Cohesin reshapes replication fork contacts to aid fork slowing and reversal

DNA replication forks can be challenged by cancer chemotherapeutic treatments, leading to accumulation of single-stranded DNA and slowdown of DNA synthesis. The marked plasticity of replication forks under replication stress ensures fork stability, damage tolerance and complete genome duplication 1 . Initiation and progression of replication forks occur in a three-dimensionally organized genome. DNA loop extrusion by the cohesin complex organizes the genome 2 and regulates the initiation and positioning of DNA replication origins 3,4 . Although transient interaction of sister forks was recently reported during unperturbed replication 5 , the functional relevance of fork contacts during replication stress and the role of cohesin in this context remain unknown. Here we show that cohesin-mediated loop extrusion rearranges nascent DNA contacts at stressed replication forks to promote genome stability. Using auxin-inducible degron 6 , separation-of-function mutants 7–9 and a newly developed Micro-C-based technique to capture chromatin contacts at nascent DNA (Repli-C), we found that loop-extruding cohesin accumulates at stalled replication forks, limiting sister-fork coupling in favour of inter-replicon contacts. This process promotes active fork slowing and reversal by preventing PRIMPOL action on single-stranded DNA 1 . These findings show that the replication stress response is not merely an accumulation of individual regulatory events, but is topologically integrated across the genome through cohesin loop extrusion. While providing a new function for loop-extruding cohesin, our results indicate the potential impact on cancer therapy of frequent cohesin mutations in tumours 10 .