Abstract

DNA replication is one of the most critical psychological process, which mainly consists of three steps: initiation, elongation, and termination. Precise regulation through all stages of DNA replication is essential for maintaining genome integrity in proliferating cells. During each cell cycle, the complete genetic information existing in the DNA needs to be duplicated and passed on to the daughter cells. The DNA replication machinery is confronted with many challenges and stresses owing to endogenous and exogenous facts that could harm the faithful duplication of the DNA. The abnormality of DNA replication could cause genomic instability and tumorigenesis. ATR is a master regulator of cells in response to DNA replication stress to safeguard genomic stability and prevent tumorigenesis. In the past years, the key functions of post-translational modification (PTM) in ATR signaling transduction has been explored. ATR signaling is complicatedly and tightly regulated by multiple PTMs, such as phosphorylation, acetylation, ubiquitination, SUMOylation and so on. Here, we summarize how the ATR signaling pathway is tightly regulated by PTM. Furthermore, we describe the critical roles of ATR signaling in DNA replication and safeguarding genomic stability. In the past years, the key functions of post-translational modification (PTM) in DNA replication has been explored. These findings benefit for us better understand the complicated mechanism of DNA replication signaling pathway.

The compositions and work model of ATR signaling in DNA replication stress

ATR signaling pathway is consisted by several key proteins as shown in Table1 (Awasthi et al., 2015; Blackford & Jackson, 2017; Maréchal & Zou, 2013; Yang, 2004). ATR (ATM and rad3-related), the central regulator of ATR signaling pathway, is one member of PI3K-Related Kinases (PIKKs) families, which are mainly consisted of five domains: HEAT repeat, FAT domain, kinase domain, PIKK regulatory domain(PRD) and FACT domain, as shown in Fig. 1. Other members of the PIKK family contain ataxia telangiectasia mutated (ATM), DNA-dependent protein kinase (DNA–PK), homolog of Caenorhabditis elegans SMG-1 (SMG1) and mammalian target of rapamycin (mTOR,also known as FRAP). Since the PIKKs family share a highly conserved catalytic domain, their target motif is very similar. For example, ATR, ATM, SMG-1 and DNA-PK preferentially phosphorylate Ser and Thr residues followed by a Gln (S/T-Q motif) (Baretic, 2019; Cimprich & Cortez, 2008; Mordes et al., 2008). ATR is one of the most important master regulators in DNA replication, DNA repair, senescence and apoptosis. Compared to ATM, which is mainly activated by DNA double-strand breaks (DSBs), ATR is a much broader responder to various types of DNA damage, including DSBs and other multiple types of DNA damage that are implicated to DNA replication (Byun et al., 2005; Kumagai et al., 2004; Walter & Newport, 2000). It is found that the HEAT repeat domain of ATR is responsible for binding with ATRIP (ATR interacting protein, the partner of ATR) and the FAT domain that provides structural assistance to the kinase domain. The kinase domain contains PRD (PIKK regulatory domain), which is responsible with binding with ToPBP1 and FATC domain (Burrows & Elledge, 2008; Lovejoy & Cortez, 2009; Wang, 2017).

Schematic of the functional domains of ATR. The ATR (homo) is consisted of 2644 amino acids and primarily divided into 5 domains, including HEAT repeat domain, FAT domain, Kinase domain, PIKK regulatory domain (PRD) and FATC domain. The HEAT domain is important for ATR binding to ATRIP

Extensive studies have suggested that ATR is activated by multiple types of DNA damage, including telomere deprotection, DSB or agents that involved in DNA replication (Maréchal & Zou, 2013; Matsuoka, 2007), as shown in Fig. 2. Firstly, DNA damage or stalled replication forks generated ssDNA that is coated and protect by RPA, and then RPA directly binds to ATRIP which is a partner of ATR and recruits it to DNA damage sites. Hence, ATR-ATRIP complex are recruited to ssDNA (Maréchal, 2014; Maréchal & Zou, 2015; Wu, 2014; Zou & Elledge, 2003). Secondly, TopBP1 which is composed of several BRCT repeats domain (9 in mammals and Xenopus, 4 in yeast) is essential to full activation of ATR–ATRIP (Lee et al. 2007; Mordes et al., 2008). The BRCT repeat domains play a key role in recognition of phosphorylated factors (Glover et al., 2004; Yu et al., 2003). Moreover, the region between BRCT6 and BRCT7 of TopBP1 is responsible for inducing the ATR-ATRIP activation (Ohashi et al., 2014; Wardlaw et al., 2014). Additionally, the specific DNA structure including dsDNA-ssDNA junction derived from stalled replication forks or DSBs resection also activates ATR signaling pathway (Ma et al., 2020; Shiotani, 2013). Except recruiting the ATR-ATRIP complex to the DNA damage sites, RPA also brings the clamp loader RAD17-RFC to the junctions (Lee & Dunphy, 2010; Yang & Zou, 2006). Further, RAD17-RFC recruits the RAD9-HUS1-RAD1 clamp (9-1-1) to the junctions (Navadgi-Patil & Burgers, 2011). The N-terminal domain of RAD9 combined with HUS1 and RAD1 forms a heterotrimer, forming a ring-shaped structure similar to PCNA (Delacroix et al., 2007; Friedrich-Heineken, 2005). The C-terminus of RAD9 is phosphorylated at Ser387 and creates a binding site for BRCT1-2 domain of TopBP1, which domain is essential for recruiting TopBP1 to dsDNA-ssDNA junctions. Finally, TopBP1 interacts with and activates ATR-ATRIP complexes, further leading to the activation and amplification of ATR-CHK1 signaling pathway (Day, 2018; Delacroix et al., 2007; Liu, 2014). In addition, it is reported that ETAA1, an RPA binding protein, is able to directly activate ATR independently of TopBP1 (Haahr, 2016). It is shown that CLASPIN functions as an adaptor between ATR-IP and CHK1, and works as a key regulator involving in ATR mediated phosphorylation and activation of CHK1 (Kumagai & Dunphy, 2000; Liu, 2006). Once activated, ATR phosphorylates multiple downstream effects, especially CHK1, facilitating ATR-CHK1 pathway to exert several key functions to promote replication fork stability and prevent genome instability.

Model of ATR-CHK1 pathway activation by primed ssDNA during replication stress. ATR-CHK1 pathway is activated in a multi-step and multi-protein participation process. Firstly, RPA binds to and protect ssDNA. Then, ATR-ATRIP is recruited to ssDNA by RPA. Besides, RPA also promotes the recruitment of RAD17-Rfc2-5 clamp loader to junctions between ssDNA and dsDNA and the loading of 9-1-1 complex (RAD9-RAD1-HUS1) checkpoint onto dsDNA. Further, TOPBP1 is recruited to dsDNA by 9-1-1 complex. Then, ATR is activated by TOPBP1 via its ATR activation domain (AAD). Red color represents ATR activation status. The activated ATR further phosphorylates and activates CHK1 with the help of Claspin, resulting in ATR-CHK1 pathway activation to execute multiple functions to promote replication fork stability and so on

The ubiquitination and deubiquitination in ATR-CHK1 signaling pathway

Ubiquitination is one of extensively investigated PTMs, which are involved in regulating multiple physiological processes in eukaryotes, including DNA damage response, proliferation, tumorigenesis and cell cycle (Mansour, 2018; Ulrich & Walden, 2010). This PTM is usually mediated by three sequential steps: activation, conjugation, and ligation, which are catalyzed by ubiquitin-activating enzyme (E1), ubiquitin-conjugating enzyme (E2), and ubiquitin ligase (E3) enzymes, respectively. Ubiquitin, a highly conserved small protein composed of 76 amino acids, is covalently attached to lysine residues of substrate proteins (Mansour, 2018). Ubiquitination is firstly identified as a signal to induce substrates degradation by ubiquitin-proteasomal degradation system. Except for the protein degradation, ubiquitination also plays a key role in signal transduction (Swatek & Komander, 2016; Wilkinson, 2000). Ubiquitination is reported to regulate ATR-CHK1 signaling pathway involving in DNA replication (Cartel & Didier, 2020; Shiotani & Zou, 2009; Yu et al., 2020; Bennett & Clarke, 2006; Ghosh & Saha, 2012; Mirsanaye et al., 2021).

RPA, a key player in ATR activation, is ubiquitinated by PRP19 and RFWD3, resulting in promoting ATR activation and homologous recombination repair (HR) upon DNA damage (Dubois, 2017; Elia, 2015). In addition, RPA ubiquitinated by RFWD3 also facilitates the removal of RPA from DNA damage site and repair at Stalled Replication Forks (Elia, 2015; Inano, 2017). The E3 ligase, HUWE1 was reported to mediate ubiquitination of CHK1 and induce its degradation by ubiquitin–proteasome system (Cassidy et al., 2020). Moreover, two Cullin RING Ligase (CRL) complexes, E3 ligases regulate the CHK1 protein level, including the SKP1-Cullin1(Cul1)-Fbx complex, which mainly ubiquitylates CHK1 in the nucleus, whereas the CDT2-Cullin4A(Cul4A)-DDB1 complex primarily ubiquitylates CHK1 in the cytoplasm during replication stress (Cartel & Didier, 2020; Cassidy et al., 2020; Zheng et al., 2016). Meanwhile, TRAF4-mediated ubiquitination of CHK1 at K132 is essential for CHK1 phosphorylation and is activated by ATR (Yu, 2020). Moreover, BTG3-mediated CHK1 K63-linked ubiquitination regulates the chromatin loading and activity of CHK1 (Cheng et al., 2013). Interestingly, HDAC6, a member of Histone deacetylases (HDAC), potentiates E3 ligase activity and is able to promote CHK1 ubiquitination, resulting in its degradation (Moses, 2020).

The E3 ligase, HERC2 is a component of the DNA replication fork complex (Izawa, 2011). It participates in regulating replication stress by USP20-CLASPIN axis (Yuan, 2014; Zhu et al., 2014). Moreover, HERC2-mediated ubiquitination of RPA regulates its phosphorylation by ATR and ubiquitin-proteasomal degradation (Wu, 2018). Ubiquitination of CLASPIN by BRCA1 regulates its degradation upon topoisomerase 1 inhibition (Sato, 2012). APC/C Cdh1 system-mediated polyubiquitylation of CLASPIN facilitates its degradation in G1 phase (Bassermann, 2008). In addition, PLK1-mediated phosphorylation of CLASPIN promotes its ubiquitination by SCF-bTrCP ubiquitin ligase, resulting in its proteasome-dependent degradation (Peschiaroli, 2006). In addition, Cdh1/APC system mediates Rad17 degradation via ubiquitin–proteasome causing the termination of checkpoint signaling and the recovery from genotoxic stress (Zhang, 2010). 9-1-1 complex is monoubiquitinated at Lys197 of the Rad9 subunit by the Rad6/Rad18 complex, which in turn facilitates the activation of 9-1-1 complex after DNA damage (Fu, 2008). hHYD (also known as UBR5), a HECT-domain ubiquitin ligase, binds with TopBP1 via its BRCT domain and ubiquitinates TopBP1, resulting in TopBP1 instability. Moreover, upon DNA damage, TopBP1 is phosphorylated, which in turn inhibits its ubiquitination (Honda, 2002).

Deubiquitination is the reverse reaction of ubiquitination, is mediated by multiple deubiquitinases, which are responsible for removing ubiquitin from the substrates, and plays a vital role in the regulation of protein stability, activity, and its subcellular localization (Farshi, 2015; Reyes-Turcu et al., 2009; Sun, Liu, et al., 2020). So far, there has been about 80 deubiqutinases identified and mainly classified in 5 distinct families, including ubiquitin-specific proteases (USPs), ubiquitin C-terminal hydrolases (UCHs), ovarian tumor proteases (OTUs), Machado–Joseph disease (MJD) protein domain proteases, and JAMM motif zinc metalloproteases (Sun, Shi, et al., 2020; Young et al., 2019). Similar to ubiquitination, deubiquitination regulates various cellular processes, including DNA damage response, cell cycle, apoptosis, DNA replication, and tumorigenesis (Brinkmann et al., 2015).

CLASPIN is a key player in ATR-CHK1 signaling and DNA replication, and is regulated by several deubiquitinases. USP28 antagonizes the APC/C Cdh1-mediated degradation of CLASPIN (Zhang et al., 2006). Additionally, USP7 and USP29 stabilize CLASPIN and reverse its degradation mediated by SCF-bTrCP (Faustrup et al., 2009; Martín, 2015). Intriguingly, our previous study and Xu lab’s work also uncovered that USP20 functions as a DUB for CLASPIN, maintaining its stability upon replication stress (Yuan, 2014; Zhu et al., 2014). USP9X deubiquitinates and stabilizes CLASPIN in an S-phase-specific manner and maintains genomic stability during DNA replication (McGarry, 2016). Intriguingly, USP20 also deubiquitinates and stabilizes Rad17 following DNA damage, resulting in replication fork stability (Shanmugam, 2014).

CHK1 is regulated by multiple deubiquitinases. USP3 removes the K63-linked ubiquitination from CHK1, which facilitates its release from chromatin. USP37 could cleave the poly-ubiquitination chains of CHK1 and increase CHK1 protein stability, which in turn facilitates cellular response to replication stress (2019). Interestingly, in addition to CLAPIN, USP7 deubiquitinates and stabilizes CHK1 (Alonso-de Vega et al., 2014). Even though RPA and 9-1-1 were reported to be ubiquitinated as mentioned above, the deubiquitinases for them remain to be explored. Taken together, the ubiquitination and deubiquitination of key regulators in the ATR-CHK1 pathway and have vital roles in the regulation of ATR signaling transduction and cellular response to DNA replication stress.

The phosphorylation in ATR-CHK1 signaling pathway

Phosphorylation, which is the most-explored PTM, plays key roles in regulating various physiological processes, including enzymatic activities, protein localization, DNA replication, gene expression, DNA damage response, and signal transduction (Ardito et al., 2017). ATR, the master of ATR-CHK1 signaling, is a kinase (Nam & Cortez, 2011). Autophosphorylation is essential for ATR activation and ATR signaling transduction, which plays an important role in constructing of a signal network in DNA replication stress to maintain replication fork stability.

Upon replication stress, RPA-coated ssDNA recruits ATR-ATRIP complex to sites of DNA damage and initiates ATR activation. Besides that, the full activation of ATR needs to trigger autophosphorylation at Thr1989 which creates a binding site for BRCT domains 7 and 8 of TopBP1 and promoting ATR substrate recognition (Liu, 2011; Nam, 2011). Hence, ATR autophosphorylation is crucial for its full activation. CHK1, a well-known substrate of ATR, is not only a key player for the checkpoint response, but also vital for the stability of DNA replication forks. Upon DNA damage, CHK1 is phosphorylated by ATR at Ser317 and Ser345 and activated to regulate cell cycle progression, maintain replication fork stability and promote faithful chromosome segregation (Goto et al., 2015; Kabeche et al., 2018). In addition, CLASPIN is phosphorylated at Thr916 and Ser945 in response to DNA replication stress, which is possibly mediated by ATR (Bennett et al., 2008). The phosphorylation is crucial to establish binding between CLAPIN and CHK1 (Bennett et al., 2008; Clarke & Clarke, 2005). Besides that, CDK7-mediated phosphorylation of CLASPIN is essential for the activation of the ATR–Chk1 signaling pathway (Yang et al., 2019).

RPA is a heterotrimeric complex composed of three subunits, RPA70, RPA32, and RPA14 (Feldkamp et al., 2014). Accumulating research has shown that the phosphorylation of RPA32 plays a vital role in RPA complex functions. RPA32 is phosphorylated on multiple N-terminal residues during replication stress (Oakley, 2009). Unperturbed, RPA32 participates in the regulation of mitosis and G1/S phase transition via its phosphorylation by cyclin-dependent kinase 1 (CDK1)/cyclin B and CDK2/cyclin A, respectively (Anantha et al., 2007). Moreover, ATR phosphorylates RPA32 at Ser33 in response to DNA damage and replication stress, which is the key and initial step for subsequent phosphorylation at Ser29 and Ser4/Ser8 phosphorylation by DNA-PKcs (Ashley, 2014; Glanzer, 2014; Shiotani, 2013).

Interestingly, ATRIP, the partner of ATR, is phosphorylated at Ser68 and Ser72 by ATR in response to genotoxic stimuli (Itakura, 2004). But, the phosphorylation is dispensable for ATR-mediated signaling. Rad9, a key component of heterotrimeric complex (the 9-1-1 complex), is phosphorylated at Ser272 by ATR and ATM in response to DNA damage, which is indispensable for CHK1 activation (Roos-Mattjus, 2002, 2003). Moreover, the Tousled-like kinase 1B (TLK1B) indirectly phosphorylates Rad9 at S328, which causes its release from stalled replication forks, leading to the instability of stalled replication forks and prolonged activation of the S-phase checkpoint (Benedetti, 2012). Rad17, a substrate of ATR, is phosphorylated at Ser635 and Ser645 upon replication interference, which is essential for ATR signaling and replication (Post, 2001; Wang, 2006). TopBP1 is directly phosphorylated by ATM at several residues, including, Ser405, Ser476, Ser1051, Ser766 and Thr975, which phosphorylation is required for cell survival in response to replication stress (Yamane et al., 2002).

The acetylation and deacetylation in ATR-CHK1 signaling pathway

Acetylation is a reversible and dynamic process and is controlled by several deacetylases and acetyltransferases. Accumulating research has shown acetylation is an important PTM exerting vital function in multiple processes, including DNA damage response, replication stress, cell cycle checkpoint, and transcription (Glozak et al., 2005; Xia et al., 2020). p300, an acetyltransferase, acetylates RPA resulting in increasing its binding affinity to ssDNA (Surendran et al., 2016). p300 is also reported as an acetyltransferase for TopBP1, which regulates its activity and function (Liu, 2014). Moreover, acetyltransferases, GCN5 and PCAF mediated RPA70 acetylation at lysine 163, which is important for nucleotide excision repair (NER) in response to UV irradiation (He et al., 2017).

Sirtuin 2 (SIRT2) interacts with and deacetylates ATRIP at Lys32 in response to replication stress, which in turn facilitates the recruitment of ATRIP to chromatin and ATR activation (Zhang, 2016). SIRT1, the mammalian homolog of yeast Sir2, is a member of the sirtuin family of type III histone deacetylases and deacetylates TopBP1, which in turn participates in regulation of DNA replication fork initiation and the intra-S-phase cell cycle checkpoint (Wang, 2014). On the other hand, Sirt1 deacetylates TopBP1 upon glucose deprivation, resulting in TopBP1-Treslin disassociation and DNA replication inhibition (Liu, 2014). In contrast, SIRT1 activity is inhibited upon genotoxic stress, resulting in an increase in TopBP1 acetylation, which is vital for the TopBP1-Rad9 interaction and activation of the ATR-Chk1 pathway. Mechanistically, the acetylation of TopBP1 induces its conformation change, which promotes its interaction with distinct partners in DNA replication and checkpoint activation (Liu, 2014). HDAC6, a member of Histone deacetylases (HDACs), antagonize GCN5 and PCAF mediated RPA70 acetylation (Zhao, 2017).

The SUMOylation and deSUMOylation in ATR-CHK1 signaling pathway

SUMOylation is achieved via the small ubiquitin-like modifier (SUMO) family of proteins. SUMO is conjugated to lysine residues of substrates by SUMO-specific E1, E2, and E3 (Yang, 2017). Like deubiquitination, SUMOylation is also a reversible dynamic process. The SUMO modification is removed by sentrin/SUMO-specific proteases (SENPs), which removal reaction is called deSUMOylation (Nayak & Müller, 2014). SUMOylation plays a vital role in regulating multiple process, including protein localization, interaction, transcription, DNA repair, and DNA replication (Celen & Sahin, 2020).

ATRIP is SUMOylated at K234 and K289, which SUMOylation facilitates its binding to multiple protein including ATR, RPA70, TopBP1, and the MRE11–RAD50–NBS1 complex (Wu, 2014; Zhou, 2020). Moreover, RPA70 is SUMOylated at Lys449 and Lys577 under replication-mediated DSBs, which results in recruitment of Rad51 to promote DNA damage repair (Dou et al., 2010). In addition, TopBP1 is highly SUMOylated in response to DNA inter-strand crosslinking damage, which SUMOylation is important in regulation of replication stress and prohibiting fork breakage to protect DNA integrity (Munk, 2017).

SENP6, a sentrin/SUMO-specific protease (SENP), is responsible for removing SUMOs from RPA70 (Li, 2018). In addition, SENP5 is able to bind to ATRIP and facilitate ATRIP deSUMOylation (Jin et al., 2016).

The methylation and crotonylation in ATR-CHK1 signaling pathway

Accumulating studies suggest that the methylation of non-histone proteins has emerged as extensive PTM, which plays as a vital regulator of multiple cellular processes, including cell survival, DNA damage response, transcription, and DNA replication (Blasi, 2021; Hamamoto et al., 2015; Rodríguez-Paredes & Lyko, 2019). Rad9 is methylated by protein arginine methyltransferase 5 (PRMT5), which plays a critical role in regulation of Chk1 activation and maintaining genome integrity (He, 2011). Crotonylation, a novel PTM, is discovered in recent year (Wan et al., 2019). RPA70 is reported to be crotonylated at Lys88, Lys379 and Lys595 in response to DNA damage, which in turn promotes its bind with ssDNA and HR factors (Yu, 2020).

Conclusions

Extensive and inspiring research of PTM of the key regulators in ATR signaling have been published (Yazinski & Zou, 2016). Herein, we simply summarized and listed ubiquitination, phosphorylation, acetylation, methylation, crotonylation, and some of their inverse processes, which are related to ATR pathway and DNA replication stress. We hope these will be benefit for us to understand the progress of this field. At the same time, we are looking forward to more in-depth and systematic research in this field since there are still a lot of questions remaining to be answered. For example, as we know most of the PTMs are a reversible and dynamic processes, however, some of these reverse processes function in DNA replication stress are still unclear. Hence, further studies need to be carried out to focusing on these regulations. In addition, these PTMs occurring seem to be spatiotemporal specific and need to be very accurate (Dantuma & Attikum, 2016), so how do cells coordinate these regulations and how they work together to make cells better cope with the DNA replication stress and safeguard genomic integrity? Further efforts need to be done to clarify the interplay among these modifiers and their contribution during DNA replication stress.

Since ATR-CHK1 signaling plays an important role in maintaining genome stability, prevent tumorigenesis and chemo/radio-resistance (Smith et al. 2010; Yazinski & Zou, 2016), targeting ATR-CHK1 signaling has been identified as an effective approach to kill cancer (Primo & Teixeira, 2019; Sørensen & Syljuåsen, 2012; Wilhelm et al., 2020). Several CHK1 inhibitors and ATR inhibitors have been developed to treat cancer (Heo, 2019; Rundle et al., 2017). Better understanding the molecular mechanism of activation of ATR-CHK1 signaling would be of valuable importance for exploring novel therapeutic target. With the study progress of PTMs of ATR signaling, it provides additional choices and targets for cancer therapy as well as biomarkers for cancer diagnosis. In addition to targeting these key components of ATR-CHK1 pathway themselves, we can also target these key proteins that mediate PTMs of ATR signaling. Inspiringly, owing to great progress had achieved in technologies and other tools, including bioinformatics workflow and proteomics, the possibility is increases to identify not only novel PTMs but also the known PTMs on previously unrecognized modified proteins in ATR signaling pathway, which will inspire us to identify novel target for disease related to aberrant regulation of DNA replication and ATR pathway.

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Acknowledgements

We gratefully acknowledge funding from National Natural Science Foundation(32070713 to J.Y., 82002985 to Y.P.C., 32090032 to J.Y.), Shanghai Pujiang program(2020PJD070 to Y.P.C.) and China Postdoctoral Science Foundation (2020M681384 to Y.P.C.).

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Affiliations

1. Key Laboratory of Arrhythmias of the Ministry of Education of China, Research Center for Translational Medicine, East Hospital, School of Life Sciences and Technology, Shanghai, 200120, China

   Yuping Chen

2. Key Laboratory of Arrhythmias of the Ministry of Education of China, Research Center for Translational Medicine, East Hospital, Tongji University School of Medicine, Shanghai, 200120, China

   Yuping Chen & Jian Yuan

3. Department of Biochemistry and Molecular Biology, Tongji University School of Medicine, Shanghai, 200120, China

   Yuping Chen & Jian Yuan

Corresponding author

Correspondence to Jian Yuan.

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The authors declare that they have no competing interests.

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Chen, Y., Yuan, J. The post translational modification of key regulators of ATR signaling in DNA replication. GENOME INSTAB. DIS. 2, 92–101 (2021). https://doi.org/10.1007/s42764-021-00036-z

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• Received09 February 2021

• Revised03 March 2021

• Accepted08 March 2021

• Published21 April 2021

• Issue DateApril 2021

• DOIhttps://doi.org/10.1007/s42764-021-00036-z

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Keywords

• Post-translational modification

• ATR pathway

• DNA replication

• DNA replication stress