The roles of inducer and catabolite repressor in the synthesis of β-galactosidase by Escherichia coli

The mechanism of the induction of β-galactosidase in Escherichia coli at 30°C was investigated by separating the phase of enzyme induction from the phase of enzyme production. This separation was accomplished by removing the inducer by filtration after 3 to 4 min of contact with the cells, that is before enzyme had been formed; the cells then produced the enzyme in inducer-free medium at an exponentially declining rate for approximately 10 min. It was found that interference with the synthesis of normal RNA by 5-fluorouracil during the phase of enzyme induction largely prevents the subsequent production of normal enzyme and causes the formation of an altered protein serologically related to β-galactosidase; on the other hand, such an interference with RNA synthesis during the phase of enzyme production does not affect the formation of the enzyme by previously induced cells. Conversely, interference with protein synthesis by starvation for an amino acid or by chloramphenicol during the phase of induction does not prevent the subsequent production of normal enzyme; but such an interference with protein synthesis during the phase of enzyme production inhibits the formation of the enzyme by previously induced cells to the same extent as that of other protein. These results indicate that during the induction phase a messenger RNA specific for β-galactosidase is produced which directs the subsequent synthesis of the enzyme in the inducer-free medium. No additional β-galactosidase messenger RNA is formed after removal of the inducer and that present in the cell decays exponentially with a half-life of approximately 2·5 min. The rate of this decay is independent of the rate of protein synthesis or the rate of energy metabolism. An excess of intracellular catabolites, produced either by the rapid catabolism of glucose or by the restriction of protein synthesis in the presence of a utilizable energy source, inhibits the induction of the enzyme but does not affect the production of the enzyme by previously induced cells. Apparently, the catabolite repressor inhibits and the inducer stimulates the synthesis of the unstable messenger RNA specific for β-galactosidase.