Site-specific recombinases (SSRs) like Cre, Dre or FLPe are valuable tools in functional genomics and have been applied in various organisms. They mediate recombination between target sites of 32-34 bp in length. The target sites, which are called loxP, rox or FRT sites are 13-14 bp palindromes separated by spacers:
loxP: 5-ATAACTTCGTATAATGTATGCTATACGAAGTTAT-3rox: 5-TAACTTTAAATAATGCCAATTATTTAAAGTTA-3FRT :5-GAAGTTCCTATTCTCTAGAAAGTATAGGAACTTC-3Cre recombinase, which was originally isolated from coliphage P1, mediates recombination between two loxP-sites through the spacer regions (e.g. removal of selectable genes). Dre was identified in a systematic search through P1-like phages for a Cre-like enzyme that had diverged sufficiently to recognize a recombination target site (RT) that is distinct from loxP.The combination of the arabinose inducible BAD promoter and the low-copy pSC101 plasmid backbone provide an excellent on-off regulation of Cre, Dre or FLPe in E. coli as proved in a test experiment.The plasmids carry a tetracyclin resistance gene for selection and are compatible with plasmids based on a ColE1 or p15A origin of replication and an ampicillin or kanamycin resistance marker.While Cre/loxP and FLPe/FRT is widely used in mouse genetics for conditional mutagenesis with many mouse lines available, a second highly efficient system like Dre/rox opens the door for more complex tasks such as a conditional mutagenesis of alternatively spliced exons. Cre/loxP can be used to remove one alternative exon and Dre/rox to remove the other one.