Technical advance: transcriptional activator TGV mediates dexamethasone-inducible and tetracycline-inactivatable gene expression

A chemically regulated gene expression system that can be switched on with dexamethasone and switched off with tetracycline was constructed. It is based on a transcriptional activator (TGV) that consists of the Tn10 encoded Tet repressor, the rat glucocorticoid receptor hormone binding domain and the transcriptional activation domain of Herpes simplex virion protein VP16. When stably expressed in transgenic tobacco plants, it mediates dexamethasone-inducible transcription from a synthetic promoter (PTop10) consisting of seven tet operators upstream of a TATA-box. Tetracycline interferes with induction by negatively regulating the DNA-binding activity of the TetR moiety of TGV. The boundaries of the expression window of the TGV-driven PTop10 reach from undetectable levels of the reporter enzyme beta-glucuronidase in the absence of dexa- methasone to induced levels reaching 15-20% of the Cauliflower Mosaic Virus 35S promoter (PCaMV35S). By modifying the sequence of PTop10, we generated a new target promoter (PTax) that is stably expressed over several generations and that can be activated to levels comparable to PCaMV35S, while yielding only slightly elevated background activities.