Cohesin regulates homology search during recombinational DNA repair.

Homologous recombination repairs DNA double-strand breaks (DSB) using an intact dsDNA molecule as a template. It entails a homology search step, carried out along a conserved RecA/Rad51-ssDNA filament assembled on each DSB end. Whether, how and to what extent a DSB impacts chromatin folding, and how this (re)organization in turns influences the homology search process, remain ill-defined. Here we characterize two layers of spatial chromatin reorganization following DSB formation in Saccharomyces cerevisiae. Although cohesin folds chromosomes into cohesive arrays of ~20-kb-long chromatin loops as cells arrest in G2/M, the DSB-flanking regions interact locally in a resection- and 9-1-1 clamp-dependent manner, independently of cohesin, Mec1ATR, Rad52 and Rad51. This local structure blocks cohesin progression, constraining the DSB region at the base of a loop. Functionally, cohesin promotes DSB-dsDNA interactions and donor identification in cis, while inhibiting them in trans. This study identifies multiple direct and indirect ways by which cohesin regulates homology search during recombinational DNA repair.