Cassette recruitment in the chromosomal Integron ofVibrio cholerae

Integrons are genetic systems conferring to bacteria a rapid adaptation capability. The integron integrase is able to capture, stockpile and shuffle novel functions embedded in cassettes. This involves the recognition of both substrates, theattIsite, and the cassette associatedattCsites. Integrons can be sedentary and chromosomally located (SCI) or, carried by conjugative plasmids (Mobile Integron, MI), hence favoring their dissemination among bacteria. Here, for the first time, we investigate the cassette recruitment in theVibrio choleraeSCI during conjugation and natural transformation. We demonstrated that horizontally transferred cassette can be recruited inside the chromosomal integron. The endogenous integrase expression is sufficiently triggered, after SOS response induction mediated by the entry of single-stranded cassettes during conjugation and natural transformation, to mediate significant cassette insertion. We demonstrate that theattIAinsertion is preferential, despite the presence of 180attCsites in the integron array. Thanks to the presence of a promoter in theattIAsite vicinity, all these newly inserted cassettes are expressed and prone to adaptive selection. We also show that the RecA protein is critical for cassette recruitment inV. choleraeSCI but not in MIs. Moreover,a contrarioto MIs, theV. choleraeSCI is not active in others bacterial hosts. MIs might have evolved from the SCIs by overcoming host factors, which would explain their large dissemination in bacteria and their role in the antibioresistance expansion.