DNA synthesis on discontinuous templates by human DNA polymerases: implications for non-homologous DNA recombination.
DNA polymerases catalyze the synthesis of DNA using a continuous uninterrupted template strand. However, it has been shown that a 3'-->5' exonuclease-deficient form of the Klenow fragment of Escherichia coli DNA polymerase I as well as DNA polymerase of Thermus aquaticus can synthesize DNA across two unlinked DNA templates. In this study, we used an oligonucleotide-based assay to show that discontinuous DNA synthesis was present in HeLa cell extracts. DNA synthesis inhibitor studies as well as fractionation of the extracts revealed that most of the discontinuous DNA synthesis was attributable to DNA polymerase alpha. Additionally, discontinuous DNA synthesis could be eliminated by incubation with an antibody that specifically neutralized DNA polymerase alpha activity. To test the relative efficiency of each nuclear DNA polymerase for discontinuous synthesis, equal amounts (as measured by DNA polymerase activity) of DNA polymerases alpha, beta, delta (+/- PCNA) and straightepsilon (+/- PCNA) were used in the discontinuous DNA synthesis assay. DNA polymerase alpha showed the most discontinuous DNA synthesis activity, although small but detectable levels were seen for DNA polymerases delta (+PCNA) and straightepsilon (- PCNA). Klenow fragment and DNA polymerase beta showed no discontinuous DNA synthesis, although at much higher amounts of each enzyme, discontinuous synthesis was seen for both. Discontinuous DNA synthesis by DNA polymerase alpha was seen with substrates containing 3 and 4 bp single-strand stretches of complementarity; however, little synthesis was seen with blunt substrates or with 1 bp stretches. The products formed from these experiments are structurally similar to that seen in vivo for non-homologous end joining in eukaryotic cells. These data suggest that DNA polymerase alpha may be able to rejoin double-strand breaks in vivo during replication.
Department of Radiation Oncology, University of California, San Francisco, CA 94143-0750, USA. aislas@lbl.gov
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D000818:Animals; D000906:Antibodies / Q000494:pharmacology; D034961:Antigens, Nuclear; D001483:Base Sequence; D016466:CHO Cells; D006224:Cricetinae; D004247:DNA / Q000096:biosynthesis* / Q000737:chemistry / Q000235:genetics*; D004265:DNA Helicases; D004256:DNA Polymerase I / Q000037:antagonists & inhibitors / Q000276:immunology / Q000378:metabolism; D051747:DNA-Activated Protein Kinase; D004268:DNA-Binding Proteins / Q000378:metabolism; D004259:DNA-Directed DNA Polymerase / Q000378:metabolism*; D004791:Enzyme Inhibitors / Q000494:pharmacology; D006367:HeLa Cells; D006801:Humans; D066298:In Vitro Techniques; D000072200:Ku Autoantigen; D008969:Molecular Sequence Data; D009500:Neutralization Tests; D009687:Nuclear Proteins / Q000378:metabolism; D019384:Nucleic Acid Synthesis Inhibitors; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism; D011995:Recombination, Genetic; D015183:Restriction Mapping; D013379:Substrate Specificity
D000906:Antibodies; D034961:Antigens, Nuclear; D004268:DNA-Binding Proteins; D004791:Enzyme Inhibitors; D009687:Nuclear Proteins; D019384:Nucleic Acid Synthesis Inhibitors; D004247:DNA; D051747:DNA-Activated Protein Kinase; C497263:PRKDC protein, human; D017346:Protein Serine-Threonine Kinases; D004256:DNA Polymerase I; D004259:DNA-Directed DNA Polymerase; D004265:DNA Helicases; C106893:XRCC5 protein, human; C000606509:Xrcc6 protein, human; D000072200:Ku Autoantigen