Fluorescent proteins (FPs) offer scientists a simple yet powerful way to tag cellular proteins and investigate protein localization, interaction, and expression. However, one caveat of FP-protein fusions (FP-chimeras) is that they undergo normal protein turnover. FP-chimeras are continuously synthesized and degraded within the cell, so at any given time, an FP-chimeric protein may be at any one of many stages of synthesis and degradation. For this reason it is virtually impossible to determine specific protein turnover or temporal expression using standard FP-chimeric proteins.
In a previous post from our Plasmids 101 series, we learned how the Cre-loxP recombination system can be used to induce site-specific recombination events, and that the orientation of the flanking loxP sites directs the Cre recombinase to invert, translocate, or excise a DNA fragment. The availability of both wild-type and mutant loxP sites has allowed scientists to leverage this system in new, creative ways. Today’s post will focus on one such strategy--the FLEx switch--which utilizes recombination elements to turn off expression of one gene, while simultaneously turning on the expression of another!