Chromosomal insertion sites for phages and plasmids.

Bacteriophages insert their DNA into host chromosomes either through transposition (as in phage Mu) or through site-specific recombination (as in phage X). Whereas Mu can insert almost anywhere along the chromosome, X has a single highly preferred chromosomal site. Certain plasmids also insert into chromosomes by site-specific recombination. The site-specific recombinases used generally belong to the integrase family, whose members show some sequence homology and conservation of reaction mechanism, indicating descent from a common ancestor (2, 8, 12, 19). This minireview examines some chromosomal sites with known nucleotide sequences. The purpose is not to prepare an exhaustive catalog but to look for common trends. The focus is on chromosomal (attB) sites, not on the phage (attP) sites. Nevertheless, interpretation requires compari-