Topoisomerase III-alpha in Double Holliday Junction Dissolution

Abstract

Topoisomerase IIIα (Top3α) is an essential component of the double Holliday junction (dHJ) dissolvasome complex in metazoans. Previous work has shown that Top3α and Bloom's helicase (Blm) are able to convergently migrate the dHJ to create solely non-crossover products, thus preserving genomic integrity. However, many questions remain about the details of this process. Using a combination of biochemical and genetic tools, including dHJ substrate assays, gel electrophoresis, EMSA, pulldowns, fly crosses, and electron microscopy, this work expands our knowledge of the dissolution reaction. Tail mutants of Top3α were created and tested in a series of in vitro assays. Through these experiments, I discovered that the C-terminus of Top3α is important for binding Blm, interacting with DNA, conveying RPA stimulation, and in vivo functionality. I also observed that dissolution is an extremely processive reaction, with no accumulation of intermediates prior to product formation. When a non-specific topoisomerase was used (Top1, a type IB), accumulation of an intermediate was evident; however, contrary to predicted models, direct observation revealed that this intermediate is not a hemicatenane structure and still requires branch migration. Modifications were also made to the dHJ substrate creation method so that multiple types of HJ substrates could be produced efficiently.