Stringent regulation of stably integrated chloramphenicol acetyl transferase genes by E. coli lac repressor in monkey cells.

Monkey cell lines that constitutively synthesize 38.6 kd lac repressor protein and bear stably integrated chloramphenicol acetyl transferase (CAT) genes linked to a lac operator-containing SV40 early promoter-enhancer were generated. When grown in medium containing isopropyl beta-D-thiogalactoside (IPTG), these cells acquired a CAT+ phenotype. In contrast, when grown in parallel in medium lacking IPTG, the cells remained CAT-. Maximum induction of CAT activity occurred after 4 days of IPTG exposure. Three days after removal of IPTG, induced cells had reverted to CAT-. Specific CAT activity increased up to 60-fold after induction, while background activity in uninduced cells was similar to or only slightly above that of parental, CAT- cells. CAT activity increased stepwise over a wide range of IPTG concentrations. Thus lac repressor-operator complexes can form on primate chromosomes and stringently block transcription from an adjoining promoter.