Figure 16.12: Mutations in lacO are constitutive and cis acting. (a) The partial diploid lacI+ lacO+ lacZ−/lacI+ lacOc lacZ+ is constitutive, producing β-galactosidase in the presence and absence of lactose. (b) The partial diploid lacI+ lacO+ lacZ+/lacI+ lacOc lacZ− is inducible (produces β-galactosidase only when lactose is present), demonstrating that the lacO gene is cis acting.