The Bloom Syndrome Complex Senses RPA-Coated Single-Stranded DNA to Restart Stalled Replication Forks

The Bloom syndrome helicase BLM interacts with topoisomerase IIIa (TOP3A), RMI1 and RMI2 to form the BTR complex, which dissolves double Holliday junctions to produce non-crossover products during homologous recombination (HR). BLM also promotes DNA-end resection and stalled replication fork restart. However, it is still unclear how the activities of the BTR complex are regulated in cells. Here, we identify multiple highly conserved short linear peptide motifs within the BTR complex that interact cooperatively with RPA. Furthermore, we demonstrate that RPA-binding is essential for stable recruitment of BLM to DNA damage sites and for stalled fork restart, but not for its roles in HR. Disruption of RPA-binding thus constitutes a novel separation-of-function mutation of the BTR complex, and suggest a model in which BTR contains the intrinsic ability to measure levels of RPA-ssDNA at replication forks, which controls its recruitment and activation in response to replication stress.