Prevalence of SOS-mediated control of integron integrase expression as an adaptive trait of chromosomal and mobile integrons

Our results showed that LexA controls the expression of a large fraction of integron integrases by binding to Escherichia coli-like LexA binding sites. In addition, the results provide experimental validation of LexA control of the integrase gene for another Vibrio chromosomal integron and for a multiresistance plasmid harboring two integrons. There was a significant correlation between lack of LexA control and predicted inactivation of integrase genes, even though experimental evidence also indicates that LexA regulation may be lost to enhance expression of integron cassettes.Ancestral-state reconstruction on an integron integrase phylogeny led us to conclude that the ancestral integron was already regulated by LexA. The data also indicated that SOS regulation has been actively preserved in mobile integrons and large chromosomal integrons, suggesting that unregulated integrase activity is selected against. Nonetheless, additional adaptations have probably arisen to cope with unregulated integrase activity. Identifying them may be fundamental in deciphering the uneven distribution of integrons in the Bacteria domain.Integrons are bacterial genetic elements capable of incorporating exogenous and promoterless open reading frames (ORF), referred to as gene cassettes, by site-specific recombination (Figure 1). First described in the late 1980s in connection with the emergence of antibiotic resistance [1], integrons always contain three functional components: an integrase gene (intI), which mediates recombination; a primary recombination site (attI); and an outward-orientated promoter (PC) [2]. Cassette integrations occur mainly at the attI site, ensuring the correct expression of newly captured cassettes by placing them under the control of the PC promoter [3,4]. To date, two main subsets of integrons have been described. On the one hand, mobile integrons, also referred to as multiresistance integrons, contain relatively few (two to eight) cassettes, and collectively en