title: Repair of O6-methylguanine adducts in human telomeric G-quadruplex DNA by O6-alkylguanine-DNA alkyltransferase.
abstract: O(6)-alkylguanine-DNA alkyltransferase (AGT) is a single-cycle DNA repair enzyme that removes pro-mutagenic O(6)-alkylguanine adducts from DNA. Its functions with short single-stranded and duplex substrates have been characterized, but its ability to act on other DNA structures remains poorly understood. Here, we examine the functions of this enzyme on O(6)-methylguanine (6mG) adducts in the four-stranded structure of the human telomeric G-quadruplex. On a folded 22-nt G-quadruplex substrate, binding saturated at 2 AGT:DNA, significantly less than the ∼ 5 AGT:DNA found with linear single-stranded DNAs of similar length, and less than the value found with the telomere sequence under conditions that inhibit quadruplex formation (4 AGT:DNA). Despite these differences, AGT repaired 6mG adducts located within folded G-quadruplexes, at rates that were comparable to those found for a duplex DNA substrate under analogous conditions. Repair was kinetically biphasic with the amplitudes of rapid and slow phases dependent on the position of the adduct within the G-quadruplex: in general, adducts located in the top or bottom tetrads of a quadruplex stack exhibited more rapid-phase repair than did adducts located in the inner tetrad. This distinction may reflect differences in the conformational dynamics of 6mG residues in G-quadruplex DNAs.
authors: Department of Molecular and Cellular Biochemistry, Center for Structural Biology, University of Kentucky, Lexington, KY 40536, USA.
authors: Lance M
authors: LM
authors: Hellman
authors: Department of Molecular and Cellular Biochemistry, Center for Structural Biology, University of Kentucky, Lexington, KY 40536, USA.
authors: Tyler J
authors: TJ
authors: Spear
authors: Department of Molecular and Cellular Biochemistry, Center for Structural Biology, University of Kentucky, Lexington, KY 40536, USA.
authors: Colton J
authors: CJ
authors: Koontz
authors: Department of Molecular and Cellular Biochemistry, Center for Structural Biology, University of Kentucky, Lexington, KY 40536, USA.
authors: Manana
authors: M
authors: Melikishvili
authors: Department of Molecular and Cellular Biochemistry, Center for Structural Biology, University of Kentucky, Lexington, KY 40536, USA michael.fried@uky.edu.
authors: Michael G
authors: MG
authors: Fried
mesh_terms: D004247:DNA / Q000737:chemistry / Q000378:metabolism; D018736:DNA Adducts / Q000378:metabolism*; D004260:DNA Repair; D054856:G-Quadruplexes; D006147:Guanine / Q000031:analogs & derivatives* / Q000378:metabolism; D006801:Humans; D019853:O(6)-Methylguanine-DNA Methyltransferase / Q000378:metabolism*; D011188:Potassium / Q000737:chemistry; D016615:Telomere / Q000737:chemistry*
chemical_list: D018736:DNA Adducts; D006147:Guanine; D004247:DNA; C008449:O-(6)-methylguanine; D019853:O(6)-Methylguanine-DNA Methyltransferase; D011188:Potassium