Multifaceted role of the Topo IIIα-RMI1-RMI2 complex and DNA2 in the BLM-dependent pathway of DNA break end resection.

BLM, a RecQ family DNA helicase mutated in Bloom's Syndrome, participates in homologous recombination at two stages: 5' DNA end resection and double Holliday junction dissolution. BLM exists in a complex with Topo IIIα, RMI1 and RMI2. Herein, we address the role of Topo IIIα and RMI1-RMI2 in resection using a reconstituted system with purified human proteins. We show that Topo IIIα stimulates DNA unwinding by BLM in a manner that is potentiated by RMI1-RMI2, and that the processivity of resection is reliant on the Topo IIIα-RMI1-RMI2 complex. Topo IIIα localizes to the ends of double-strand breaks, thus implicating it in the recruitment of resection factors. While the single-stranded DNA binding protein RPA plays a major role in imposing the 5' to 3' polarity of resection, Topo IIIα also makes a contribution in this regard. Moreover, we show that DNA2 stimulates the helicase activity of BLM. Our results thus uncover a multifaceted role of the Topo IIIα-RMI1-RMI2 ensemble and of DNA2 in the DNA resection reaction.