Université de Limoges, Faculté de Médecine, EA3175, Limoges, France.
PLoS Genet. 2010 Jan;6(1):e1000793. doi: 10.1371/journal.pgen.1000793. Epub 2010 Jan 8.
Class 1 integrons are widespread genetic elements that allow bacteria to capture and express gene cassettes that are usually promoterless. These integrons play a major role in the dissemination of antibiotic resistance among Gram-negative bacteria. They typically consist of a gene (intI) encoding an integrase (that catalyzes the gene cassette movement by site-specific recombination), a recombination site (attI1), and a promoter (Pc) responsible for the expression of inserted gene cassettes. The Pc promoter can occasionally be combined with a second promoter designated P2, and several Pc variants with different strengths have been described, although their relative distribution is not known. The Pc promoter in class 1 integrons is located within the intI1 coding sequence. The Pc polymorphism affects the amino acid sequence of IntI1 and the effect of this feature on the integrase recombination activity has not previously been investigated. We therefore conducted an extensive in silico study of class 1 integron sequences in order to assess the distribution of Pc variants. We also measured these promoters' strength by means of transcriptional reporter gene fusion experiments and estimated the excision and integration activities of the different IntI1 variants. We found that there are currently 13 Pc variants, leading to 10 IntI1 variants, that have a highly uneven distribution. There are five main Pc-P2 combinations, corresponding to five promoter strengths, and three main integrases displaying similar integration activity but very different excision efficiency. Promoter strength correlates with integrase excision activity: the weaker the promoter, the stronger the integrase. The tight relationship between the aptitude of class 1 integrons to recombine cassettes and express gene cassettes may be a key to understanding the short-term evolution of integrons. Dissemination of integron-driven drug resistance is therefore more complex than previously thought.
Class 1 整合子是广泛存在的遗传元件，使细菌能够捕获和表达通常没有启动子的基因盒。这些整合子在革兰氏阴性菌中抗生素耐药性的传播中起着重要作用。它们通常由一个基因（intI）编码一个整合酶（催化基因盒通过位点特异性重组移动）、一个重组位点（attI1）和一个启动子（Pc）组成，负责插入基因盒的表达。Pc 启动子偶尔可以与第二个启动子 P2 结合，已经描述了几种 Pc 变体，具有不同的强度，尽管它们的相对分布尚不清楚。Class 1 整合子中的 Pc 启动子位于 intI1 编码序列内。Pc 多态性影响 IntI1 的氨基酸序列，而这种特征对整合酶重组活性的影响尚未被研究过。因此，我们进行了广泛的 Class 1 整合子序列的计算机模拟研究，以评估 Pc 变体的分布。我们还通过转录报告基因融合实验测量了这些启动子的强度，并估计了不同 IntI1 变体的切除和整合活性。我们发现，目前有 13 种 Pc 变体，导致 10 种 IntI1 变体，它们的分布非常不均匀。有五个主要的 Pc-P2 组合，对应五个启动子强度，以及三个主要的整合酶显示出相似的整合活性，但切除效率却非常不同。启动子强度与整合酶切除活性相关：启动子越弱，整合酶越强。Class 1 整合子重组和表达基因盒的能力之间的紧密关系可能是理解整合子短期进化的关键。整合子驱动的药物耐药性的传播比以前想象的要复杂。
