You have created transgenic mice that carry the gene encoding green fluorescent protein (GFP) under the control of a ubiquitous promoter. The GFP-encoding gene, however, is inactive unless recombination by Cre removes a blocking sequence between the promoter and the gene. These mice have been mated with those harboring the Cre recombinase under the control of the insulin promoter. The Cre recombinase is fused to the estrogen receptor, which keeps the fusion in the cytosol unless it binds to tamoxifen, in which case it allows nuclear entry. After mating, you select adult mice harboring both of these transgenes and inject them with either tamoxifen or water. Which of the following would you expect to observe later as a result when you examine brain and pancreas tissue samples under a fluorescence microscope?

Answer 1

If adult mice harboring both the transgenes GFP under the control of a ubiquitous promoter and Cre recombinase under the control of the insulin promoter are injected with tamoxifen, the fusion of Cre recombinase and estrogen receptor will enable nuclear entry, leading to recombination of the GFP gene. This will remove the blocking sequence between the promoter and the gene, and activate the GFP gene. As a result, under a fluorescence microscope, you would expect to observe fluorescence only in the brain tissue, as the insulin promoter controls the expression of Cre recombinase in the pancreas cells, not in the brain cells.

On the other hand, if the mice are injected with water instead of tamoxifen, the Cre recombinase will remain in the cytosol and will be unable to enter the nucleus. Therefore, the blocking sequence will remain intact, and the GFP gene will remain inactive. Consequently, under a fluorescence microscope, you would not observe any fluorescence in either the brain or the pancreas tissue.