Mini-reviewMaking the best of the loose ends: Mre11/Rad50 complexes and Sae2 promote DNA double-strand break resection
Introduction
The repair of DNA double-strand breaks (DSBs) through mechanisms of homologous recombination initiates with the processing of the broken DNA strands, primarily removal of the 5′ strand [1], [2]. This process of 5′ strand resection is essential for the eventual establishment of a Rad51 filament on the 3′ strand, which performs the homology search for a target to use as a template for replication [3]. In eukaryotes, the 3′ single-stranded DNA (ssDNA) generated during resection is first coated by the RPA complex before exchange for Rad51 by mediator proteins [4], and plays an important biological role in recruitment and activation of Mec1(ATR) in the replication checkpoint [5]. In budding yeast, both resection and checkpoint activation occur much more efficiently in S and G2 phases of the cell cycle when sister chromatids are present [6], [7]. 5′ strand resection is thus a critical event in the initiation of homologous recombination as well as in S phase cell cycle checkpoint control in eukaryotic cells. Resection enzymes and their roles in vivo have recently been reviewed in DNA Repair [8], thus this review focuses primarily on the mechanisms by which the Mre11/Rad50/Xrs2(Nbs1) (MRX(N)) complex and Sae2(CtIP) facilitate DNA end processing.
Section snippets
DSB resection in budding yeast
DSB resection was observed decades ago [1] but only in the last few years have the molecular details of this process become more apparent [8], [9], [10]. Most of these details have been elucidated in S. cerevisiae, where resection of DSBs has primarily been measured during meiotic recombination, or at sites of inducible endonuclease-generated breaks in vegetatively growing cells. The extent of resection can vary depending on the presence of functional strand invasion machinery, as well as the
MRX and Sae2 introduction
Mre11/Rad50 complexes are found in all organisms and are important for efficient DSB repair, as well as for signaling of DSBs that occurs through the Tel1(ATM) protein kinase in eukaryotes [15], [16]. The Mre11 protein is in the lambda phosphatase family of phosphoesterases, and recombinant Mre11 proteins from several divergent species exhibit exo- and endonuclease activity in vitro [17], [18], [19], [20], [21], [22], [23], [24]. Exonuclease activity in manganese occurs in the 3′ to 5′
Summary and future questions
We now understand much more about the mechanisms of DSB resection than we did a few years ago. The enzymes responsible for long-range resection in S. cerevisiae have been identified as Exo1 and Dna2/Sgs1/Rmi1/Top3 [13], [55], [56], [57], and it appears that at least Exo1 function in resection has been conserved in human cells [118]. The RecQ helicase BLM also has been shown to stimulate the activity of human Exo1 in vitro [119], and to affect DSB resection in human cells [57], suggesting that
Conflict of interest statement
The authors declare that there are no conflicts of interest.
Acknowledgments
I am grateful to Sang Eun Lee, Gregory Ira, John Petrini, and members of the Paull laboratory for their critical reading of this manuscript. Research performed in the Paull laboratory that is cited in this review was supported by a grant from the National Institutes of Health (R01 CA094008).
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