The MRN complex (MRX complex in yeast) is a protein complex consisting of Mre11, Rad50 and Nbs1 (also known as Nibrin  in humans and as Xrs2 in yeast). In eukaryotes, the MRN/X complex plays an important role in the initial processing of double-strand DNA breaks prior to repair by homologous recombination or non-homologous end joining. The MRN complex binds avidly to double-strand breaks both in vitro and in vivo and may serve to tether broken ends prior to repair by non-homologous end joining or to initiate resection prior to repair by homologous recombination. The MRN complex also participates in activating the checkpoint kinase ATM in response to DNA damage. Production of short single-strand oligonucleotides by Mre11 endonuclease activity has been implicated in ATM activation by the MRN complex.
Evolutionary ancestry and biologic function
The MRN complex has been mainly studied in eukaryotes. However, recent work shows that two of the three protein components of this complex, Mre11 and Rad50, are also conserved in extant prokaryotic archaea. This finding suggests that key components of the eukaryotic MRN complex are derived by evolutionary descent from the archaea. In the archaeon Sulfolobus acidocaldarius, the Mre11 protein interacts with the Rad50 protein and appears to have an active role in the repair of DNA damages experimentally introduced by gamma radiation. Similarly, during meiosis in the eukaryotic protist Tetrahymena Mre11 is required for repair of DNA damages, in this case double-strand breaks, by a process that likely involves homologous recombination.
Role in human disease
Mutations in the human Nbs1 subunit of the MRN complex have been implicated in the rare genetic disorder Nijmegen Breakage Syndrome.
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