Double-strand breaks arising by replication through a nick are repaired by cohesin-dependent sister-chromatid exchange

Molecular studies on double- strand break ( DSB) repair in mitosis are usually performed with enzymatically induced DSBs, but spontaneous DSBs might arise because of replication failures, for example when replication encounters nicks. To study repair of replication- born DSBs, we defined a system in Saccharomyces cerevisiae for the induction of a site- specific single- strand break. We show that a 21- base pair ( bp) HO site is cleaved at only one strand by the HO endonuclease, with the resulting nick being converted into a DSB by replication during the S phase. Repair of such replication- born DSBs occurs by sister- chromatid exchange ( SCE). We provide molecular evidence that cohesins are required for repair of replication- born DSBs by SCE, as determined in smc3, scc1 and scc2 mutants, but not for other recombinational repair events. This work opens new perspectives to understand the importance of single- strand breaks as a source of recombination and the relevance of cohesion in the repair of replication- born DSBs.