Top3α Is Required during the Convergent Migration Step of Double Holliday Junction Dissolution

Although Blm and Top3 alpha are known to form a minimal dissolvasome that can uniquely undo a double Holliday junction structure, the details of the mechanism remain unknown. It was originally suggested that Blm acts first to create a hemicatenane structure from branch migration of the junctions, followed by Top3 alpha performing strand passage to decatenate the interlocking single strands. Recent evidence suggests that Top3 alpha may also be important for assisting in the migration of the junctions. Using a mismatch-dHJ substrate (MM-DHJS) and eukaryotic Top1 (in place of Top3 alpha), we show that the presence of a topoisomerase is required for Blm to substantially migrate a topologically constrained Holliday junction. When investigated by electron microscopy, these migrated structures did not resemble a hemicatenane. However, when Blm is together with Top3 alpha, the dissolution reaction is processive with no pausing at a partially migrated structure. Potential mechanisms are discussed.