Repetitive short-pulsed illumination efficiently activates photoactivatable-Cre as continuous illumination in embryonic stem cells and pre-implantation embryos of transgenic mouse.

The Cre-loxP system has been widely used for specific DNA recombination which induces gene inactivation or expression. Recently, photoactivatable-Cre (PA-Cre) proteins have been developed as a tool for spatiotemporal control of the enzymatic activity of Cre recombinase. Here, we generated transgenic mice bearing a PA-Cre gene and systematically investigated the conditions of photoactivation for the PA-Cre in embryonic stem cells (ESCs) derived from the transgenic mice and in a simple mathematical model. Cre-mediated DNA recombination was induced in 16% of the PA-Cre ESCs by 6 hr continuous illumination. We show that repetitive pulsed illumination efficiently induced DNA recombination with low light energy as efficient as continuous illumination in the ESCs (96 ± 15% of continuous illumination when pulse cycle was 2 s), which was also supported by a minimal mathematical model. DNA recombination by the PA-Cre was also successfully induced in the transgenic mouse pre-implantation embryos under the developed conditions. These results suggest that strategies based on repetitive pulsed illumination are efficient for the activation of photoactivatable Cre and, possibly other photo-switchable proteins.
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