<p>Bacteria of the genus <em>Deinococcus</em> are perhaps the most resilient life forms ever discovered, demonstrating extreme resistance to ionizing radiation, ultraviolet radiation, desiccation, and a variety of mutagenic chemical agents. The most studied member of this genus, <em>D. radiodurans</em>, has been observed to rapidly reassemble its genome following severe fragmentation by hundreds of γ-radiation induced double-strand DNA breaks. Amongst the numerous factors contributing to DNA repair, a single-stranded annealing protein, DdrB, is believed to play an important role during the initial phases of recovery. The work described in this thesis represents the first structural characterization of DdrB, revealing a novel fold for single-stranded DNA binding. Together with biochemical data delineating the DNA-binding interface, two crystal structures of the DdrB/ssDNA complex were also solved, providing a comprehensive illustration of this interaction. Quaternary assemblies observed in these crystal structures also informed on the potential contribution of higher-order nucleoprotein complexes to the function of DdrB in single-stranded annealing. Most significantly, a face-to-face assembly of DdrB/ssDNA complexes provided insight into the mechanism by which DdrB mediates annealing of DNA, which may represent a common mechanism shared by other single-stranded annealing proteins.<strong></strong></p> / Doctor of Philosophy (PhD)