Constitutive O mutations

• Z⁺ dominant to Z^-

• Y⁺ dominant to Y^-              

• O^C – operator is constitutive

• O⁺ - operator is inducible

Constitutive effect of  O^C is restricted to the structural genes on the same chromosome: “cis-acting”

Strain 4: wild type-Y⁺ cis to wild type- O⁺: permease activity is inducible

               mutant- Z⁺ cis to mutant- O^C : β-galactosidase activity is constitutive

This proves that the operator makes no product of its own, it only regulates the expression of the structural genes linked to it.