Figure 1.

(A) Diagram of the dual regulated expression vector pDuoRex3 containing convergently oriented P_PIR-YFP-pA_I and P_hCMV*-1-CFP-pA_II expression units (P_PIR-YFP-pA_I-/-pA_II-CFP-P_hCMV*-1). (B) pDuoRex3 was transfected into CHO-TWIN1₁₀₈, which stably harbors both tTA and PIT-dependent transactivators, and expression of YFP and CFP was assessed 48 h following transfection by fluorescence microscopy in the presence (+) or absence (–) of either tetracycline (TET; 2 µg/ml) or pristinamycin I (PI; 2 µg/ml). In the absence of both antibiotics (–TET/–PI) simultaneous induction of both responsive promoters results in reciprocal transcriptional silencing of the convergently oriented expression units.