The Genetic Control and Cytoplasmic Expression of “Inducibility” in the Synthesis of β-galactosidase by E. Coli †

A number of extremely closely linked mutations have been found to affect the synthesis of β -galactosidase in E. coli . Some of these ( z mutations) are expressed by loss of the capacity to synthesize active enzyme. Others ( i mutations) allow the enzyme to be synthesized constitutively instead of inducibly as in the wild type. The study of galactosidase synthesis in heteromerozygotes of E. coli indicates that the z and i mutations belong to different cistrons. Moreover the constitutive allele of the i cistron is recessive over the inducible allele. The kinetics of expression of the i + (inducible) character suggest that the i gene controls the synthesis of a specific substance which represses the synthesis of ( β -galactosidase. The constitutive state results from loss of the capacity to synthesize active repressor.