Certain mutations in the regulator gene of the lac system in E. coli result in maximal synthesis of the lac proteins (β-galactosidase, etc.) even in the absence of the inducer (lactose). Provide an explanation for this observation.

Certain mutations in the repressor gene or operator sequence make the operon constitutive.

Explanation:

Lac operon is not expressed in the absence of lactose sugar. The repressor gene codes for the repressor protein which in turn binds to the operator region. The operator region of lac operon serves as a binding site for the repressor protein. Binding of repressor to the operator region of lac operon does not allow the RNA polymerase to transcribe the genes and the expression of operon is prevented.

Certain mutations in the repressor gene or operator sequence make the operon transcriptionally active even in the absence of lactose. A mutation in the repressor gene that results in the synthesis of non-functional repressor protein or no repressor protein would make the operon constitute. Similarly, a mutation in the operator sequence that does not allow an otherwise functional repressor protein to bind it would also result in the expression of the operon in the absence of lactose.