The MRN-MDC1 complex plays a central role in the DNA damage response (DDR) and repair. Using Proteomics of Isolated Chromatin Fragments (PICh), we identified DDR factors, such as MDC1, among those that become highly associated with a genomic locus upon transcriptional activation. Purification of endogenous MDC1, in the absence of exogenous DNA damage, revealed its interaction with factors involved in gene expression and co-transcriptional RNA processing, in addition to DDR factors. ChIP-seq analysis showed that MDC1 interacting factors, MRE11 and NBS1 subunits of MRN, were co-localized throughout the genome and notably at TSSs and gene bodies of actively transcribing genes. Blockade of transcriptional elongation showed that binding of MRN was dependent on the RNAPII transcriptional complex rather than transcription per se. Depletion of MRN increased RNAPII abundance at TSSs and gene bodies of MRE11/NBS1-bound genes. Prolonged exposure of cells to either MRE11- or NBS1- depletion led to single nucleotide polymorphism formation across actively transcribing, MRE11 or NBS1 target genes. These data support a model by which association of the MRN complex with the transcriptional machinery constitutively scans active genes for transcription-induced DNA damage to preserve the integrity of the coding genome.

中文翻译：

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染色质相关的MRN复合物可保护高度转录的基因免受基因组不稳定的影响

MRN-MDC1复合体在DNA损伤反应（DDR）和修复中起着核心作用。使用分离的染色质片段的蛋白质组学（PICh），我们鉴定了在转录激活后与基因组基因位高度相关的DDR因子，例如MDC1。在没有外源DNA损伤的情况下，内源MDC1的纯化显示了它与DDR因子以及与基因表达和共转录RNA加工相关的因子的相互作用。ChIP-seq分析显示MDC1相互作用因子，MRN的MRE11和NBS1亚基共定位在整个基因组中，特别是在TSS和活跃转录基因的基因体中。转录伸长的阻断表明MRN的结合依赖于RNAPII转录复合物而不是转录本身。MRN的耗竭会增加TSS和MRE11 / NBS1结合基因的基因体的RNAPII丰度。细胞长时间暴露于MRE11或​​NBS1耗尽会导致跨主动转录的MRE11或​​NBS1目标基因形成单核苷酸多态性。这些数据支持一个模型，通过该模型，MRN复杂体与转录机制的结合可组成性地扫描活性基因以寻找转录诱导的DNA损伤，从而保留编码基因组的完整性。