INDUCIBLE SITE-DIRECTED RECOMBINATION IN MOUSE EMBRYONIC STEM-CELLS

The site-directed recombinase Cre can be employed to delete or express genes in cell lines or animals, Clearly, the ability to control remot ely the activity of this enzyme would be highly desirable, To this end we have constructed expression vectors for fusion proteins consisting of the Cre recombinase and a mutated hormone-binding domain of the mu rine oestrogen receptor, The latter still binds the anti-oestrogen dru g tamoxifen but no longer 17 beta-oestradiol. We show here that in emb ryonic stem cells expressing such fusion proteins, tamoxifen can effic iently induce Cre-mediated recombination, thereby activating a stably integrated LacZ reporter gene, In the presence of either 10 mu M tamox ifen or 800 nM 4-hydroxy-tamoxifen, recombination of the LacZ gene is complete within 3-4 days, By placing a tamoxifen-binding domain on bot h ends of the Cre protein, the enzymatic activity of Cre can be even m ore tightly controlled, Transgenic mice expressing such an tamoxifen-i nducible Cre enzyme may thus provide a new and useful genetic tool to mutate or delete genes at specific times during developement or in adu lt animals.