Integrons: agents of bacterial evolution

Abstract

Integrons are assembly platforms that incorporate exogenous open reading frames through site-specific recombination and convert them to functional genes by ensuring their correct expression. Although integrons were discovered through their involvement in the development of multiple antibiotic resistance in Gram-negative pathogens when carried in transposons, their role in genome evolution has been extended with the discovery of other, often larger, integron structures as genuine components of the genomes of many γ-proteobacterial species. This Review discusses the structural differences among the different types of integron — those carried in mobile DNA elements and the chromosomal superintegrons — as well as their evolutionary history and phylogenetic relationships. The different functions encoded by the integron gene cassettes are reviewed, with emphasis on those from superintegrons and other chromosomal integrons from environmental bacteria. The dynamics of the intraspecies and interspecies variation of the large cassette arrays of superintegrons, and their role in the increase in antibiotic resistance, are discussed. Finally, the specific recombination reactions occurring in these elements are reviewed, and a novel model involving a single-stranded substrate for recombination for cassette insertion and deletion is proposed.