The Bloom syndrome complex senses RPA-coated single-stranded DNA to restart stalled replication forks

The Bloom syndrome helicase BLM interacts with topoisomerase III alpha (TOP3A), RMI1 and RMI2 to form the BTR complex, which dissolves double Holliday junctions to produce non-crossover homologous recombination (HR) products. BLM also promotes DNA-end resection, restart of stalled replication forks, and processing of ultra-fine DNA bridges in mitosis. How these activities of the BTR complex are regulated in cells is still unclear. Here, we identify multiple conserved motifs within the BTR complex that interact cooperatively with the single-stranded DNA (ssDNA)-binding protein RPA. Furthermore, we demonstrate that RPA-binding is required for stable BLM recruitment to sites of DNA replication stress and for fork restart, but not for its roles in HR or mitosis. Our findings suggest a model in which the BTR complex contains the intrinsic ability to sense levels of RPA-ssDNA at replication forks, which controls BLM recruitment and activation in response to replication stress. The BLM helicase interacts with the topoisomerase TOP3A and RMI1 to form the BTR complex. Here, the authors reveal that this complex contains multiple binding sites for the single-stranded DNA-binding complex RPA, and that RPA-binding stimulates BLM recruitment to stalled replication forks to promote their restart after replication stress.