Bloom syndrome is a rare autosomal recessive disorder that is caused by mutations in the BLM gene. BLM associates with TopoIIIα and RMI1 to form a complex that is essential to maintain genome integrity. This complex catalyzes a dissolution reaction that resolves recombination intermediates containing two Holliday junctions without crossing over of genetic material. Dissolution activity is remarkable because it accounts for the in vivo role of BLM-TopoIIIα-RMI1 in suppressing sister chromatid exchanges. To further understand the biochemical roles that each member of the BLM complex plays in dissolution, I generated single-stranded catenanes that resemble the proposed intermediates at the latest steps of dissolution. Using this substrate, I demonstrated that TopoIIIα is a single-stranded DNA decatenase that is specifically stimulated by BLM and RMI1. Interaction between TopoIIIα and RMI1 is essential for the optimal decatenase activity. Furthermore, binding of RPA to single-stranded DNA substrate inhibits TopoIIIα decatenase activity. However, complex formation between BLM, TopoIIIα and RMI enables TopoIIIα to displace RPA and catalyze decatenation. Since the decatenase activity is presumed to be involved in many aspects of DNA metabolism, I investigated the roles of RMI1 and TopoIIIα in DNA replication in vivo. Using the molecular combing technique, I showed that RMI1 functions downstream of BLM to promote normal replication fork progression. In addition, BLM, TopoIIIα and RMI1 colocalize with one another in response to replication stress. Finally, interaction between TopoIIIα and RMI1 is essential for nuclear localization of the complex and for the complex to promote recovery from replication stress. This work defines molecular functions for RMI1 and TopoIIIα in DNA replication and repair, providing insight into their roles as suppressors of genome instability.

Bloom syndrome is a rare autosomal recessive disorder that is caused by mutations in the BLM gene. BLM associates with TopoIIIα and RMI1 to form a complex that is essential to maintain genome integrity. This complex catalyzes a dissolution reaction that resolves recombination intermediates containing two Holliday junctions without crossing over of genetic material. Dissolution activity is remarkable because it accounts for the in vivo role of BLM-TopoIIIα-RMI1 in suppressing sister chromatid exchanges. To further understand the biochemical roles that each member of the BLM complex plays in dissolution, I generated single-stranded catenanes that resemble the proposed intermediates at the latest steps of dissolution. Using this substrate, I demonstrated that TopoIIIα is a single-stranded DNA decatenase that is specifically stimulated by BLM and RMI1. Interaction between TopoIIIα and RMI1 is essential for the optimal decatenase activity. Furthermore, binding of RPA to single-stranded DNA substrate inhibits TopoIIIα decatenase activity. However, complex formation between BLM, TopoIIIα and RMI enables TopoIIIα to displace RPA and catalyze decatenation. Since the decatenase activity is presumed to be involved in many aspects of DNA metabolism, I investigated the roles of RMI1 and TopoIIIα in DNA replication in vivo. Using the molecular combing technique, I showed that RMI1 functions downstream of BLM to promote normal replication fork progression. In addition, BLM, TopoIIIα and RMI1 colocalize with one another in response to replication stress. Finally, interaction between TopoIIIα and RMI1 is essential for nuclear localization of the complex and for the complex to promote recovery from replication stress. This work defines molecular functions for RMI1 and TopoIIIα in DNA replication and repair, providing insight into their roles as suppressors of genome instability.