Mutations in lacO are constitutive and cis acting

(a) The partial diploid lacI+ lacO+ lacZ~/lacI+ lacO lacZ+ is constitutive, producing p-galactosidase in the presence and absence of lactose.

(b) The partial diploid lacI+ lacO+ lacZ+/lacI+ lacO lacZ~ is inducible (produces p-galactosidase only when lactose is present), demonstrating that the lacO gene is cis acting.

lacO+ operator

Lactose

Inactive repressor lacO+ operator

Lactose

Inactive repressor

lacOc operator

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