MECHANISM OF CATABOLITE REPRESSION OF TRYPTOPHANASE SYNTHESIS IN ESCHERICHIA-COLI

Repression of tryptophanase (tryptophan indole-lyase) by glucose and i ts nonmetabolizable analogue methyl alpha-glucoside has been studied e mploying a series of isogenic strains of Escherichia coli lacking cycl ic AMP phosphodiesterase and altered for two of the proteins of the ph osphoenolpyruvate :sugar phosphotransferase system (PTS), Enzyme I and University, Stanford, Enzyme IIA(Glc). Basal activity of tryptophanas e was depressed mildly by inclusion of glucose in the growth medium, b ut inducible tryptophanase synthesis was subject to strong glucose rep ression in the parental strain, which exhibited normal PTS enzyme acti vities. Methyl alpha-glucoside was without effect in this strain. Loss of Enzyme I decreased sensitivity to repression by glucose but enhanc ed sensitivity to repression by methyl alpha-glucoside. Loss of Enzyme IIA(Glc) activity largely abolished repression by methyl alpha-glucos ide 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 alpha-glucosi de in the parental strain. Inhibition by both sugars was alleviated by partial loss of Enzyme I. Inhibition by methyl alpha-glucoside appear ed 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 re vealed a good correlation with intensity of repression. The results su ggest that while basal tryptophanase synthesis is relatively insensiti ve to catabolite repression, inducible synthesis is subject to strong repression by two distinct mechanisms, one dependent on enzyme IIA(Glc ) of the PTS and the other independent of this protein. Both mechanism s are attributable to depressed rates of cyclic AMP synthesis. No evid ence for a cyclic-AMP-independent mechanism of catabolite repression w as obtained.