RPA-ssDNA co-phase separation facilitates RAD51 enrichment during homologous recombination.

Homologous recombination (HR) is pivotal for the maintenance of genome integrity. During HR, replacing replication protein A (RPA) with the recombinase RAD51 on single-stranded DNA (ssDNA) is crucial in forming the presynaptic complex for homology search. However, how RAD51 identifies legitimate RPA-coated ssDNA involved in HR to prepare for the replacement remains elusive. Here, we develop an innovative approach to generate ssDNA for the single-molecule measurements of this transaction. We find that human RPA can undergo phase separation and co-condense with coated ssDNA, a process primarily mediated by the N-terminal DNA-binding domain of RPA70 (RPA70N). The resulting RPA-ssDNA co-condensates form stable nucleoprotein assemblies that withstand disruptive forces of tens of piconewtons. Intriguingly, both in vitro and in vivo evidence indicates that these co-condensates act as hubs for the local enrichment of RAD51. Consequently, deletion or sequestration of RPA70N readily suppresses the phase separation ability of RPA, reduces RAD51 enrichment, and ultimately compromises DNA double-strand break (DSB) repair. Our work defines the key determinants underlying RPA-mediated recombinase enrichment during the early stage of HR-directed DSB repair.