SPECIFIC METABOLIC REPRESSION OF THREE INDUCED ENZYMES IN ESCHERICHIA COLI

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Abstract
Metabolic repression of the inducible enzymes, ([beta]-galactosi-dase, tryptophanase and D-serine deaminase) were examined in E. coli. The repression of the first enzyme can be dissociated from the other 2 with a continuous-culture apparatus. There are specific repressor effects: galactose is an effective repressor for [beta]-galactosidase but not for the other enzymes; puruvate represses tryptophanase and serine deaminase but not [beta]-galactosidase. In wild-type E. coli, glucose represses all 3 enzymes. A mutant was selected in which there is no glucose effect for any of the enzymes but in which each enzyme remains specifically repressible. [beta]-Galactosidase synthesis in galactose-negative mutants was examined. In kinaseless and transferaseless strains, the enzyme is repressed both by glucose and galactose. In epimeraseless strains, the glucose effect is largely abolished. This indicates that the ability to form uridine-diphosphate galactose is an essential part of the glucose effect. Metabolic repression of an enzyme is specifically produced by an end-product of the enzyme action or some closely related compound, and glucose acts as a repressor only when it produces sufficient amounts of the specific repressor. The regulation of synthesis of induced degradative enzymes by end-product repression is analogous to that in biosynthetic pathways.