3 years ago

Single-Molecule Imaging Reveals How Mre11-Rad50-Nbs1 Initiates DNA Break Repair

Single-Molecule Imaging Reveals How Mre11-Rad50-Nbs1 Initiates DNA Break Repair
DNA double-strand break (DSB) repair is essential for maintaining our genomes. Mre11-Rad50-Nbs1 (MRN) and Ku70-Ku80 (Ku) direct distinct DSB repair pathways, but the interplay between these complexes at a DSB remains unclear. Here, we use high-throughput single-molecule microscopy to show that MRN searches for free DNA ends by one-dimensional facilitated diffusion, even on nucleosome-coated DNA. Rad50 binds homoduplex DNA and promotes facilitated diffusion, whereas Mre11 is required for DNA end recognition and nuclease activities. MRN gains access to occluded DNA ends by removing Ku or other DNA adducts via an Mre11-dependent nucleolytic reaction. Next, MRN loads exonuclease 1 (Exo1) onto the free DNA ends to initiate DNA resection. In the presence of replication protein A (RPA), MRN acts as a processivity factor for Exo1, retaining the exonuclease on DNA for long-range resection. Our results provide a mechanism for how MRN promotes homologous recombination on nucleosome-coated DNA.

Graphical abstract

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Teaser

Myler et al. directly observe how Mre11-Rad50-Nbs1 (MRN) recognizes DNA breaks, removes Ku, and promotes DNA resection to initiate homologous recombination. Overall, their results provide a mechanism for double-strand break repair initiation at protein-occluded DNA ends.

Publisher URL: www.sciencedirect.com/science

DOI: S1097276517305762

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