Schematic representation of the Cumate switch. A. Sequences encompassing the promoter regions of p-cym and p-cmt: P1 and P2 are sequences from the promoter region of two bacterial operons regulated by CymR. The arrows represent putative binding sites for a helix-turn-helix CymR repressor. B. Repressor configuration: The bacterial repressor, CymR, can bind to the operator sequence (CuO) placed downstream of CMV5, a strong viral promoter that is active in mammalian cells. Once bound, CymR blocks transcription from the CMV5 promoter. CymR bound to cumate is unable to bind to CuO. Transcription from CMV5 can proceed unhindered. C. Activator configuration: A chimaeric transactivator, cTA can activate transcription from a minimal CMV promoter by binding to six repeats of the putative DNA recognition sequence (6X-CuO) placed upstream of the promoter in the absence of cumate. Upon cumate addition, the activator no longer binds DNA and therefore is no longer able to activate transcription from the basal promoter. D. Reverse activator configuration: A chimaeric transactivator, rcTA can activate transcription from a minimal CMV promoter by binding to six repeats of the putative DNA recognition sequence (6X-CuO) placed upstream of the promoter in the presence of cumate.