A new route for integron cassette dissemination among bacterial genomes

Integrons are genetic elements found exclusively in bacteria. They are well known for their role in disseminating antibiotic resistance genes among pathogens and more generally for enabling bacteria to rapidly adapt to changing environmental conditions. Integrons constitute a natural system to capture, stockpile, shuffle, express and disseminate genes embedded in cassettes. All these events are governed by the integron integrase through site-specific recombination between integron att sites (attC and attI sites). Here, we demonstrate that integron integrase can efficiently catalyze the insertion of cassettes in bacterial genomes, outside the att sites carried by the integron system. Surprisingly, analysis of more than 5 × 105 independent clones revealed that the genome recombination sites differ greatly, in terms of sequence and structure, from both classical attC and attI recombination sites. We named these new sites attG. Notably, among these a few are driving integration at very high rates. We also showed that, once inserted in genomes, cassettes can be expressed if located near a bacterial promoter. Moreover, even if occurring at low frequency, genome inserted cassettes can be excised precisely or imprecisely, inducing in this latter case, chromosomal modifications. These results unveil a new route for antibiotic resistance dissemination and expand the role of integrons in bacterial genome evolution.