Site-specific recombination

Recombination can occur between regions of DNA with apparently no homology, where only a few base pairs are the same. This is an important mechanism in the XerC/XerD- mediated resolution of the two circular molecules of DNA in E. coli chromosomal replication and does not require RecA. A similar method is used to integrate bacteriophage λ DNA into its host’s genome. The integration site is a 15 bp sequence present in both the host and λ DNA. The l integrase makes cuts on both strands about 15 bp apart. Integration host factor (IHF) catalyzes recombination between the homologous sites and the λ genome is inserted into that of E. coli. The reverse of the process is catalyzed by the phage-encoded excisionase, stimulated by host IHF and Xis.