During transcription, the mRNA may hybridize with DNA, forming an R loop, which can be physiological or pathological, constituting in this case a source of genomic instability. To understand the mechanism by which eukaryotic cells prevent harmful R loops, we used human activation-induced cytidine deaminase (AID) to identify genes preventing R loops. A screening of 400 Saccharomyces cerevisiae selected strains deleted in nuclear genes revealed that cells lacking the Mlp1/2 nuclear basket proteins show AID-dependent genomic instability and replication defects that were suppressed by RNase H1 overexpression. Importantly, DNA–RNA hybrids accumulated at transcribed genes in mlp1/2 mutants, indicating that Mlp1/2 prevents R loops. Consistent with the Mlp1/2 role in gene gating to nuclear pores, artificial tethering to the nuclear periphery of a transcribed locus suppressed R loops in mlp1∆ cells. The same occurred in THO-deficient hpr1∆ cells. We conclude that proximity of transcribed chromatin to the nuclear pore helps restrain pathological R loops.

中文翻译：

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染色质与核孔的物理接近性可防止有害的R环积累，从而有助于维持基因组稳定性

在转录过程中，mRNA可以与DNA杂交，形成一个R环，该环可以是生理性的或病理性的，在这种情况下构成基因组不稳定的来源。为了了解真核细胞防止有害的R环的机制，我们使用了人类激活诱导的胞苷脱氨酶（AID）来鉴定防止R环的基因。筛选了400株在酵母基因中缺失的酿酒酵母菌株后发现，缺少Mlp1 / 2核篮子蛋白的细胞显示出AID依赖性基因组不稳定性和复制缺陷，这些缺陷被RNase H1过表达抑制。重要的是，DNA-RNA杂种会积累在mlp1 / 2的转录基因上突变体，表明Mlp1 / 2阻止了R环。与Mlp1 / 2在门控核孔基因中的作用一致，对转录位点核周边的人工束缚抑制了mlp1 ∆细胞中的R环。在THO缺陷型hpr1 ∆细胞中也发生了同样的情况。我们得出的结论是，转录的染色质与核孔的接近有助于抑制病理性R环。