REGULATION OF CRE RECOMBINASE ACTIVITY BY THE SYNTHETIC STEROID RU-486

To create a strategy for inducible gene targeting we developed a Cre-l ox recombination system which responds to the synthetic steroid RU 486 . Several fusions between Cre recombinase and the hormone binding doma in (HBD) of a mutated human progesterone receptor, which binds RU 486 but not progesterone, were constructed. When tested in transient expre ssion assays recombination activities of all fusion proteins were resp onsive to RU 486, but not to the endogenous steroid progesterone. Howe ver, the observed induction of recombination activity by the synthetic steroid varied between the different fusion proteins. The fusion with the highest activity in the presence of RU 486 combined with low back ground activity in the absence of the steroid was tested after stable expression in fibroblast and embryonal stem (ES) cells. We could demon strate that its recombination activity was highly dependent on RU 486. Since the RU 486 doses required to activate recombination were consid erably lower than doses displaying anti-progesterone effects in mice, this system could be used as a valuable tool for inducible gene target ing.