Studies on the properties of P1 site-specific recombination: Evidence for topologically unlinked products following recombination

Bacteriophage P1 encodes its own site-specific recombination system consisting of a site at which recombination takes place called IoxP and a recombinase called Cre. A number of λ and plasmid substrates containing two IoxP sites have been constructed. Using these substrates we have shown both in vivo and in vitro that a fully functional IoxP site is composed of no more than 60 bp. In vitro, when an extract containing Cre is used, recombination between IoxP sites on supercoiled, nickedcircle or linear DNA occurs efficiently. The most surprising result from the in vitro studies is that 50% of the products of recombination between IoxP sites on a supercoiled DNA substrate are present as free supercoiled circles. The ability to produce free products starting with a supercoiled substrate suggests a rather unique property of Cre-mediated Iox recombination, the implications of which are discussed in terms of possible effects of the protein on the topology of the DNA molecule.