The relation of catabolite repression to the induction system for β-galactosidase in Escherichia coli

Abstract

The effect of glucose on the constitutive synthesis of β-galactosidase following conjugation of an Escherichia coli Hfr strain carrying lac i+z+ genes with different F− strains was investigated. It was found that the addition of glucose to a glycerol-containing medium reduces the rate of synthesis of the enzyme to approximately one-half, not only in the merozygotes formed from two i−z− strains, but also in the merozygotes formed from a strain in which the i and z genes are deleted. In merozygotes formed from a third i−z− strain, glucose reduces β-galactosidase synthesis by 90%. This exceptionally strong effect of glucose is a trait of the recipient strain and is not specific for β-galactosidase. Identical results were found in analogous F-duction crosses. It was concluded that the i+z+/lacdel merozygotes are susceptible to catabolite repression immediately after the entry of i+z+ genes. Therefore, it appears that the i product has no role in catabolite repression. An oc mutation was also studied and found not to limit catabolite repression. It is suggested that the system by which the level of a catabolite controls the synthesis of messenger RNA specific for β-galactosidase in E. coli is distinct from the induction system for the lac operon.