S. cerevisiae RAD50, MRE11, and XRS2 genes are required for telomere maintenance, cell cycle checkpoint signaling, meiotic recombination, and the efficient repair of DNA double-strand breaks (DSB)s by homologous recombination and nonhomologous end-joining (NHEJ). Here, we demonstrate that the complex formed by Rad50, Mre11, and Xrs2 proteins promotes intermolecular DNA joining by DNA ligase IV (Dnl4) and its associated protein Lif1. Our results show that the Rad50/Mre11/Xrs2 complex juxtaposes linear DNA molecules via their ends to form oligomers and interacts directly with Dnl4/Lif1. We also demonstrate that Rad50/Mre11/Xrs2-mediated intermolecular DNA joining is further stimulated by Hdf1/Hdf2, the yeast homolog of the mammalian Ku70/Ku80 heterodimer. These studies reveal specific functional interplay among the Hdf1/Hdf2, Rad50/Mre11/Xrs2, and Dnl4/Lif1 complexes in NHEJ.
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