In eukaryotic cells, RAD52 protein plays a central role in genetic recombination and DNA repair by (i) promoting the annealing of complementary single-stranded DNA and (ii) stimulation of the RAD51 recombinase. The single-strand annealing domain resides in the N-terminal region of the protein and is highly conserved, whereas the nonconserved RAD51-interaction domain is located in the C-terminal region. An N-terminal fragment of human RAD52 (residues 1-209) has been purified to homogeneity and, similar to the full-size protein (residues 1-418), shown to promote singlestrand annealing in vitro. We have determined the crystal structure of this single-strand annealing domain at 2.7 Å. The structure reveals an undecameric (11) subunit ring with extensive subunit contacts. A large, positively charged groove runs along the surface of the ring, readily suggesting a mechanism by which RAD52 presents the single strand for reannealing with complementary single-stranded DNA.recombination ͉ DNA repair ͉ crystallography T he ability of a cell to survive agents that promote genome breakage requires efficient recombinational repair systems. In lower eukaryotes, repair involves the RAD52 epistasis group of genes including RAD50, RAD51, RAD52, RAD54, RAD55, RAD57, RAD59, MRE11, and XRS2 (1,2). A key member of this group, RAD52, encodes a protein that plays a dual role in recombination by (i) promoting the annealing of complementary single strands (3-5) and (ii) stimulating in vitro recombination reactions catalyzed by the RAD51 recombinase (6-8).Human RAD52 protein shares many properties with its yeast counterpart including DNA strand annealing and RAD51 stimulation activities (9-11). The human protein has been visualized by electron microscopy, and a low-resolution three-dimensional structure has been determined (12, 13). Human RAD52 consists of seven subunits that are organized in the form of a ring with a large central channel. The Saccharomyces cerevisiae RAD52 protein has been shown also to form rings, but no detailed structure is available presently (4).The mechanism by which RAD52 promotes single-stranded DNA (ssDNA) annealing is unknown. However, when complexes formed between RAD52 and ssDNA were probed with hydroxyl radicals, a unique repeating four-nucleotide hypersensitivity pattern was observed (14). Sequence-independent hypersensitivity was observed over Ϸ36 nucleotides and was phased precisely from the terminal nucleotide. These results led to the proposal that RAD52 binds ssDNA via specific interactions with the terminal base, leading to the formation of a precisely organized ssDNA-RAD52 complex in which the DNA lies on an exposed surface of the protein ring.Sequence comparisons, site-directed mutagenesis, and biochemical studies indicate that the highly conserved N-terminal region of RAD52 possesses ssDNA annealing activities, whereas the RAD51-interaction domain is located toward the nonconserved C-terminal region (15-18). Furthermore, the RAD59 protein shares sequence homology with the N-terminal region of RAD5...

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DNA gyrase supercoils DNA in bacteria. The fact that it is essential in all bacteria and absent from eukaryotes makes it an ideal drug target. We discuss the action of coumarin and quinolone drugs on gyrase. In the case of coumarins, the drugs are known to be competitive inhibitors of the gyrase ATPase reaction. From a combination of structural and biochemical studies, the molecular details of the gyrase-coumarin complex are well established. In the case of quinolones, the drugs are thought to act by stabilising a cleavage complex between gyrase and DNA that arrests polymerases in vivo. The exact nature of the gyrase-quinolone-DNA complex is not known; we propose a model for this complex based on structural and biochemical data.