Human Topoisomerase IIIα Is a Single-stranded DNA Decatenase That Is Stimulated by BLM and RMI1

Human topoisomerase III alpha is a type IA DNA topoisomerase that functions with BLM and RMI1 to resolve DNA replication and recombination intermediates. BLM, human topoisomerase III alpha, and RMI1 catalyze the dissolution of double Holliday junctions into noncrossover products via a strand-passage mechanism. We generated single-stranded catenanes that resemble the proposed dissolution intermediate recognized by human topoisomerase III alpha. We demonstrate that human topoisomerase III alpha is a single-stranded DNA decatenase that is specifically stimulated by the BLM-RMI1 pair. In addition, RMI1 interacts with human topoisomerase III alpha, and the interaction is required for the stimulatory effect of RMI1 on decatenase activity. Our data provide direct evidence that human topoisomerase III alpha functions as a decatenase with the assistance of BLM and RMI1 to facilitate the processing of homologous recombination intermediates without crossing over as a mechanism to preserve genome integrity.