Repair of O6-methylguanine adducts in human telomeric G-quadruplex DNA by O6-alkylguanine-DNA alkyltransferase.
Bottom Line: 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.This distinction may reflect differences in the conformational dynamics of 6mG residues in G-quadruplex DNAs.
Affiliation: Department of Molecular and Cellular Biochemistry, Center for Structural Biology, University of Kentucky, Lexington, KY 40536, USA.Show MeSH
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Mentions: Circular dichroism (CD) spectroscopy was used to further characterize the folding of these DNAs. In TE + 75 mM KCl buffer, the unmodified 22wt DNA and sequences with 6mG at residues G1, G3, G4 and G6 had CD maxima near 295 nm and minima near 235 nm (Figure 3A). These features correspond well to spectra previously observed for antiparallel strand quadruplex DNA (38). They also resemble previously-determined spectra of a folded form of the human sequence (24). In contrast, spectra for telomere sequences with 6mG at positions G2, G5 and G11 have distinct maxima near 260 nm (Figure 3B); maxima in this wavelength range have been described for parallel G-quadruplex DNAs (38,39). Finally, the CD spectra of 22wtx and G5x DNAs in K+-free buffers differ significantly from those obtained in the presence of K+, indicating the presence of conformations that are distinct from the quadruplex fold (Figure 3C). Spectra obtained in TE buffer and TE buffer containing 75mM TEA–HCl were similar, indicating that the corresponding conformations are not unique to low [salt] conditions.
Affiliation: Department of Molecular and Cellular Biochemistry, Center for Structural Biology, University of Kentucky, Lexington, KY 40536, USA.