12.11 Mutations in lacO are constitutive and cis acting. (a) The partial diploid lacl+ lacO+ lacZ−/lacl+ lacOc lacZ+ is constitutive, producing β-galactosidase in the presence and absence of lactose. (b) The partial diploid lacl+ lacO+ lacZ+/lacl+ lacOc lacZ− is inducible (produces β-galactosidase only when lactose is present), demonstrating that the lacO gene is cis acting.