Eukaryotic conditional expression system

Existing conditional expression systems can be classified in two major categories that are based either on the induction or on the de-repression of transcription. The system described here combines both mechanisms, since a unique transcription factor can be shifted from a repression to a stimulation activity by simply changing its ligand. The resulting advantage of this system is the complete absence of basal expression before active induction. The principle of this method is based on the unexpected ability of the chimeric protein containing the DNA-binding domain of the yeast Gal4 transcription factor fused to the COOH half of the estradiol receptor (GalER), to act as a repressor when bound to the drug 4OH-tamoxifen, in the context of a previously described optimized Gal4-responsive promoter. The efficacy of this system has been assessed in transient expression assays using the chloramphenicol acetyl transferase (CAT), and in situ, through the activity of a Gal4 responsive beta-galactosidase gene.