O6-alkylguanine-DNA alkyltransferase (AGT) is a single-cycle DNA repair enzyme that removes pro-mutagenic O6-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 O6-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.
Digital Object Identifier (DOI)
National Institutes of Health (NIH) [GM-070662 to M.G.F.]. Funding for open access charge: NIH [GM-070662 to M.G.F.].
Hellman, Lance M.; Spear, Tyler J.; Koontz, Colton J.; Melikishvili, Manana; and Fried, Michael G., "Repair of O6-Methylguanine Adducts in Human Telomeric G-Quadruplex DNA by O6-Alkylguanine-DNA Alkyltransferase" (2014). Molecular and Cellular Biochemistry Faculty Publication. 75.