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Derivatives of ColE1 cer show altered topological specificity in site‐specific recombination.

ColE1 contains a 250‐bp sequence (cer) which is required in cis for the conversion of plasmid dimers to monomers. Recombination between cer and parB (a dimer resolution site from plasmid CloDF13) occurs in vivo at low frequency. The properties of the resulting hybrid sites have been studied. The type I hybrid closely resembles wild‐type cer. It supports intramolecular recombination and requires the products of the chromosomal xerA, xerB and xerC genes together with the 250‐bp site. In contrast, the type II hybrid (although differing from type I by only 2 bp) functions independently of the topological relationship of the participating sites, supporting both inter‐ and intramolecular recombination. Furthermore, recombination between type II sites is independent of the products of the xerA and xerB genes and requires a site of less than 50 bp.