Jové Thomas, Da Re Sandra, Tabesse Aurore, Gassama-Sow Amy, Ploy Marie-Cécile
INSERM, CHU Limoges, UMR 1092, Université LimogesLimoges, France.
Unité de Bactériologie Expérimentale, Institut Pasteur de DakarDakar, Senegal.
Front Microbiol. 2017 Aug 15;8:1499. doi: 10.3389/fmicb.2017.01499. eCollection 2017.
Integrons are powerful bacterial genetic elements that permit the expression and dissemination of antibiotic-resistance gene cassettes. They contain a promoter Pc that allows the expression of gene cassettes captured through site-specific recombination catalyzed by IntI, the integron-encoded integrase. Class 1 and 2 integrons are found in both clinical and environmental settings. The regulation of and of Pc promoters has been extensively studied in class 1 integrons and the regulatory role of the SOS response on expression has been shown. Here we investigated class 2 integrons. We characterized the P promoter and showed that expression is not regulated via the SOS response. We also showed that, unlike class 1 integrons, class 2 integrons possess not one but two active Pc promoters that are located within the region that seem to contribute equally to gene cassette expression. Class 2 integrons mostly encode an inactive truncated integrase, but the rare class 2 integrons that encode an active integrase are associated with less efficient Pc2 promoter variants. We propose an evolutionary model for class 2 integrons in which the absence of repression of the integrase gene expression led to mutations resulting in either inactive integrase or Pc variants of weaker activity, thereby reducing the potential fitness cost of these integrons.
整合子是强大的细菌遗传元件，可使抗生素抗性基因盒得以表达和传播。它们含有一个启动子Pc，能让通过整合子编码的整合酶IntI催化的位点特异性重组捕获的基因盒得以表达。1类和2类整合子在临床和环境中均有发现。1类整合子中Pc启动子的调控已得到广泛研究，并且已证明SOS应答对其表达具有调控作用。在此，我们对2类整合子展开了研究。我们对P启动子进行了表征，并表明其表达不受SOS应答调控。我们还表明，与1类整合子不同，2类整合子并非只有一个，而是有两个位于区域内的活性Pc启动子，它们似乎对基因盒表达的贡献相同。2类整合子大多编码一种无活性的截短整合酶，但编码活性整合酶的罕见2类整合子与活性较弱的Pc2启动子变体相关。我们提出了一个关于2类整合子的进化模型，其中整合酶基因表达缺乏抑制导致了突变，产生了无活性的整合酶或活性较弱的Pc变体，从而降低了这些整合子潜在的适应性成本。
