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Repair of O6-methylguanine adducts in human telomeric G-quadruplex DNA by O6-alkylguanine-DNA alkyltransferase.

Hellman LM, Spear TJ, Koontz CJ, Melikishvili M, Fried MG - Nucleic Acids Res. (2014)

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.

View Article: PubMed Central - PubMed

Affiliation: Department of Molecular and Cellular Biochemistry, Center for Structural Biology, University of Kentucky, Lexington, KY 40536, USA.

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Schematic diagrams of G-quadruplex DNAs. (A) Backbone diagram of a three-quartet quadruplex in the antiparallel ‘basket’ conformation. Guanine residues are numbered; other residues are located at the vertices of the connecting line segments. The planes of the G-quartets are shown as rectangles in projection. (B) Backbone diagram of a three-quartet quadruplex in the all-parallel ‘propeller’ conformation. Drawing conventions as in (A). (C) Schematic representation of hydrogen bonding and ion binding within a single G-quartet. Here X represents the sugar-phosphate backbone. Redrawn from (23). (D) Schematic diagram showing potential effect of O6-methylation of a single guanine on hydrogen bonding and ion binding within a G-quartet. Here X represents the sugar-phosphate backbone.
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Figure 1: Schematic diagrams of G-quadruplex DNAs. (A) Backbone diagram of a three-quartet quadruplex in the antiparallel ‘basket’ conformation. Guanine residues are numbered; other residues are located at the vertices of the connecting line segments. The planes of the G-quartets are shown as rectangles in projection. (B) Backbone diagram of a three-quartet quadruplex in the all-parallel ‘propeller’ conformation. Drawing conventions as in (A). (C) Schematic representation of hydrogen bonding and ion binding within a single G-quartet. Here X represents the sugar-phosphate backbone. Redrawn from (23). (D) Schematic diagram showing potential effect of O6-methylation of a single guanine on hydrogen bonding and ion binding within a G-quartet. Here X represents the sugar-phosphate backbone.

Mentions: Much less is known about AGT function with non-canonical DNA structures such as the G-quadruplexes that can form in DNA containing the human telomeric repeat sequence (TTAGGG)x (20,21), and that protect chromosome ends and play roles in telomere maintenance (22). G-quadruplex structures can form if one guanine in a stack of quartets is methylated at the O6-position, although the methyl group interferes with hydrogen bonding within the quartet, and quite possibly also the binding of a counterion near the center of the affected quartet (Figure 1C, 23). The result is destabilization of the quadruplex, with respect to the corresponding unmethylated structure (24). Together these features suggest that there might be a role for AGT activity in the maintenance of the unmethylated form of the telomere sequence. On the other hand, differences in charge density, geometry and base-stacking energies that distinguish quadruplex from duplex structures led us to predict that AGT would be unable to bind and repair O6-methylguanines in G-quadruplex structures. The experiments described below tested these predictions.


Repair of O6-methylguanine adducts in human telomeric G-quadruplex DNA by O6-alkylguanine-DNA alkyltransferase.

Hellman LM, Spear TJ, Koontz CJ, Melikishvili M, Fried MG - Nucleic Acids Res. (2014)

Schematic diagrams of G-quadruplex DNAs. (A) Backbone diagram of a three-quartet quadruplex in the antiparallel ‘basket’ conformation. Guanine residues are numbered; other residues are located at the vertices of the connecting line segments. The planes of the G-quartets are shown as rectangles in projection. (B) Backbone diagram of a three-quartet quadruplex in the all-parallel ‘propeller’ conformation. Drawing conventions as in (A). (C) Schematic representation of hydrogen bonding and ion binding within a single G-quartet. Here X represents the sugar-phosphate backbone. Redrawn from (23). (D) Schematic diagram showing potential effect of O6-methylation of a single guanine on hydrogen bonding and ion binding within a G-quartet. Here X represents the sugar-phosphate backbone.
© Copyright Policy - creative-commons
Related In: Results  -  Collection

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Show All Figures
getmorefigures.php?uid=PMC4150771&req=5

Figure 1: Schematic diagrams of G-quadruplex DNAs. (A) Backbone diagram of a three-quartet quadruplex in the antiparallel ‘basket’ conformation. Guanine residues are numbered; other residues are located at the vertices of the connecting line segments. The planes of the G-quartets are shown as rectangles in projection. (B) Backbone diagram of a three-quartet quadruplex in the all-parallel ‘propeller’ conformation. Drawing conventions as in (A). (C) Schematic representation of hydrogen bonding and ion binding within a single G-quartet. Here X represents the sugar-phosphate backbone. Redrawn from (23). (D) Schematic diagram showing potential effect of O6-methylation of a single guanine on hydrogen bonding and ion binding within a G-quartet. Here X represents the sugar-phosphate backbone.
Mentions: Much less is known about AGT function with non-canonical DNA structures such as the G-quadruplexes that can form in DNA containing the human telomeric repeat sequence (TTAGGG)x (20,21), and that protect chromosome ends and play roles in telomere maintenance (22). G-quadruplex structures can form if one guanine in a stack of quartets is methylated at the O6-position, although the methyl group interferes with hydrogen bonding within the quartet, and quite possibly also the binding of a counterion near the center of the affected quartet (Figure 1C, 23). The result is destabilization of the quadruplex, with respect to the corresponding unmethylated structure (24). Together these features suggest that there might be a role for AGT activity in the maintenance of the unmethylated form of the telomere sequence. On the other hand, differences in charge density, geometry and base-stacking energies that distinguish quadruplex from duplex structures led us to predict that AGT would be unable to bind and repair O6-methylguanines in G-quadruplex structures. The experiments described below tested these predictions.

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.

View Article: PubMed Central - PubMed

Affiliation: Department of Molecular and Cellular Biochemistry, Center for Structural Biology, University of Kentucky, Lexington, KY 40536, USA.

Show MeSH
Related in: MedlinePlus