The Integron Database

What are integrons, integrases and gene cassettes?

Integrons are genetic systems that allow bacteria to capture and express gene cassettes (Fig.1). They typically consist of an intI gene encoding for an integrase that catalyses the incorporation or excision of gene cassettes by site-specific recombination, a recombination site attI and a promoter responsible for the expression of inserted gene cassettes (for a review see Mazel, 2006).

Typically, gene cassettes (Gc) consist of a promoterless open reading frame (orf) and a recombination site (attC, also named as 59-base element) necessary for integration (Hall et al., 1999; Holmes et al 2003). They can exist free as circular molecules or mobilised in integrons (Hall et al., 1999; Holmes et al 2003).

Integrons were first reported in clinical isolates in the 1980s (Hall et al., 1999) and have been extensively studied in clinical environments since then, due to their association with other mobile genetic elements and multiresistance phenotypes (Leverstein-van Hall et al., 2003). However, in the last decade special attention has been given to integrons from natural environments.

Fig. 1. Mechanism of integration of gene cassettes into the integron.

Recommended literature:

Boucher Y, Labbate M, Koenig J, Stokes H (2007) Integrons: mobilizable platforms that promote genetic diversity in bacteria. Trends Microbiol. 15:301-309.

Gillings M, Boucher Y, Labbate M, Holmes A, Krishnan S, Holley M, Stokes H (2008) The evolution of class 1 integrons and the rise of antibiotic resistance. J. Bacteriol.190:5095–5100.

Hall R, Collis C (1995) Mobile gene cassettes and integrons:capture and spread of genes by site-specific recombination. Mol. Microbiol.15:593-600.

Hall R, Collis C, Kim M, Partridge S, Recchia G, Stokes H (1999) Mobile gene cassettes and integrons in evolution. Ann. New York Acad. Sci. 870:68–80.

Mazel D (2006) Integrons: agents of bacterial evolution. Nat. Rev. Microbiol. 4:608-620.

Ochman H, Lawrence J, Groisman E (2000) Lateral gene transfer and the nature of bacterial innovation. Nature 405:299–304.

Rowe-Magnus D, Mazel D (1999) Resistance gene capture. Curr. Opin. Microbiol. 2:483-488.