Human Oxoguanine Glycosylase 1 Removes Solution Accessible 8‑Oxo-7,8-dihydroguanine Lesions from Globally Substituted Nucleosomes Except in the Dyad Region

Persistent DNA damage is responsible for mutagenesis, aging, and disease. Repair of the prototypic oxidatively damaged guanine lesion 8-oxo-7,8-dihydroguanine (8-oxoG) is initiated by oxoguanine glycosylase (hOGG1 in humans). In this work, we examine hOGG1 activity on DNA packaged as it is in chromatin, in a nucleosome core particle (NCP). We use synthetic methods to generate a population of NCPs with G to 8-oxoG substitutions and evaluate the global profile of hOGG1 repair in packaged DNA. For several turns of the helix, we observe that solution accessible 8-oxoGs are sites of activity for hOGG1. At the dyad axis, however, hOGG1 activity is suppressed, even at lesions predicted to be solution accessible by hydroxyl radical footprinting (HRF). We predict this diminished activity is due to the properties of the DNA unique to the dyad axis and/or the local histone environment. In contrast to the dyad axis, the DNA ends reveal hOGG1 activity at sites predicted by HRF to be both solution accessible and inaccessible. We attribute the lack of correlation between hOGG1 activity and solution accessibility at the ends of the DNA to transient unwrapping of the DNA from the protein core, thus exposing the inward-facing lesions

Comparative analysis of the end-joining activity of several DNA ligases

<div><p>DNA ligases catalyze the repair of phosphate backbone breaks in DNA, acting with highest activity on breaks in one strand of duplex DNA. Some DNA ligases have also been observed to ligate two DNA fragments with short complementary overhangs or blunt-ended termini. In this study, several wild-type DNA ligases (phage T3, T4, and T7 DNA ligases, <i>Paramecium bursaria</i> chlorella virus 1 (PBCV1) DNA ligase, human DNA ligase 3, and <i>Escherichia coli</i> DNA ligase) were tested for their ability to ligate DNA fragments with several difficult to ligate end structures (blunt-ended termini, 3′- and 5′- single base overhangs, and 5′-two base overhangs). This analysis revealed that T4 DNA ligase, the most common enzyme utilized for <i>in vitro</i> ligation, had its greatest activity on blunt- and 2-base overhangs, and poorest on 5′-single base overhangs. Other ligases had different substrate specificity: T3 DNA ligase ligated only blunt ends well; PBCV1 DNA ligase joined 3′-single base overhangs and 2-base overhangs effectively with little blunt or 5′- single base overhang activity; and human ligase 3 had highest activity on blunt ends and 5′-single base overhangs. There is no correlation of activity among ligases on blunt DNA ends with their activity on single base overhangs. In addition, DNA binding domains (Sso7d, hLig3 zinc finger, and T4 DNA ligase N-terminal domain) were fused to PBCV1 DNA ligase to explore whether modified binding to DNA would lead to greater activity on these difficult to ligate substrates. These engineered ligases showed both an increased binding affinity for DNA and increased activity, but did not alter the relative substrate preferences of PBCV1 DNA ligase, indicating active site structure plays a role in determining substrate preference.</p></div

Schematic representation of DNA ligase fusions.

<p>All DNA ligases contain a catalytic core NTase domain (blue) and an OBD (red), which are fairly well conserved. Many ligases also have additional domains, such as the N-terminal ZnF (yellow) and DBD (green) in Human Lig3 and the N-terminal domain (NTD) of T4 DNA ligase (purple). Wild type PBCV1 ligase, which contains only the core NTase and OBD domains, was chosen for fusion to other binding domains: Sso7d (white) at both the N- and C-termini, the hLig3 ZnF domain, and the T4 DNA ligase NTD.</p

Wild type DNA ligase blunt/cohesive capillary electrophoresis assay.

<p>Bar graphs depict the fraction of either ligated DNA (product, blue) or abortive adenylylation (App, red) produced in a 20-minute sealing reaction with the indicated DNA substrate. Reactions included 1 μM of the DNA ligase, 100 nM of the substrate and reaction conditions consisting of either T4 DNA ligase reaction buffer (50 mM Tris-HCl pH 7.5 @ 25°C, 1 mM ATP and 10 mM MgCl₂) or NEBNext^® Quick Ligation reaction buffer (66 mM Tris pH 7.6 @ 25°C, 10 mM MgCl₂, 1 mM DTT, 1 mM ATP, 6% Polyethylene glycol (PEG 6000)). Ligation assays performed with T4 DNA ligase (A), T3 DNA ligase (B), PBCV1 DNA ligase (C) and hLig3 (D), respectively Experiments were performed in triplicate; the plotted value is the average and the error bars represent the standard deviation across replicates.</p

Wild type DNA ligase λ DNA digest ligation assay.

<p>Agarose gel electrophoresis of λ DNA cut by EcoRV (A/T Blunt, <b>1</b>), NruI (G/C Blunt, <b>2</b>), BstNI (5′ SBO, <b>3</b>), Hpy188I (3′SBO, <b>4</b>), NdeI (2 BO, <b>5</b>) and BamHI (4 BO, <b>6</b>), generating DNA fragments with ligatable ends. 0.5 ng of the cut DNA was ligated in the presence of T4 ligase reaction buffer (50 mM Tris-HCl pH 7.5 @ 25°C, 1 mM ATP and 10 mM MgCl₂) or NEBNext^® Quick Ligation reaction buffer (66 mM Tris pH 7.6 @ 25°C, 10 mM MgCl2, 1 mM DTT, 1 mM ATP, 6% polyethylene glycol (PEG 6000)) and 7 μM of the indicated DNA ligase for 1 hour at 25°C. Ligation assays performed with T4 DNA ligase (A), T3 DNA ligase (B), PBCV1 DNA ligase (C) and, hLig3 (D), respectively. E) Gel of restriction enzyme digested λ DNA samples as well as a schematic depiction of each substrate. The DNA fragments were visualized using ethidium bromide stain.</p

Measured binding constants.

<p>Measured binding constants.</p

DNA substrates.

<p>Substrates are shown aligned as annealed. A boldface and underlined nucleotide indicates a 3’-fluorescent label; 3′<u><b>C</b></u> is labeled with FAM, 3′<u><b>A</b></u> with ROX. SBO substrates combine both the FAM and ROX fragments in a single reaction.</p