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Shekhar Madhuker Kumta. / Thesis (Ph.D.)--Chinese University of Hong Kong, 1998. / Includes bibliographical references (p. 175-190). / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Mode of access: World Wide Web.

Homologous recombination (HR) is a precise mechanism for repairing harmful DNA double-strand breaks. The process has been extensively studied in microbial species leading to identification of the major proteins, HR models and homology requirements. Much less is known about HR in mammalian systems, especially early HR events. Our laboratory has recently devel-oped an assay that detects the new DNA synthesis that accompanies the early homology search and strand invasion steps of HR (the 3’ extension assay). The hypothesis that homology require-ments for the early steps of HR may differ from those identified in other HR assays was tested. Plasmids bearing varying amounts of homology to the chromosomal immunoglobulin μ target locus gene were constructed and tested in the 3’ extension assay. The homology require-ments for the 3’ extension assay were somewhat lower than might be expected based on other HR assays. An approximately linear relationship between homology length and 3’ extension was also established on each side of the double-strand break. The effect of excess Rad51, an essential protein involved in early HR, was also measured with respect to homology, leading to the dis-covery that increased Rad51 resulted in an increase in 3’ extension events independent of ho-mology. In summary, 3’ extension generates a potentially unstable, short-lived HR intermediate that has less dependence on homology than a completed HR product. Homology plays a role in the initiation of HR, but it may be more important in the stabilization of the intermediate than the actual generation of the early HR product detected in the 3’ extension assay. / CIHR

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<p>Topoisomerase III&alpha; (Top3&alpha;) is an essential component of the double Holliday junction (dHJ) dissolvasome complex in metazoans. Previous work has shown that Top3&alpha; 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 Top3&alpha; were created and tested in a series of <italic>in vitro</italic> assays. Through these experiments, I discovered that the C-terminus of Top3&alpha; is important for binding Blm, interacting with DNA, conveying RPA stimulation, and <italic>in vivo</italic> 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.</p> / Dissertation