Single-nucleotide base excision repair DNA polymerase activity in C

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  • Single-nucleotide base excision repair DNApolymerase activity in C. elegans in theabsence of DNA polymerase bKenjiro Asagoshi1, Wade Lehmann2, Elena K. Braithwaite3, Lucas Santana-Santos4,

    Rajendra Prasad1, Jonathan H. Freedman3, Bennett Van Houten2,4 and

    Samuel H. Wilson1,*

    1Laboratory of Structural Biology, 2Laboratory of Molecular Genetics, 3Laboratory of Toxicology andPharmacology, NIEHS, National Institutes of Health, Research Triangle Park, NC 27709, USA and4Department of Pharmacology and Chemical Biology, University of Pittsburgh School of Medicine andThe University of Pittsburgh Cancer Institute, Hillman Cancer Center, Pittsburgh, PA 15213, USA

    Received February 8, 2011; Revised August 22, 2011; Accepted August 23, 2011


    The base excision DNA repair (BER) pathway knownto occur in Caenorhabditis elegans has not beenwell characterized. Even less is known about theDNA polymerase (pol) requirement for the gap-fillingstep during BER. We now report on characterizationof in vitro uracil-DNA initiated BER in C. elegans.The results revealed single-nucleotide (SN)gap-filling DNA polymerase activity and completeBER. The gap-filling polymerase activity was notdue to a DNA polymerase b (pol b) homolog, or toanother X-family polymerase, since computer-based sequence analyses of the C. elegansgenome failed to show a match for a pol b-likegene or other X-family polymerases. Activity gelanalysis confirmed the absence of pol b in the C.elegans extract. BER gap-filling polymeraseactivity was partially inhibited by both dideoxy-nucleotide and aphidicolin. The results are consist-ent with a combination of both replicativepolymerase(s) and lesion bypass/BER polymerasepol y contributing to the BER gap-filling synthesis.Involvement of pol y was confirmed in experimentswith extract from pol y null animals. The presence ofthe SN BER in C. elegans is supported by theseresults, despite the absence of a pol b-like enzymeor other X-family polymerase.


    Throughout the life cycle of an organism, damage ingenomic DNA is produced from endogenous and exogen-ous sources, ultimately threatening genomic integrity. Forexample, uracil is generated from deamination of cytosineduring normal cellular processes and, if unrepaired, will bemutagenic by pairing with adenine during the next cycle ofDNA replication. Base excision repair (BER) purifies thegenome of DNA base damage such as uracil. In the ca-nonical mammalian single-nucleotide (SN) BER pathwayfor uracil-DNA, uracil-DNA glycosylase (UDG) recog-nizes and removes the base, leaving an apurinic/apyrimidinic (AP) site. AP endonuclease (APE) incisesthe DNA strand 50 to the AP site to create a SN gapwith 30-OH and 50-deoxyribose phosphate (dRP) at themargins. DNA polymerase (pol) b removes the 50-dRPgroup from the gap and fills the SN gap with a correctnucleotide. Finally, DNA ligase seals the resulting nick tocomplete the repair pathway (14).

    Caenorhabditis elegans is a popular model organism forinvestigating development, neurological maturation, agingand genome instability in developmental biology research(510). Although C. elegans is known to possess repairhomologs to some of the proteins in mammalian DNArepair pathways (1114), BER has not been wellelucidated. Several C. elegans BER repair enzymes havebeen identified and characterized, including: UDG foruracil removal (15); the nth1 glycosylase for oxidized pyr-imidine base removal (16) and two APEs, APN-1 and

    *To whom correspondence should be addressed. Tel: +1 919 541 4701; Fax: +1 919 541 4724; Email: wilson5@niehs.nih.govPresent addresses:Kenjiro Asagoshi, Department of Biochemistry and Biophysics, Lineberger Comprehensive Cancer Center, University of North Carolina at ChapelHill, Chapel Hill, NC 27599, USA.Wade Lehmann, Office of Science and Technology, Office of Water U.S. Environmental Protection Agency, Washington, DC 20460, USA.

    670681 Nucleic Acids Research, 2012, Vol. 40, No. 2 Published online 14 September 2011doi:10.1093/nar/gkr727

    Published by Oxford University Press 2011.This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (, which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

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  • EXO-3, for AP site incision and processing (17). Inaddition to these enzymes, C. elegans also is known toexpress pol y, a lesion bypass polymerase and back-upBER polymerase in vertebrate systems (1821). Eventhough a complete BER system has not been reported inC. elegans, the existence of the above noted enzymesstrongly suggested the presence of BER in the organism.

    In this study, we further evaluated BER in C. elegans.To identify gap-filling DNA polymerases in C. elegans, wefirst conducted computer-based analyses of the genomicsequence. Computer-based sequence alignment analysesidentified several DNA polymerases, including a pol yhomolog, but failed to identify a pol b homolog. In spiteof the absence of a pol b homolog, we found robustin vitro BER in the extract prepared from L1 stageC. elegans and then examined the DNA polymerase re-quirement for the BER reaction using uracil-DNA as sub-strate. The DNA polymerase activity in the extract wascharacterized using inhibitors, and the activity was notconsistent with a mammalian pol b-like enzyme, but wasconsistent with a combination of replicative polymeraseand pol y-like enzyme. The results illustrate the presenceof complete BER in C. elegans and are discussed in thecontext of enzymes that fulfill the DNA polymerase re-quirement in the absence of a pol b homolog.



    Radioactive [g-32P]ATP (7000Ci/mmol) was fromMP Biomedicals (Irvine, CA, USA). [a-32P]dCTP(3000Ci/mmol), [a-32P] dTTP (3000Ci/mmol), [a-32P]Cordycepin (3000Ci/mmol) from PerkinElmer LifeSciences (Waltham, MA, USA), dCTP, ddCTP andMicroSpin G-25 columns were from GE Healthcare(Piscataway, NJ, USA). OptiKinase and terminal deoxy-nucleotidyl transferase were obtained from USBCorporation (Cleveland, OH, USA). Protease inhibitorcocktail was from Roche Diagnostic Corp.(Indianapolis, IN, USA). Human pol b (22,23), UDG(24), APE (25), DNA ligase I (26) and human pol ydomain (27) were purified as described. Activated calfthymus DNA, dimethyl sulfoxide (DMSO) and aphi-dicolin were obtained from Sigma-Aldrich (St Louis,MO, USA).

    DNA substrates

    The primer-template duplex DNA substrate for primerextension assays was constructed by annealing two syn-thetic oligodeoxynucleotides (Midland Certified ReagentCompany, Inc., Midland, TX, USA): 15PRM, 50-CTGCAGCTGATGCGC-30 and 37COM, 50-GCCGTACCCGGGGATCCGTACCGCGCATCAGCTGCAG-30.The uracil-containing DNA substrate for BER wasprepared by annealing two synthetic oligonucleotides:35U, 50-GCCCTGCAGGTCGAUTCTAGAGGATCCCCGGGTAC-30 and 35COM, 50-GTACCCGGGGATCCTCTAGAGTCGACCTGCAGGGC-30.

    Preparation of L1 stage C. elegans extract

    Caenorhabditis elegans strains Bristol N2 (wild-type) andtm2572 (polq-1 mutant) were obtained from theCaenorhabditis Genetic Center (Minneapolis, MN, USA)and the National Bioresource Project (Tokyo, Japan), re-spectively. The tm2572 insertiondeletion strain wasback-crossed three times into the Bristol N2 wild-typestrain and screened by nested PCR using the followingprimer sets: external primers (forward 50-GGT GCACCA TGA TAG GTA TT-30 and reverse 50-TGT ACCATC GAA AAA GCA GC) internal primers (forward50-TTA CGA CAG TGA CAC CAC AA-30 and reverse50-CGA TTC GTC TCG TGG TGC AC-30). Wild-typenematodes yielded an amplification product of 1776 bp,whereas, the mutant allele produced a 1078 bp product.Synchronized populations of wild-type nematodes were

    prepared by sodium hydroxide/hypochlorite treatment, aspreviously described (28). Adult animals were cultured on150mm nematode growth medium agar plates and eggswere obtained by established methods using a sodiumhypochlorite and sodium hydroxide mixture (29). The re-sulting eggs were washed in sterile K-medium (30), resus-pended in complete K media (31) and cultured at 20Covernight in a vented tissue culture flask to allow theeggs to hatch and for growth arrest at the L1 stage.After the overnight incubation, a portion of thecomplete K media was streaked onto an LB-agar plateto test for bacterial contamination. No bacterial colonyformation was identified post-egg preparation.L1 nematodes were washed with K media and pelleted

    by centrifugation to remove the majority of culture me-dium. The compressed nematode pellet was resuspendedin two volumes of sonication buffer and then sonicatedthree times for 30 s each, using a Bioruptor at 4C.Initially, three sonication buffers were tested in prepar-ation of whole-cell extracts: Buffer A {50mM TrisHCl,pH 7.4, 14mM b-mercaptoethanol, 100mM NaCl, 1mMEDTA, 10% glycerol and two mini-EDTA-free proteaseinhibitor tablets [modified from Morinaga et al. (16)]},Buffer B {25mM TrisHCl, pH 7.4, 1mM DTT, 5mMKCl, 1.5mM MgCl2 and two mini-EDTA-free proteaseinhibitor tablets [modified from Nakamura et al. (15)]}or Buffer C (50mM TrisHCl, pH 7.4, 500mM NaCl,1mM EDTA, 1mM DTT, 0.5% Triton X-100, 20%glycerol and two mini-EDTA-free protease inhibitortablets). The resulting extract was clarified by centrifuga-tion at 14 000 rpm at 4C for 30min. The clear super-natant fraction was divided into aliquots, quick-frozenin liquid nitrogen and stored at 80C until use. Theprotein concentration of the extract was determined byBio-Rad protein assay using BSA as standard.

    Activity gel analysis of DNA polymerase inC. elegans extract

    The activity gel assay was performed essentially asdescribed previously by Karawya et al. (32). Briefly, theprote


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