PDF(Pubmed)
Abstract：
Bacterial defence systems are tightly regulated to avoid autoimmunity. In Type I restriction-modification (R-M) systems, a specific mechanism called restriction alleviation (RA) controls the activity of the restriction module. In the case of the Escherichia coli Type I R-M system EcoKI, RA proceeds through ClpXP-mediated proteolysis of restriction complexes bound to non-methylated sites that appear after replication or reparation of host DNA. Here, we show that RA is also induced in the presence of plasmids carrying EcoKI recognition sites, a phenomenon we refer to as plasmid-induced RA. Further, we show that the anti-restriction behavior of plasmid-borne non-conjugative transposons such as Tn5053, previously attributed to their ardD loci, is due to plasmid-induced RA. Plasmids carrying both EcoKI and Chi sites induce RA in RecA- and RecBCD-dependent manner. However, inactivation of both RecA and RecBCD restores RA, indicating that there exists an alternative, RecA-independent, homologous recombination pathway that is blocked in the presence of RecBCD. Indeed, plasmid-induced RA in a RecBCD-deficient background does not depend on the presence of Chi sites. We propose that processing of random dsDNA breaks in plasmid DNA via homologous recombination generates non-methylated EcoKI sites, which attract EcoKI restriction complexes channeling them for ClpXP-mediated proteolysis.
摘要：
细菌防御系统受到严格调节以避免自身免疫。在I型限制修改(R-M)系统中，一种称为限制缓解(RA)的特定机制控制限制模块的活动.在大肠杆菌I型R-M系统EcoKI的情况下，RA通过ClpXP介导的限制性复合物的蛋白水解进行，该复合物与宿主DNA复制或修复后出现的非甲基化位点结合。这里,我们表明，RA也在携带EcoKI识别位点的质粒的存在下被诱导，我们称之为质粒诱导的RA的现象。Further,我们表明，质粒携带的非接合转座子如Tn5053的抗限制性行为，以前归因于它们的ardD基因座，是由于质粒诱导的RA。携带EcoKI和Chi位点的质粒以RecA和RecBCD依赖性方式诱导RA。然而,RecA和RecBCD的失活可恢复RA，表明存在另一种选择，RecA独立，在RecBCD存在下被阻断的同源重组途径。的确,在RecBCD缺陷背景下质粒诱导的RA不依赖于Chi位点的存在。我们建议通过同源重组处理质粒DNA中的随机dsDNA断裂会产生非甲基化的EcoKI位点，这吸引了EcoKI限制复合物，将它们引导用于ClpXP介导的蛋白水解。
