INHIBITION BY GLUCOSE OF THE INDUCED SYNTHESIS OF THE β-GALACTOSIDE-ENZYME SYSTEM OF ESCHERICHIA COLI . ANALYSIS OF MAINTENANCE

Abstract

An E. coli clone growing in the presence of glucose, to which is added a [beta]-galactoside inducer, will not be induced to make either the galactoside-permease (Y) or [beta]-galactosidase (Z). The same clone, pre-induced in the presence of inducer, will continue to make permease and enzyme, after glucose is added. These studies have shown that a given culture growing in a fixed medium can exist indefinitely in alternative steady states and that it can be shifted from one stable state to the other by transitory variations in the environment. A model which accounts for these findings is based upon the facts that (a) Y is induced by an internal galactoside the accumulation of which is catalyzed by Y, and (b) glucose, in order to inhibit, enters by a distinct and independent route. In such a situation, the state of non-synthesis is stable because the [beta]-galactoside cannot reach the site of Y synthesis and overcome the glucose inhibition in a cell which lacks Y. The state of synthesis is stable because the initial presence of Y provides the internal inducer necessary for the continued synthesis of Y in a glucose environment. The synthesis of Z simply mirrors the synthesis of Y since both entities are induced directly or indirectly by the same substance, namely the product >f Y-action.