Abstract
In all the eukaryotic cells, nucleolytic processing (resection) of a double strand DNA break (DSB) is a key step to channel the repair of the lesion toward the homologous recombination, at the expenses of the non-homologous end joining (NHEJ). The coordinated action of several nucleases and helicases generates 3′ single strand (ss) DNA, which is covered by RPA and recombination factors. Molecular details of the process have been first dissected in the model organism Saccharomyces cerevisiae. When DSB ends are occupied by KU, a central component of the NHEJ, the Mre11-Rad50-Xrs2 (MRX) nuclease complex (MRN in human), aided by the associated factors Sae2 (CTIP in human), initiates the resection process, inducing a nick close to the DSB ends. Then, starting from the nick, the nucleases Mre11, Exo1, Dna2, in cooperation with Sgs1 helicase (BLM in human), degrade DNA strand in both the directions, creating the 3′ ssDNA filament. Multiple levels of regulation of the break processing ensure faithful DSB repair, preventing chromosome rearrangements, and genome instability. Here we review the DSB resection process and its regulation in the context of chromatin. Particularly, we focus on proteins that limit DSB resection, acting as physical barriers toward nucleases and helicases. Moreover, we also take into consideration recent evidence regarding functional interplay between DSB repair and RNA molecules nearby the break site.
Highlights
Specialty section: This article was submitted to Cellular Biochemistry, a section of the journal Frontiers in Molecular Biosciences
In all the eukaryotic cells, nucleolytic processing of a double strand DNA break (DSB) is a key step to channel the repair of the lesion toward the homologous recombination, at the expenses of the non-homologous end joining (NHEJ)
When DSB ends are occupied by Ku70-80 complex (KU), a central component of the NHEJ, the Mre11-Rad50-Xrs2 (MRX) nuclease complex (MRN in human), aided by the associated factors Sae2 (CTIP in human), initiates the resection process, inducing a nick close to the DSB ends
Summary
DSBs are classically defined as broken chromosomes, uncapped telomere ends and reversed forks are bound and processed by the same factors. Recent in vitro data indicate that BLM promotes the EXO1 resection processivity, too (Soniat et al, 2019) This nick-dependent mechanism for resection is activated in S and G2/M phases through the CDK1-dependent phosphorylation of Sae (CTIP in human) (Huertas et al, 2008; Huertas and Jackson, 2009), which associates with the Mre complex. Following DSB-induced phosphorylation by ATM, 53BP1 recruits RIF1, the Shielding complex and the CST/ Pol α-Prim complex that fills in the resected DNA end, restoring dsDNA and allowing NHEJ [see a recent review here (Setiaputra and Durocher, 2019)] It is an open debate whether 53BP1 and its partners exert their function to limit resection directly as a physical barrier to nucleases or indirectly reconstituting processed DNA ends (Setiaputra and Durocher, 2019). In human cells the 5′ −3′ translocase HELB limits EXO1 and DNA2/BLM nuclease activity (Tkac et al, 2016)
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