MECHANISM OF GLUCOSE INHIBITION OF BETA-GALACTOSIDASE BIOSYNTHESIS IN RESTING CULTURES OF ESCHERICHIA COLI

Protein phosphorylation and allosteric control of inducer exclusion and catabolite repression by the bacterial phosphoenolpyruvate: sugar phosphotransferase system

The bacterial phosphotransferase system (PTS) functions in a variety of regulatory capacities. One of the best characterized of these is the process by which the PTS regulates inducer uptake and catabolite repression. Early genetic and physiological evidence supported a mechanism whereby the phosphorylation state of an enzyme of the PTS, the enzyme III specific for glucose (IIIGlc), allosterically inhibits the activities of a number of permeases and catabolic enzymes, the lactose, galactose, melibiose, and maltose permeases, as well as glycerol kinase. Extensive biochemical evidence now supports this model. Evidence is also available showing that substrate binding to those target proteins enhances their affinities for IIIGlc. In the case of the lactose permease, this positively cooperative interaction represents a well documented example of transmembrane signaling, demonstrated both in vivo and in vitro. Although the PTS-mediated regulation of cyclic AMP synthesis (catabolite repression) is not as well defined from a mechanistic standpoint, a model involving allosteric activation of adenylate cyclase by phospho-IIIGlc, together with the evidence supporting it, is presented. These regulatory mechanisms may prove to be operative in gram-positive as well as gram-negative bacteria, but the former organisms may have introduced variations on the theme by covalently attaching IIIGlc-like moieties to some of the target permeases and catabolic enzymes. It appears likely that the general process of PTS-catalyzed protein phosphorylation-dephosphorylation will prove to be important to the regulation of numerous bacterial physiological processes, including chemotaxis, intermediary metabolism, gene transcription, and virulence.

Pool sizes of metabolic intermediates and their relation to glucose repression of beta-galactosidase synthesis in Escherichia coli

1. The intermediary metabolism of two strains of Escherichia coli has been examined. One strain (Q22) exhibits acute transient repression of beta-galactosidase synthesis when glucose is supplied to cells growing on glycerol; the other strain (W3110) does not. The two strains do not differ genetically in their lac operons. 2. Strain Q22 uses about twice as much glucose as strain W3110 per unit of cell mass produced. 3. Pentose phosphate-cycle activity in the presence of glucose is much stronger in strain Q22 than in strain W3110. 4. In strain Q22 the pool sizes of glucose 6-phosphate, 6-phosphogluconate, fructose 1,6-diphosphate and NADPH increase when glucose is added to cells growing on glycerol, and beta-galactosidase synthesis is severely inhibited. After about 1hr. the synthesis of beta-galactosidase is partly resumed, and the pool sizes of the four compounds fall. ATP, NADH and several other phosphorylated compounds show no concentration changes. 5. These concentration changes do not occur in strain W3110, in which beta-galactosidase synthesis is only rather weakly repressed by glucose. 6. It is suggested that repression of enzyme synthesis by glucose requires the rapid operation of the pentose phosphate cycle, and is mediated by one of the four substances whose concentration rises and later falls in strain Q22. A definite choice of effector from among these four possibilities cannot at present be made.