title: O6-methylguanine-induced replication blocks.
abstract: The ability of Klenow polymerase I, phage T7 polymerase (Sequenase), human polymerase alpha, and human polymerase beta to synthesize past (bypass) O6-methylguanine (O6-meG) lesions was studied in the presence of MgCl2 and MnCl2. An end-labeled 16-mer primer was annealed to the 3' end of gel-purified oligodeoxyribonucleotide templates (45-mers), each containing a single O6-meG in place of one G in the sequence -G1G2CG3G4T-. Extension products were analyzed by denaturing polyacrylamide gel electrophoresis and autoradiography. A fraction of the products extended by Klenow fragment terminated either opposite or one base before O6-meG located at sites 1 and 3. Termination occurred primarily one base before O6-meG located at sites 2 and 4. The remaining fractions that bypassed the lesions represented full-length product. In control reactions, the O6-meG-containing templates were annealed with complementary 45-mers, repaired with O6-alkylguanine DNA-alkyltransferase, annealed with an excess of labeled primer, and extended by Klenow fragment. Full-length extension of > 90% was observed with each template. Primer extension past O6-meG by DNA polymerase alpha and Sequenase was partially blocked in a manner which varied with the site of O6-meG in the template while primer extension by DNA polymerase beta was completely blocked (< 2% full length extension) with O6-meG at sites 1-4. Substitution of MnCl2 for MgCl2 in the reaction mixture greatly increased the bypass of O6-meG by Klenow fragment and DNA polymerase alpha but not Sequenase or DNA polymerase beta. The increased ability of Klenow fragment to bypass O6-meG in the presence of MnCl2 was found to result from an increased incorporation of G (O6-meG at sites 1 and 2) and A (O6-meG at sites 1, 2, and 3) opposite the lesion. The results indicate that O6-meG can block in vitro polymerization by several DNA polymerases and are consistent with the observed cytotoxic effects of methylating agents on mammalian cells.
authors.affiliation: Department of Pharmacology and Toxicology, Philadelphia College of Pharmacy and Science, PA 19104, USA.
authors.forename: J M
authors.initials: JM
authors.lastname: Voigt
authors.forename: M D
authors.initials: MD
authors.lastname: Topal
mesh_terms: D001483:Base Sequence; D004260:DNA Repair; D004261:DNA Replication / Q000187:drug effects*; D004259:DNA-Directed DNA Polymerase / Q000494:pharmacology; D006147:Guanine / Q000031:analogs & derivatives* / Q000494:pharmacology; D006801:Humans; D008274:Magnesium / Q000494:pharmacology; D008345:Manganese / Q000494:pharmacology; D008969:Molecular Sequence Data
chemical_list: D008345:Manganese; D006147:Guanine; C008449:O-(6)-methylguanine; D004259:DNA-Directed DNA Polymerase; D008274:Magnesium