Group II introns are mobile retroelements, capable of invading new sites in DNA. They are self-splicing ribozymes that complex with an intron-encoded protein to form a ribonucleoprotein that targets DNA after splicing. These molecules can invade DNA site-specifically, through a process known as retrohoming, or can invade ectopic sites through retrotransposition. Retrotransposition, in particular, can be strongly influenced by both environmental and cellular factors. To investigate host factors that influence retrotransposition, we performed random insertional mutagenesis using the ISS1 transposon to generate a library of over 1000 mutants in Lactococcus lactis, the native host of the Ll.LtrB group II intron. By screening this library, we identified 92 mutants with increased retrotransposition frequencies (RTP-ups). We found that mutations in amino acid transport and metabolism tended to have increased retrotransposition frequencies. We further explored a subset of these RTP-up mutants, the most striking of which is a mutant in the ribosomal RNA methyltransferase rlmH, which exhibited a reproducible 20-fold increase in retrotransposition frequency. In vitro and in vivo experiments revealed that ribosomes in the rlmH mutant were defective in the m3Ψ modification and exhibited reduced binding to the intron RNA. Taken together, our results reinforce the importance of the native host organism in regulating group II intron retrotransposition. In particular, the evidence from the rlmH mutant suggests a role for ribosome modification in limiting rampant retrotransposition.

中文翻译：

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rRNA 甲基化作为宿主抵御猖獗的 II 组内含子逆转录转座的防御


II 组内含子是可移动的逆转录元件，能够侵入 DNA 中的新位点。它们是自剪接核酶，与内含子编码的蛋白质复合，形成剪接后靶向 DNA 的核糖核蛋白。这些分子可以通过称为逆向归巢的过程特异性侵入DNA位点，或者可以通过逆向转座侵入异位位点。尤其是逆转录转座，可能受到环境和细胞因素的强烈影响。为了研究影响逆转录转座的宿主因素，我们使用 ISS1 转座子进行随机插入诱变，以在乳酸乳球菌（Ll.LtrB II 组内含子的天然宿主）中生成包含 1000 多个突变体的文库。通过筛选该文库，我们鉴定了 92 个逆转录转座频率 (RTP-ups) 增加的突变体。我们发现氨基酸转运和代谢的突变往往会增加逆转录转座频率。我们进一步探索了这些 RTP-up 突变体的一个子集，其中最引人注目的是核糖体 RNA 甲基转移酶 rlmH 的突变体，其逆转录转座频率表现出可重复的 20 倍增加。体外和体内实验表明，rlmH 突变体中的核糖体存在 m3Ψ 修饰缺陷，并且与内含子 RNA 的结合减少。综上所述，我们的结果强调了天然宿主生物在调节 II 组内含子逆转录转座中的重要性。特别是，来自 rlmH 突变体的证据表明核糖体修饰在限制猖獗的逆转录转座中发挥作用。