Integrons

Abstract

Integrons are genetic entities that are defined by their ability to capture members of a large family of small mobile elements known as ‘gene cassettes’. A site-specific recombinase (IntI) belonging to the tyrosine recombinase or integrase family is encoded by the integron, and IntI recombinases incorporate cassettes into a specific site that is part of the integron and adjacent to the intI gene. Integrons thus share two characteristic features, an intI gene that encodes the integrase and an attI site recognized by the integrase. The IntI integrase also recognizes a family of sites called attC sites found in the gene cassettes. Recombination between an attC site and an attI site incorporates the cassette into the integron. Integrons can capture more than one cassette and the arrays of cassettes found in integrons include from one to hundreds of cassettes. The third key feature of at least the best-studied integrons is a promoter, Pc, which faces toward the cassettes and directs transcription of the promoterless genes in the cassettes. Thus, integrons are natural cloning vehicles acting as agents of gene capture and as expression vectors for the captured genes. A large family of integrons, each with a unique IntI and attI combination, have been identified, indicating a long evolutionary history. Although IntI can recognize only their cognate attI site, they can all recognize the attC sites of cassettes and hence share them.