Cre recombinase

Cre recombinase, often abbreviated to Cre, is a Type I topoisomerase from P1 bacteriophage that catalyzes site-specific recombination of DNA between loxP sites. The enzyme does not require any energy cofactors and Cre-mediated recombination quickly reaches equilibrium between substrate and reaction products. The loxP recognition element is a 34 base pair (bp) sequence composed of two 13 bp inverted repeats flanking an 8 bp spacer region which confers directionality. Recombination products are dependent on the location and relative orientation of the loxP sites. Two DNA species containing single loxP sites will be fused whilst DNA between loxP sites in the same orientation will be excised in circular form and DNA between opposing loxP sites will be inverted with respect to the rest of the DNA.[1]

Cre recombinase is used as a tool to modify genes and chromosomes. In this approach the Cre recombinase is used to delete a segment of DNA flanked by LoxP sites (aka 'floxed') in an experimental animal. It has been used to generate animals with mutations limited to certain cell types (tissue-specific knockout) or animals with mutations that can be activated by drug administration (inducible knockout) in a number of transgenic species.^[1] The availability of transgenic lines with tissue specific or inducible Cre expression permits researchers to inactivate or activate a gene of interest simply by breeding a floxed animal to pre-existing Cre-transgenics. One example of an inducible Cre recombinase system is the Cre-ER (ER = Estrogen Receptor) system in which intraperitoneal injection of tamoxifen will cause dose-dependent excision of the floxed site (i.e. will inactivate the gene of choice).^[2]

References

1. ^ Akagi K, Sandig V, Vooijs M, Van der Valk M, Giovannini M, Strauss M, Berns A (May 1997). "Cre-mediated somatic site-specific recombination in mice". Nucleic Acids Res. 25 (9): 1766–73. doi:10.1093/nar/25.9.1766. PMC 146656. PMID 9108159. http://nar.oxfordjournals.org/cgi/pmidlookup?view=long&pmid=9108159. 
2. ^ Hayashi S, McMahon AP (April 2002). "Efficient recombination in diverse tissues by a tamoxifen-inducible form of Cre: a tool for temporally regulated gene activation/inactivation in the mouse". Dev. Biol. 244 (2): 305–18. doi:10.1006/dbio.2002.0597. PMID 11944939.