MRN and Topoisomerase IIIα-RMI1/2 synchronize DNA resection motor proteins

DNA resection--the nucleolytic processing of broken DNA ends--is the first step of homologous recombination. Resection is catalyzed by the resectosome, a multi-enzyme complex that includes BLM helicase, DNA2 or EXO1 nucleases, and additional DNA-binding proteins. Although the molecular players have been known for over a decade, how the individual proteins work together to regulate DNA resection remain unknown. Using single-molecule imaging, we characterized the roles of MRN and TOP3A-RMI1/2 during long-range DNA resection. BLM partners with TOP3A-RMI1/2 to form the BTRR complex (or BLM dissolvasome). TOP3A-RMI1/2 aids BLM in initiating DNA unwinding, and along with MRN, stimulates DNA2-mediated resection. Furthermore, MRN promotes the association between BTRR and DNA, and synchronizes BLM and DNA2 translocation to prevent BLM from pausing during resection. Together, this work provides direct observation of how MRN and DNA2 harness the BTRR complex to resect DNA efficiently and how TOP3A-RMI1/2 regulates BLMs helicase activity to promote efficient DNA repair.