Cre Recombinase (NEB #M0298) is a Type I topoisomerase from bacteriophage P1 that catalyzes the site-specific recombination of DNA between loxP sites (1). The loxP recognition element is a 34 base pair (bp) sequence comprised of two 13 bp inverted repeats flanking an 8 bp spacer region which confers directionality (3). Recombination products depend on the location and relative orientation of the loxP sites. Two DNA species containing single loxP sites will be fused. DNA between directly repeated loxP sites will be excised in circular form while DNA between opposing loxP sites will be inverted with respect to external sequences.
pLox2+ (linearized) (NEB #N0417) is a substrate for Cre Recombinase (NEB #M0298). It is 3.65 kb in length and has a loxP site approximately 350 bp from each end. Between the loxP sites lies an origin of replication and an ampicillin resistance gene. Recombination between these loxP sites by Cre Recombinase produces a circular, ampicillin-resistant plasmid (which migrates at approximately 1.7 kb on a 0.8% agarose gel) and an 850 bp DNA fragment.