Consistent with this idea, it has been shown that Sae2 and MRX are not essential for resection and downstream HR at clean DSBs in yeast (76C79)

Consistent with this idea, it has been shown that Sae2 and MRX are not essential for resection and downstream HR at clean DSBs in yeast (76C79). blocked and clean DSBs is initiated via unique mechanisms. INTRODUCTION DNA double-strand breaks (DSBs) are arguably the most hazardous forms of DNA damage in cells, which can be caused by ionizing radiation, reactive oxygen species, chemotherapeutic drugs and collapse of replication forks, or induced during genome engineering with CRISPR, ZFN and TALEN technologies (1C3). DSBs also occur as programmed recombination events during meiosis and V(D)J recombination in lymphocyte development (4,5). Regardless of their origin, DSBs present a serious threat and can lead to genomic instability or cell death if not properly repaired. To cope with this problem, cells have developed a highly sophisticated mechanism called DNA damage response (DDR) to detect, signal and repair Raddeanin A these breaks (6C8). DSBs are repaired mainly by two largely competing pathways: non-homologous end joining (NHEJ) and homologous recombination (HR) (9C11). While NHEJ can occur throughout the cell cycle, HR is mainly limited to S and G2 phases when a homologous copy of the damaged region is usually available (11C14). The choice between these repair pathways is usually dictated by end resection, a DNA processing mechanism that selectively degrades the 5 strand DNA from your ends to generate long 3 ssDNA overhangs required for HR in S and G2 phases of the cell cycle. By transforming dsDNA ends into ssDNA structures, resection promotes HR and averts NHEJ (11,15C18). DSB resection also controls the checkpoint responses that coordinate DNA repair with other cellular processes such as cell cycle progression and gene expression (19C22). Checkpoint responses are controlled by ATM and ATR Raddeanin A protein kinases, both of which are activated by Rabbit Polyclonal to DDX3Y DSBs (23C25). Whereas ATM activation occurs on double-strand DNA structure adjacent to the DNA break ends, ATR is usually activated around the ssDNA structure generated by resection (26C28). Consequently, DSB resection promotes the ATR checkpoint pathway and attenuates the ATM checkpoint pathway (29C31). Thus, resection governs both DNA repair and checkpoint signaling in the DSB damage response. DSB resection entails multiple enzymatic activities including nucleases and helicases and is tightly regulated to ensure genomic stability (17,32). Studies in multiple organisms such as yeasts, extracts (43,66C74). How resection of clean DSBs with free endswhich can be generated at collapsed replication forks or by endonucleases or malignancy drugsis initiated remains an outstanding question. Because Sae2CMRX in yeast can carry out limited resection of a clean DSB generated by HO endonuclease in the absence of the Exo1 and Dna2 pathways, it was Raddeanin A suggested that resection of clean DSBs is also initiated by Sae2CMRX (CtIPCMRN) (33,34). However, this observation does not address which nuclease initiates resection of clean DSBs when all resection activities are present in cells. Unlike blocked DSBs, clean DSBs can be directly resected by Dna2 and Exo1 (together with other factors) (48,50C52,57,58,75), raising the possibility that these nucleases could initiate resection at these breaks. Consistent with this idea, it has been shown that Sae2 and MRX are not essential for resection and downstream HR at clean DSBs in yeast (76C79). In the Raddeanin A cytosolic extract the nuclease activity of MRN is usually dispensable for the overall resection of clean DSBs, which is in sharp contrast to 5 blocked ends where Raddeanin A the nuclease activity of MRN is absolutely essential (80). Similarly, the catalytic function of CtIP has also been shown to be dispensable for resection of clean DSBs.