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Yi G, Sung Y, Kim C, Ra JS, Hirakawa H, Kato T, Fujimori A, Kim H, Takata KI. DNA polymerase θ-mediated repair of high LET radiation-induced complex DNA double-strand breaks. Nucleic Acids Res 2023; 51:2257-2269. [PMID: 36805268 PMCID: PMC10018357 DOI: 10.1093/nar/gkad076] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/19/2022] [Revised: 01/21/2023] [Accepted: 01/25/2023] [Indexed: 02/22/2023] Open
Abstract
DNA polymerase θ (POLQ) is a unique DNA polymerase that is able to perform microhomology-mediated end-joining as well as translesion synthesis (TLS) across an abasic (AP) site and thymine glycol (Tg). However, the biological significance of the TLS activity is currently unknown. Herein we provide evidence that the TLS activity of POLQ plays a critical role in repairing complex DNA double-strand breaks (DSBs) induced by high linear energy transfer (LET) radiation. Radiotherapy with high LET radiation such as carbon ions leads to more deleterious biological effects than corresponding doses of low LET radiation such as X-rays. High LET-induced DSBs are considered to be complex, carrying additional DNA damage such as AP site and Tg in close proximity to the DSB sites. However, it is not clearly understood how complex DSBs are processed in mammalian cells. We demonstrated that genetic disruption of POLQ results in an increase of chromatid breaks and enhanced cellular sensitivity following treatment with high LET radiation. At the biochemical level, POLQ was able to bypass an AP site and Tg during end-joining and was able to anneal two single-stranded DNA tails when DNA lesions were located outside the microhomology. This study offers evidence that POLQ is directly involved in the repair of complex DSBs.
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Affiliation(s)
- Geunil Yi
- Center for Genomic Integrity, Institute for Basic Science, Ulsan 44919, Republic of Korea
- Department of Biological Sciences, Ulsan National Institute of Science and Technology, Ulsan 44919, Republic of Korea
| | - Yubin Sung
- Center for Genomic Integrity, Institute for Basic Science, Ulsan 44919, Republic of Korea
| | - Chanwoo Kim
- Department of Biomedical Engineering, Ulsan National Institute of Science and Technology, Ulsan 44919, Republic of Korea
| | - Jae Sun Ra
- Center for Genomic Integrity, Institute for Basic Science, Ulsan 44919, Republic of Korea
| | - Hirokazu Hirakawa
- Department of Charged Particle Therapy Research, Institute for Quantum Medical Science, Chiba 263-8555, Japan
| | - Takamitsu A Kato
- Department of Environmental & Radiological Health Sciences, Colorado State University, Colorado 80523, USA
| | - Akira Fujimori
- Department of Charged Particle Therapy Research, Institute for Quantum Medical Science, Chiba 263-8555, Japan
| | - Hajin Kim
- Center for Genomic Integrity, Institute for Basic Science, Ulsan 44919, Republic of Korea
- Department of Biomedical Engineering, Ulsan National Institute of Science and Technology, Ulsan 44919, Republic of Korea
| | - Kei-ichi Takata
- Center for Genomic Integrity, Institute for Basic Science, Ulsan 44919, Republic of Korea
- Department of Biological Sciences, Ulsan National Institute of Science and Technology, Ulsan 44919, Republic of Korea
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2
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Multifaceted Nature of DNA Polymerase θ. Int J Mol Sci 2023; 24:ijms24043619. [PMID: 36835031 PMCID: PMC9962433 DOI: 10.3390/ijms24043619] [Citation(s) in RCA: 4] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/10/2022] [Revised: 01/26/2023] [Accepted: 02/02/2023] [Indexed: 02/15/2023] Open
Abstract
DNA polymerase θ belongs to the A family of DNA polymerases and plays a key role in DNA repair and damage tolerance, including double-strand break repair and DNA translesion synthesis. Pol θ is often overexpressed in cancer cells and promotes their resistance to chemotherapeutic agents. In this review, we discuss unique biochemical properties and structural features of Pol θ, its multiple roles in protection of genome stability and the potential of Pol θ as a target for cancer treatment.
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3
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Abstract
DNA polymerase θ (Pol θ) is a DNA repair enzyme widely conserved in animals and plants. Pol θ uses short DNA sequence homologies to initiate repair of double-strand breaks by theta-mediated end joining. The DNA polymerase domain of Pol θ is at the C terminus and is connected to an N-terminal DNA helicase-like domain by a central linker. Pol θ is crucial for maintenance of damaged genomes during development, protects DNA against extensive deletions, and limits loss of heterozygosity. The cost of using Pol θ for genome protection is that a few nucleotides are usually deleted or added at the repair site. Inactivation of Pol θ often enhances the sensitivity of cells to DNA strand-breaking chemicals and radiation. Since some homologous recombination-defective cancers depend on Pol θ for growth, inhibitors of Pol θ may be useful in treating such tumors.
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Affiliation(s)
- Richard D Wood
- Department of Epigenetics and Molecular Carcinogenesis, The University of Texas MD Anderson Center, Houston, Texas, USA;
| | - Sylvie Doublié
- Department of Microbiology and Molecular Genetics, University of Vermont, Burlington, Vermont, USA;
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4
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Lv Q, Han S, Wang L, Xia J, Li P, Hu R, Wang J, Gao L, Chen Y, Wang Y, Du J, Bao F, Hu Y, Xu X, Xiao W, He Y. TEB/POLQ plays dual roles in protecting Arabidopsis from NO-induced DNA damage. Nucleic Acids Res 2022; 50:6820-6836. [PMID: 35736216 PMCID: PMC9262624 DOI: 10.1093/nar/gkac469] [Citation(s) in RCA: 2] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/29/2021] [Revised: 05/07/2022] [Accepted: 06/10/2022] [Indexed: 12/24/2022] Open
Abstract
Nitric oxide (NO) is a key player in numerous physiological processes. Excessive NO induces DNA damage, but how plants respond to this damage remains unclear. We screened and identified an Arabidopsis NO hypersensitive mutant and found it to be allelic to TEBICHI/POLQ, encoding DNA polymerase θ. The teb mutant plants were preferentially sensitive to NO- and its derivative peroxynitrite-induced DNA damage and subsequent double-strand breaks (DSBs). Inactivation of TEB caused the accumulation of spontaneous DSBs largely attributed to endogenous NO and was synergistic to DSB repair pathway mutations with respect to growth. These effects were manifested in the presence of NO-inducing agents and relieved by NO scavengers. NO induced G2/M cell cycle arrest in the teb mutant, indicative of stalled replication forks. Genetic analyses indicate that Polθ is required for translesion DNA synthesis across NO-induced lesions, but not oxidation-induced lesions. Whole-genome sequencing revealed that Polθ bypasses NO-induced base adducts in an error-free manner and generates mutations characteristic of Polθ-mediated end joining. Our experimental data collectively suggests that Polθ plays dual roles in protecting plants from NO-induced DNA damage. Since Polθ is conserved in higher eukaryotes, mammalian Polθ may also be required for balancing NO physiological signaling and genotoxicity.
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Affiliation(s)
- Qiang Lv
- College of Life Sciences, Capital Normal University, Beijing 100048, China
| | - Shuang Han
- College of Life Sciences, Capital Normal University, Beijing 100048, China
| | - Lei Wang
- College of Life Sciences, Capital Normal University, Beijing 100048, China
- Department of Biological Sciences, Mississippi State University, Mississippi State, MS 39762, USA
| | - Jinchan Xia
- College of Life Sciences, Capital Normal University, Beijing 100048, China
| | - Peng Li
- College of Life Sciences, Capital Normal University, Beijing 100048, China
| | - Ruoyang Hu
- College of Life Sciences, Capital Normal University, Beijing 100048, China
| | - Jinzheng Wang
- College of Life Sciences, Capital Normal University, Beijing 100048, China
| | - Lei Gao
- College of Life Sciences, Capital Normal University, Beijing 100048, China
| | - Yuli Chen
- College of Life Sciences, Capital Normal University, Beijing 100048, China
| | - Yu Wang
- College of Life Sciences, Capital Normal University, Beijing 100048, China
| | - Jing Du
- College of Life Sciences, Capital Normal University, Beijing 100048, China
| | - Fang Bao
- College of Life Sciences, Capital Normal University, Beijing 100048, China
| | - Yong Hu
- College of Life Sciences, Capital Normal University, Beijing 100048, China
| | - Xingzhi Xu
- College of Life Sciences, Capital Normal University, Beijing 100048, China
- Guangdong Key Laboratory for Genome Stability & Disease Prevention and Carson International Cancer Center, Shenzhen University School of Medicine, Shenzhen, Guangdong 518060, China
| | - Wei Xiao
- College of Life Sciences, Capital Normal University, Beijing 100048, China
- Department of Biochemistry, Microbiology and Immunology, University of Saskatchewan, Saskatoon, SK S7N 5E5, Canada
| | - Yikun He
- College of Life Sciences, Capital Normal University, Beijing 100048, China
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5
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Yudkina AV, Zharkov DO. Miscoding and DNA Polymerase Stalling by Methoxyamine-Adducted Abasic Sites. Chem Res Toxicol 2022; 35:303-314. [PMID: 35089032 DOI: 10.1021/acs.chemrestox.1c00359] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Apurinic/apyrimidinic (AP) sites appear in DNA spontaneously and as intermediates of base excision DNA repair. AP sites are noninstructive lesions: they strongly block DNA polymerases, and if bypassed, the nature of the incorporated dNMP is mostly guided by the interactions within the polymerase-DNA active site. Many DNA polymerases follow the "A-rule", preferentially incorporating dAMP opposite to natural AP sites. Methoxyamine (MX), a small molecule, efficiently reacts with the aldehyde moiety of natural AP sites, thereby preventing their cleavage by APEX1, the major human AP endonuclease. MX is currently regarded as a possible sensitizer of cancer cells toward DNA-damaging drugs. To evaluate the mutagenic potential of MX, we have studied the utilization of various dNTPs by five DNA polymerases of different families encountering MX-AP adducts in the template in comparison with the natural aldehydic AP site. The Klenow fragment of Escherichia coli DNA polymerase I strictly followed the A-rule with both natural AP and MX-adducted AP sites. Phage RB69 DNA polymerase, a close relative of human DNA polymerases δ and ε, efficiently incorporated both dAMP and dGMP. DNA polymerase β mostly incorporated dAMP and dCMP, preferring dCMP opposite to the natural AP site and dAMP opposite to the MX-AP site, while DNA polymerase λ was selective for dGMP, apparently via the primer misalignment mechanism. Finally, translesion DNA polymerase κ also followed the A-rule for MX-AP and additionally incorporated dCMP opposite to a natural AP site. Overall, the MX-AP site, despite structural differences, was similar to the natural AP site in terms of the dNMP misincorporation preference but was bypassed less efficiently by all polymerases except for Pol κ.
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Affiliation(s)
- Anna V Yudkina
- SB RAS Institute of Chemical Biology and Fundamental Medicine, 8 Lavrentieva Avenue, Novosibirsk 630090, Russia
| | - Dmitry O Zharkov
- SB RAS Institute of Chemical Biology and Fundamental Medicine, 8 Lavrentieva Avenue, Novosibirsk 630090, Russia.,Novosibirsk State University, 2 Pirogova Street, Novosibirsk 630090, Russia
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6
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Whitaker AM, Stark WJ, Flynn TS, Freudenthal BD. Molecular and structural characterization of disease-associated APE1 polymorphisms. DNA Repair (Amst) 2020; 91-92:102867. [PMID: 32454397 DOI: 10.1016/j.dnarep.2020.102867] [Citation(s) in RCA: 12] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/26/2020] [Revised: 04/22/2020] [Accepted: 04/23/2020] [Indexed: 12/14/2022]
Abstract
Under conditions of oxidative stress, reactive oxygen species (ROS) continuously assault the structure of DNA resulting in oxidation and fragmentation of the nucleobases. When the nucleobase structure is altered, its base-pairing properties may also be altered, promoting mutations. Consequently, oxidative DNA damage is a major source of the mutation load that gives rise to numerous human maladies, including cancer. Base excision repair (BER) is the primary pathway tasked with removing and replacing mutagenic DNA base damage. Apurinic/apyrimidinic endonuclease 1 (APE1) is a central enzyme with AP-endonuclease and 3' to 5' exonuclease functions during BER, and therefore is key to maintenance of genome stability. Polymorphisms, or SNPs, in the gene encoding APE1 (APEX1) have been identified among specific human populations and result in variants of APE1 with modified function. These defects in APE1 potentially result in impaired DNA repair capabilities and consequently an increased risk of disease for individuals within these populations. In the present study, we determined the X-ray crystal structures of three prevalent disease-associated APE1 SNPs (D148E, L104R, and R237C). Each APE1 SNP results in unique localized changes in protein structure, including protein dynamics and DNA binding contacts. Combined with comprehensive biochemical characterization, including pre-steady-state kinetic and DNA binding analyses, variant APE1:DNA complex structures with both AP-endonuclease and exonuclease substrates were analyzed to elucidate how these SNPs might perturb the two major repair functions employed by APE1 during BER.
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Affiliation(s)
- Amy M Whitaker
- Department of Biochemistry and Molecular Biology, University of Kansas Medical Center, Kansas City, KS,66160, USA
| | - Wesley J Stark
- Department of Biochemistry and Molecular Biology, University of Kansas Medical Center, Kansas City, KS,66160, USA
| | - Tony S Flynn
- Department of Biochemistry and Molecular Biology, University of Kansas Medical Center, Kansas City, KS,66160, USA
| | - Bret D Freudenthal
- Department of Biochemistry and Molecular Biology, University of Kansas Medical Center, Kansas City, KS,66160, USA.
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Raetz AG, Banda DM, Ma X, Xu G, Rajavel AN, McKibbin PL, Lebrilla CB, David SS. The DNA repair enzyme MUTYH potentiates cytotoxicity of the alkylating agent MNNG by interacting with abasic sites. J Biol Chem 2020; 295:3692-3707. [PMID: 32001618 DOI: 10.1074/jbc.ra119.010497] [Citation(s) in RCA: 8] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/06/2019] [Revised: 01/22/2020] [Indexed: 11/06/2022] Open
Abstract
Higher expression of the human DNA repair enzyme MUTYH has previously been shown to be strongly associated with reduced survival in a panel of 24 human lymphoblastoid cell lines exposed to the alkylating agent N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). The molecular mechanism of MUTYH-enhanced MNNG cytotoxicity is unclear, because MUTYH has a well-established role in the repair of oxidative DNA lesions. Here, we show in mouse embryonic fibroblasts (MEFs) that this MNNG-dependent phenotype does not involve oxidative DNA damage and occurs independently of both O6-methyl guanine adduct cytotoxicity and MUTYH-dependent glycosylase activity. We found that blocking of abasic (AP) sites abolishes higher survival of Mutyh-deficient (Mutyh -/-) MEFs, but this blockade had no additive cytotoxicity in WT MEFs, suggesting the cytotoxicity is due to MUTYH interactions with MNNG-induced AP sites. We found that recombinant mouse MUTYH tightly binds AP sites opposite all four canonical undamaged bases and stimulated apurinic/apyrimidinic endonuclease 1 (APE1)-mediated DNA incision. Consistent with these observations, we found that stable expression of WT, but not catalytically-inactive MUTYH, enhances MNNG cytotoxicity in Mutyh -/- MEFs and that MUTYH expression enhances MNNG-induced genomic strand breaks. Taken together, these results suggest that MUTYH enhances the rapid accumulation of AP-site intermediates by interacting with APE1, implicating MUTYH as a factor that modulates the delicate process of base-excision repair independently of its glycosylase activity.
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Affiliation(s)
- Alan G Raetz
- Department of Chemistry, University of California, Davis, California 95616
| | - Douglas M Banda
- Department of Chemistry, University of California, Davis, California 95616
| | - Xiaoyan Ma
- Department of Chemistry, University of California, Davis, California 95616
| | - Gege Xu
- Department of Chemistry, University of California, Davis, California 95616
| | - Anisha N Rajavel
- Department of Chemistry, University of California, Davis, California 95616
| | - Paige L McKibbin
- Department of Chemistry, University of California, Davis, California 95616
| | - Carlito B Lebrilla
- Department of Chemistry, University of California, Davis, California 95616
| | - Sheila S David
- Department of Chemistry, University of California, Davis, California 95616.
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8
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Redrejo-Rodríguez M, Ordóñez CD, Berjón-Otero M, Moreno-González J, Aparicio-Maldonado C, Forterre P, Salas M, Krupovic M. Primer-Independent DNA Synthesis by a Family B DNA Polymerase from Self-Replicating Mobile Genetic Elements. Cell Rep 2018; 21:1574-1587. [PMID: 29117562 PMCID: PMC5695915 DOI: 10.1016/j.celrep.2017.10.039] [Citation(s) in RCA: 25] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [Key Words] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/10/2017] [Revised: 09/19/2017] [Accepted: 10/11/2017] [Indexed: 01/06/2023] Open
Abstract
Family B DNA polymerases (PolBs) play a central role during replication of viral and cellular chromosomes. Here, we report the discovery of a third major group of PolBs, which we denote primer-independent PolB (piPolB), that might be a link between the previously known protein-primed and RNA/DNA-primed PolBs. PiPolBs are encoded by highly diverse mobile genetic elements, pipolins, integrated in the genomes of diverse bacteria and also present as circular plasmids in mitochondria. Biochemical characterization showed that piPolB displays efficient DNA polymerization activity that can use undamaged and damaged templates and is endowed with proofreading and strand displacement capacities. Remarkably, the protein is also capable of template-dependent de novo DNA synthesis, i.e., DNA-priming activity, thereby breaking the long-standing dogma that replicative DNA polymerases require a pre-existing primer for DNA synthesis. We suggest that piPolBs are involved in self-replication of pipolins and may also contribute to bacterial DNA damage tolerance.
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Affiliation(s)
- Modesto Redrejo-Rodríguez
- Centro de Biología Molecular "Severo Ochoa," Consejo Superior de Investigaciones Científicas and Universidad Autónoma de Madrid, Universidad Autónoma, Cantoblanco, 28049 Madrid, Spain.
| | - Carlos D Ordóñez
- Centro de Biología Molecular "Severo Ochoa," Consejo Superior de Investigaciones Científicas and Universidad Autónoma de Madrid, Universidad Autónoma, Cantoblanco, 28049 Madrid, Spain
| | - Mónica Berjón-Otero
- Centro de Biología Molecular "Severo Ochoa," Consejo Superior de Investigaciones Científicas and Universidad Autónoma de Madrid, Universidad Autónoma, Cantoblanco, 28049 Madrid, Spain
| | - Juan Moreno-González
- Centro de Biología Molecular "Severo Ochoa," Consejo Superior de Investigaciones Científicas and Universidad Autónoma de Madrid, Universidad Autónoma, Cantoblanco, 28049 Madrid, Spain
| | - Cristian Aparicio-Maldonado
- Centro de Biología Molecular "Severo Ochoa," Consejo Superior de Investigaciones Científicas and Universidad Autónoma de Madrid, Universidad Autónoma, Cantoblanco, 28049 Madrid, Spain
| | - Patrick Forterre
- Institut Pasteur, Unité Biologie Moléculaire du Gène chez les Extrêmophiles, Paris, France
| | - Margarita Salas
- Centro de Biología Molecular "Severo Ochoa," Consejo Superior de Investigaciones Científicas and Universidad Autónoma de Madrid, Universidad Autónoma, Cantoblanco, 28049 Madrid, Spain.
| | - Mart Krupovic
- Institut Pasteur, Unité Biologie Moléculaire du Gène chez les Extrêmophiles, Paris, France.
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9
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Laverty DJ, Greenberg MM. In Vitro Bypass of Thymidine Glycol by DNA Polymerase θ Forms Sequence-Dependent Frameshift Mutations. Biochemistry 2017; 56:6726-6733. [PMID: 29243925 PMCID: PMC5743609 DOI: 10.1021/acs.biochem.7b01093] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/01/2023]
Abstract
Unrepaired DNA lesions block replication and threaten genomic stability. Several specialized translesion polymerases, including polymerase θ (Pol θ), contribute to replicative bypass of these lesions. The role of Pol θ in double-strand break repair is well-understood, but its contribution to translesion synthesis is much less so. We describe the action of Pol θ on templates containing thymidine glycol (Tg), a major cytotoxic, oxidative DNA lesion that blocks DNA replication. Unrepaired Tg lesions are bypassed in human cells by specialized translesion polymerases by one of two distinct pathways: high-fidelity bypass by the combined action of Pol κ and Pol ζ or weakly mutagenic bypass by Pol θ. Here we report that in vitro bypass of Tg by Pol θ results in frameshift mutations (deletions) in a sequence-dependent fashion. Steady-state kinetic analysis indicated that one- and two-nucleotide deletions are formed 9- and 6-fold more efficiently, respectively, than correct, full-length bypass products. Sequencing of in vitro bypass products revealed that bypass preference decreased in the following order on a template where all three outcomes were possible: two-nucleotide deletion > correct bypass > one-nucleotide deletion. These results suggest that bypass of Tg by Pol θ results in mutations opposite the lesion, as well as frameshift mutations.
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Affiliation(s)
- Daniel J. Laverty
- Department of Chemistry, Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218
| | - Marc M. Greenberg
- Department of Chemistry, Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218
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10
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Liu X, Zou X, Li H, Zou Z, Yang J, Wang C, Wu S, Zhang H. Bypass of an Abasic Site via the A-Rule by DNA Polymerase of Pseudomonas aeruginosa Phage PaP1. Chem Res Toxicol 2017; 31:58-65. [PMID: 29183115 DOI: 10.1021/acs.chemrestox.7b00287] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/31/2022]
Affiliation(s)
- Xiaoying Liu
- School
of Public Health, Xinjiang Medical University, Urumqi 830011, China
- Public
Health Laboratory Sciences and Toxicology, West China School of Public
Health, Sichuan University, Chengdu 610041, China
| | - Xiaoli Zou
- Public
Health Laboratory Sciences and Toxicology, West China School of Public
Health, Sichuan University, Chengdu 610041, China
| | - Huangyuan Li
- Key
Laboratory of Environment and Health among Universities and Colleges
in Fujian, School of Public Health, Fujian Medical University, Minhou
County, Fuzhou 350108, China
| | - Zhenyu Zou
- Public
Health Laboratory Sciences and Toxicology, West China School of Public
Health, Sichuan University, Chengdu 610041, China
| | - Jie Yang
- Public
Health Laboratory Sciences and Toxicology, West China School of Public
Health, Sichuan University, Chengdu 610041, China
| | - Chenlu Wang
- School
of Public Health, Xinjiang Medical University, Urumqi 830011, China
| | - Shunhua Wu
- School
of Public Health, Xinjiang Medical University, Urumqi 830011, China
| | - Huidong Zhang
- Public
Health Laboratory Sciences and Toxicology, West China School of Public
Health, Sichuan University, Chengdu 610041, China
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