MRX increases chromatin accessibility at stalled replication forks to promote nascent DNA resection and cohesin loading

The recovery of stalled replication forks depends on the controlled resection of nascent DNA and on the loading of cohesin. These processes operate in the context of nascent chromatin, but the impact of nucleosome structure on fork restart remains poorly understood. Here, we show that the Mre11-Rad50-Xrs2 (MRX) complex acts together with the chromatin modifiers Gcn5 and Set1 and the histone remodelers RSC, Chd1 and Isw1 to promote chromatin remodeling at stalled forks. Increased chromatin accessibility facilitates the resection of nascent DNA by the Exo1 nuclease and the Sgs1 and Chl1 DNA helicases. Importantly, increased ssDNA promotes the recruitment of cohesin to arrested forks in a Scc2-Scc4-dependent manner. Altogether, these results indicate that MRX cooperates with chromatin modifiers to orchestrate the action of remodelers, nucleases and DNA helicases, promoting the resection of nascent DNA and the loading of cohesin, two key processes involved in the recovery of arrested forks.

停滞复制叉的修复依赖于新生DNA的可控切除以及黏连蛋白的加载。上述过程均发生于新生染色质的环境中，但核小体结构对复制叉重启的影响迄今仍未被充分阐明。本研究显示，Mre11-Rad50-Xrs2（MRX）复合物可与染色质修饰因子Gcn5、Set1以及组蛋白重塑因子RSC、Chd1和Isw1协同作用，促进停滞复制叉处的染色质重塑。增强的染色质可及性可通过Exo1核酸酶以及Sgs1、Chl1 DNA解旋酶，推动新生DNA的切除过程。尤为重要的是，单链DNA（single-stranded DNA, ssDNA）的积累会以Scc2-Scc4依赖的方式，介导黏连蛋白向停滞复制叉的招募。综上，本研究结果表明，MRX可与染色质修饰因子协同配合，统筹重塑因子、核酸酶与DNA解旋酶的功能活动，进而促进新生DNA的切除与黏连蛋白的加载——这两项过程皆是停滞复制叉修复中的关键环节。