Mobile gene cassettes and integrons in evolution

Integrons and the site-specific recombination systems encoded by them provide a simple mechanism for the addition of new genes to bacterial chromosomes. Although there is substantial divergence among the four known integron encoded integrases, they all recognize the recombination sites, known as 59-base elements, that are associated with genes that are packaged in gene cassettes. In contrast, the integron-associated recombination sites, attI sites, are preferentially recognized by the cognate integrase.