the phosphoenolpyruvate:sugar phosphotransferase system (PTS), Enzyme I and Enzyme IIAGIc. Basal activity of tryptophanase was depressed mildly by inclusion of glucose in the growth medium, but inducible tryptophanase synthesis was subject to strong glucose repression in the parental strain, which exhibited normal PTS enzyme activities. Methyl α-glucoside was without effect in this strain. Loss of Enzyme I decreased sensitivity to repression by glucose but enhanced sensitivity to repression by methyl α-glucoside. Loss of Enzyme IIAGIc activity largely abolished repression by methyl α-glucoside but had a less severe effect on glucose repression. The repressive effects of both sugars were fully reversed by inclusion of cyclic AMP in the growth medium. Tryptophan uptake under the same conditions was inhibited weakly by glucose and more strongly by methyl α-glucoside in the parental strain. Inhibition by both sugars was alleviated by partial loss of Enzyme I. Inhibition by methyl α-glucoside appeared to be largely due to energy competition and was not responsible for repression of tryptophanase synthesis. Measurement of net production of cyclic AMP as well as intracellular concentrations of cyclic AMP revealed a good correlation with intensity of repression. The results suggest that while basal tryptophanase synthesis is relatively insensitive to catabolite repression, inducible synthesis is subject to strong repression by two distinct mechanisms, one dependent on enzyme IIAGIc of the PTS and the other independent of this protein. Both mechanisms are attributable to depressed rates of cyclic AMP synthesis. No evidence for a cyclic-AMP-independent mechanism of catabolite repression was obtained.

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