Cassette recruitment in the chromosomal Integron of Vibrio cholerae

Abstract

Integrons confer a rapid adaptation capability to bacteria. Integron integrases are able to capture and shuffle novel functions embedded in cassettes. Here, we investigated cassette recruitment in the Vibrio cholerae chromosomal integron during horizontal transfer. We demonstrated that the endogenous integrase expression is sufficiently triggered, after SOS response induction mediated by the entry of cassettes during conjugation and natural transformation, to mediate significant cassette insertions. These insertions preferentially occur at the attIA site, despite the presence of about 180 attC sites in the integron array. Thanks to the presence of a promoter in the attIA site vicinity, all these newly inserted cassettes are expressed and prone to selection. We also showed that the RecA protein is critical for cassette recruitment in the V. cholerae chromosomal integron but not in mobile integrons. Moreover, unlike the mobile integron integrases, that of V. cholerae is not active in other bacteria. Mobile integrons might have evolved from the chromosomal ones by overcoming host factors, explaining their large dissemination in bacteria and their role in antibioresistance expansion.

Unbridled Integrons: A Matter of Host Factors

“…Similarly to what has been shown for conjugation, the chitin-induced uptake of exogenous DNA was sufficient to induce the expression of the integrase through the SOS [20]. In a recent report, for the first time, we provided evidences for cassette recruitment within the SCI of V. cholerae while relying solely on the SOS-dependent induction of IntIA [56]. Indeed, the efficiency of integration of a cassette provided as exogenous DNA in a chitin containing medium approached that of a cassette provided by conjugation without over-expression of IntIA (approximately 10 −6 ).…”

“…One of the consequences is to induce cassette mobility within the host integron. In particular, in recipient V. cholerae cells, cassette shuffling within the SCI can be observed at quite high rates [56].…”

“…Deletion of the endogenous integrase did not allow for the observation of a single recombination event, proving that the induction of the endogenous integrase through natural transformation is enough to perform efficient recruitment in this natural setting. Interestingly, apart from the integration of the incoming cassette, the induction of IntIA through SOS response also caused cassette rearrangements within the SCI at a high frequency [56]. Natural competence has often been viewed as a stress response by itself, allowing the concerned organism to efficiently acquire exogeneous DNA as a way to adapt quickly to a new environment [61].…”

The Natural History of Integrons

“…The second is the lack of species-specific factors needed for IntI1 to catalyse cassette insertion, as is needed for other integron integrases [45]. For example, the endogenous IntI of V. cholerae is not active in other bacterial species as it relies on multiple V. choleraespecific host factors [45].…”

“…The second is the lack of species-specific factors needed for IntI1 to catalyse cassette insertion, as is needed for other integron integrases [45]. For example, the endogenous IntI of V. cholerae is not active in other bacterial species as it relies on multiple V. choleraespecific host factors [45]. It is unknown whether the chromosomal ancestor of class 1 integrons required such host factors and subsequently overcame these constraints during its evolution, or if they were never required for IntI1 activity.…”