Homology-directed repair involves multiple strand invasion cycles in fission yeast

Homology-directed repair of DNA double-strand breaks (DSBs) can be a highly faithful pathway. Non-crossover repair dominates in mitotically growing cells, likely through a preference for synthesis-dependent strand annealing (SDSA). While genetic studies highlight a key role for the RecQ helicase BLM/Rqh1 (in human and S. pombe cells, respectively) in promoting noncrossover repair, how homology-directed repair mechanism choice is orchestrated in time and space is not well understood. Here, we develop a microscopy-based assay in living fission yeast to determine the dynamics and kinetics of an engineered, site-specific interhomologue repair event. We observe highly efficient homology search and homology-directed repair in this system. Surprisingly, we find that the initial distance between the DSB and the donor sequence does not correlate with the duration of repair. Instead, we observe that repair is likely to involve multiple site-specific and Rad51-dependent co-localization events between the DSB and donor sequence, suggesting that efficient interhomologue repair in fission yeast often involves multiple strand invasion events. By contrast, we find that loss of Rqh1 leads to successful repair through a single strand invasion event, suggesting that multiple strand invasion cycles reflect ongoing SDSA. However, failure to repair is also more likely in rqh1Δ cells, which could reflect increased strand invasion at non-homologous sites. This work has implications for the molecular etiology of Bloom syndrome, caused by mutations in BLM and characterized by aberrant sister chromatid crossovers and inefficient repair.