If not properly processed and repaired, DNA double-strand breaks (DSBs) can give rise to deleterious chromosome rearrangements, which could ultimately lead to the tumor phenotype 1, 2. DSB ends are resected in a 5′ to 3′ fashion in cells, to yield single-stranded DNA for the recruitment of factors critical for DNA damage checkpoint activation and repair by homologous recombination2. The resection process involves redundant pathways consisting of nucleases, DNA helicases, and associated proteins3. Being guided by recent genetic studies 4-6 , we have reconstituted the first eukaryotic ATP-dependent DNA end resection machinery comprising the Saccharomyces cerevisiae Mre11-Rad50-Xrs2 (MRX) complex, the Sgs1-Top3-Rmi1 (STR) complex, Dna2 protein and the heterotrimeric single-strand DNA binding protein RPA. We show that DNA strand separation during end resection is mediated by the Sgs1 helicase function, in a manner that is enhanced by Top3-Rmi1 and MRX. In congruence with genetic observations 6 , while the Dna2 nuclease activity is critical for resection, the Mre11 nuclease activity is dispensable. By examining the top3 Y356F allele and its encoded protein, we provide evidence that the topoisomerase activity of Top3, although critical for the suppression of crossover recombination 2,7 , is not needed for resection either in cells or in the reconstituted system. Our results also unveil a multi-faceted role of RPA, in the sequestration of ssDNA generated by DNA unwinding, enhancement of 5′ strand incision, and protection of the 3′ strand. Our reconstituted system should serve as a useful model for delineating the mechanistic intricacy of the DNA break resection process in eukaryotes.The 3′ ssDNA strands derived from DSB resection attract RPA, which promotes the recruitment of checkpoint proteins to effect cell cycle arrest 8 . With the aid of a recombination mediator protein, such as yeast Rad52 or human BRCA2, the Rad51 recombinase displaces RPA from the ssDNA to assemble into a right-handed helical polymer capable of initiating DSB repair by homologous recombination 1,2 . Genetic studies in yeast have shown that DSB resection proceeds in two steps. The MRX complex plays a 3 To whom correspondences and request for materials should be addressed: Gregory Ira: gira@bcm.edu, Patrick Sung: patrick.sung@yale.edu. role in initiation, while the Sgs1 helicase, its associated proteins Top3 and Rmi1, and the helicase/nuclease Dna2, whose nuclease activity is needed for Okazaki fragment processing 9,10 , constitute the DNA motor-driven path of long-range resection. Exo1, a 5′-3′ exonuclease, defines a redundant resection means 4-6 . Here we reconstitute the Sgs1/Dna2-dependent DNA resection machinery and present results germane for understanding its mechanistic underpinnings.The requisite factors, namely, Sgs1, Top3-Rmi1 (TR) complex, MRX complex, Dna2, and RPA were purified and analyzed (see Supplementary Fig. 2 and the Supplementary Information). As shown in Figure 1a, the combination of these factors degraded a 1.9-kb ...

DNA strand exchange plays a central role in genetic recombination across all kingdoms of life, but the physical basis for these reactions remains poorly defined. Using single-molecule imaging, we found that bacterial RecA and eukaryotic Rad51 and Dmc1 all stabilize strand exchange intermediates in precise three-nucleotide steps. Each step coincides with an energetic signature (0.3 kBT) that is conserved from bacteria to humans. Triplet recognition is strictly dependent on correct Watson-Crick pairing. Rad51, RecA, and Dmc1 can all step over mismatches, but only Dmc1 can stabilize mismatched triplets. This finding provides insight into why eukaryotes have evolved a meiosis-specific recombinase. We propose that canonical Watson-Crick base triplets serve as the fundamental unit of pairing interactions during DNA recombination.

Summary The tumor suppressor BRCA2 is thought to facilitate the handoff of ssDNA from replication protein A (RPA) to the RAD51 recombinase during DNA break and replication fork repair by homologous recombination. However, we find that RPA-RAD51 exchange requires the BRCA2 partner DSS1. Biochemical, structural, and in vivo analyses reveal that DSS1 allows the BRCA2-DSS1 complex to physically and functionally interact with RPA. Mechanistically, DSS1 acts as a DNA mimic to attenuate the affinity of RPA for ssDNA. A mutation in the solvent-exposed acidic domain of DSS1 compromises the efficacy of RPA-RAD51 exchange. Thus, by targeting RPA and mimicking DNA, DSS1 functions with BRCA2 in a two-component homologous recombination mediator complex in genome maintenance and tumor suppression. Our findings may provide a paradigm for understanding the roles of DSS1 in other biological processes.

The BLAP75 protein combines with the BLM helicase and topoisomerase (Topo) III␣ to form an evolutionarily conserved complex, termed the BTB complex, that functions to regulate homologous recombination. BLAP75 binds DNA, associates with both BLM and Topo III␣, and enhances the ability of the BLM-Topo III␣ pair to branch migrate the Holliday junction (HJ) or dissolve the double Holliday junction (dHJ) structure to yield non-crossover recombinants. Here we seek to understand the relevance of the biochemical attributes of BLAP75 in HJ processing. With the use of a series of BLAP75 protein fragments, we show that the evolutionarily conserved N-terminal third of BLAP75 mediates complex formation with BLM and Topo III␣ and that the DNA binding activity resides in the C-terminal third of this novel protein. Interestingly, the N-terminal third of BLAP75 is just as adept as the full-length protein in the promotion of dHJ dissolution and HJ unwinding by BLMTopo III␣. Thus, the BLAP75 DNA binding activity is dispensable for the ability of the BTB complex to process the HJ in vitro. Lastly, we show that a BLAP75 point mutant (K166A), defective in Topo III␣ interaction, is unable to promote dHJ dissolution and HJ unwinding by BLM-Topo III␣. This result provides proof that the functional integrity of the BTB complex is contingent upon the interaction of BLAP75 with Topo III␣.Bloom syndrome is a rare, hereditary disorder characterized by proportional dwarfism, light sensitivity, immunodeficiency, male infertility, and high incidence of various types of cancer (1). Cells derived from patients with Bloom syndrome display a high degree of chromosomal instability, marked by a dramatic increase in the frequency of sister chromatid exchanges that arise from the crossing over of chromatid arms during resolution of homologous recombination (HR) 3 intermediates (2, 3). These results indicate an important function of BLM, the protein mutated in Bloom syndrome, in the suppression of HRmediated crossover events.BLM is one of the five RecQ-like DNA helicases in humans (4, 5). Consistent with its HR regulatory role, BLM is able to dissociate various DNA structures that resemble HR intermediates, such as the D-loop and Holliday junction (HJ) (6 -8). Importantly, BLM cooperates with Topo III␣, a Type 1A topoisomerase, to catalyze the resolution of the double Holliday junction (dHJ) intermediate to generate exclusively non-crossover recombinants, in a process termed "dHJ dissolution" (9). Remarkably, BLM also acts to disrupt the Rad51 presynaptic filament and can stimulate DNA repair synthesis by DNA polymerase (10). All these noted HR-related functions of BLM are strictly dependent on its ATPase activity (9 -11). The ability of BLM to unwind HR intermediates, to mediate the dismantling of the Rad51 presynaptic filament, and to catalyze Topo III␣-dependent dHJ dissolution is likely important for the regulation of HR to limit the formation of crossovers and prevent genome rearrangements induced by crossover HR events (10, 12, 13), whereas the DN...

NUCKS1 (nuclear casein kinase and cyclin-dependent kinase substrate 1) is a 27 kD chromosomal, vertebrate-specific protein, for which limited functional data exist. Here, we demonstrate that NUCKS1 shares extensive sequence homology with RAD51AP1 (RAD51 associated protein 1), suggesting that these two proteins are paralogs. Similar to the phenotypic effects of RAD51AP1 knockdown, we find that depletion of NUCKS1 in human cells impairs DNA repair by homologous recombination (HR) and chromosome stability. Depletion of NUCKS1 also results in greatly increased cellular sensitivity to mitomycin C (MMC), and in increased levels of spontaneous and MMC-induced chromatid breaks. NUCKS1 is critical to maintaining wild type HR capacity, and, as observed for a number of proteins involved in the HR pathway, functional loss of NUCKS1 leads to a slow down in DNA replication fork progression with a concomitant increase in the utilization of new replication origins. Interestingly, recombinant NUCKS1 shares the same DNA binding preference as RAD51AP1, but binds to DNA with reduced affinity when compared to RAD51AP1. Our results show that NUCKS1 is a chromatin-associated protein with a role in the DNA damage response and in HR, a DNA repair pathway critical for tumor suppression.