| Topological selectivity in site-specific recombination

Abstract In conservative site-specific recombination, DNA molecules are broken in both strands at two separate predetermined points, and the ends are rejoined to new partners, without any synthesis or degradation of the DNA (Fig. la). The reactions are catalysed by specialized recombinase proteins and may involve other protein accessory factors. Many site-specific recombination systems have been identified in prokaryotes and eukaryotes, and several systems have been reconstituted in vitro. In studying these systems, one of the main aims has been to attempt to understand the mechanisms by which selectivity for recombination between sites in particular relationships is achieved. It has become apparent that DNA topology plays a crucial role in this selectivity. In vivo, site-specific recombination has a variety of functions, including switching of gene expression, control of plasmid copy number, resolution of plasmid multimers to monomers, bacteriophage integration and excision, and transposon co-integrate resolution.