FRTs

FRTs, or FLP Recognition Targets, are specific DNA sequences used in the FLP-FRT site-specific recombination system. Derived from the yeast Saccharomyces cerevisiae, FRT sites direct recombination events when bound by FLP recombinase. The system is widely used in genetic engineering to manipulate DNA in a controlled, spatially and temporally restricted manner.

The canonical FRT site is 34 base pairs long, with two 13-base pair inverted repeats separated by an 8-base pair core. Recombination occurs between two FRT sites on the same DNA molecule when FLP recombinase is present; the relative orientation determines the outcome: same orientation yields excision of the intervening sequence, while opposite orientation results in inversion.

Applications: In organisms such as Drosophila and mice, FLP-FRT enables conditional knockouts, mosaic analysis, and lineage tracing. It can be used with tissue-specific or inducible FLP expression to target recombination to particular cells or times. The system is often used alongside Cre-loxP for more complex genetic strategies.

Variants and practical considerations: Several FLP variants have been developed to improve activity in different contexts, including codon-optimized and temperature-tolerant forms. Design considerations include placing two FRT sites in the same or opposite orientation, the distance between sites, and chromatin context, all of which influence efficiency. Delivery methods include transgenic lines, plasmids, and viral vectors.

Relation to other systems: The FLP-FRT system is one of several site-specific recombination systems, notably Cre-loxP. Used together, these systems enable intersectional genetics for more precise control.