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.
Highlights
Mobile Genetic Elements (MGE) widely contribute to the evolution of bacterial genomes notably by conferring adaptive traits such as the ability to resist antibiotic treatments (Partridge et al, 2018)
Recombination assays that were performed in V. cholerae aimed at evaluating cassette insertion in recombination sites carried on plasmids (Biskri et al, 2005)
We demonstrated that the expression of endogenous integrase is dependant of the SOS system since no recombination event was detected below the detection limit of 1.5 × 10-8 in the N16961 lexAind- strain in which the SOS response is not inducible
Summary
Mobile Genetic Elements (MGE) widely contribute to the evolution of bacterial genomes notably by conferring adaptive traits such as the ability to resist antibiotic treatments (Partridge et al, 2018). Integrons are MGE that are considered as major contributors in the rise of multi-resistance in Gram-negative bacteria These genetic systems were discovered in the late 80s and described as platforms involved in the capture, stockpiling and expression of antibiotic resistance genes, embedded in structures termed “cassettes” (Stokes and Hall, 1989). This superintegron is located on the chromosome 2 of V. cholerae and contains 180 gene cassettes coding mainly for proteins with no homologs in the databases or for proteins of unknown function In contrast to their mobile counterpart and to refer at their location, such structures are termed Sedentary Chromosomal Integrons (SCI). Large SCIs such as those found in genomes of Vibrio species, could constitute a reservoir of gene cassettes that can be captured and spread by MIs (Loot et al, 2017; Rowe-Magnus et al, 2002)
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