RMI1 promotes DNA replication fork progression and recovery from replication fork stress.

RMI1 is a member of an evolutionarily conserved complex composed of BLM and topoisomerase III alpha (TopoIII alpha). This complex exhibits strand passage activity in vitro, which is likely important for DNA repair and DNA replication in vivo. The inactivation of RMI1 causes genome instability, including elevated levels of sister chromatid exchange and accelerated tumorigenesis. Using molecular combing to analyze DNA replication at the single-molecule level, we show that RMI1 is required to promote normal replication fork progression. The fork progression defect in RMI1-depleted cells is alleviated in cells lacking BLM, indicating that RMI1 functions downstream of BLM in promoting replication elongation. RMI1 localizes to subnuclear foci with BLM and TopoIII alpha in response to replication stress. The proper localization of the complex requires a BLM-TopoIII alpha-RMI1 interaction and is essential for RMI1 to promote recovery from replication stress. These findings reveal direct roles of RMI1 in DNA replication and the replication stress response, which could explain the molecular basis for its involvement in suppressing sister chromatid exchange and tumorigenesis.