Cohesin regulates homology search during recombinational DNA repair

Summary Homologous recombination (HR) is a ubiquitous DNA double-strand break (DSB) repair mechanism that promotes cell survival. It entails a potentially genome-wide homology search step, carried out along a conserved RecA/Rad51-ssDNA nucleoprotein filament (NPF) assembled on each DSB ends 1–3 . This search is subdued to NPF-dsDNA collision probability, dictated in part by chromatin conformation 2,4 . In contrast to the extensive knowledge about chromatin composition and mobility changes elicited by the DNA damage checkpoint (DDC) 5–7 , whether, how, and to which extent a DSB impacts spatial chromatin organization, and whether this organization in turns influences the homology search process, remains ill-defined 8,9 . Here we characterize two layers of spatial chromatin reorganization following DSB formation in S. cerevisiae. While cohesin folds chromosomes into cohesive arrays of 10-20 kb long chromatin loops as cells arrest in G2/M 10,11 , the DSB-flanking regions locally interact in a resection- and 9-1-1 clamp-dependent manner, independently of cohesin and HR proteins. This local structure blocks cohesin progression, constraining the extending NPF at loop base. Functionally this organization promotes side-specific cis DSB-dsDNA interactions that scales with loop expansion span, and provides a kinetic advantage for identification of intra- over inter-chromosomal homologies. We propose that cohesins regulate homology search by promoting cis dsDNA over-sampling, both upon loop expansion-coupled unidimensional dsDNA scanning, NPF trapping, and chromosome individualization, largely independent of their role in sister chromatid cohesion.