Research Spotlight: The impact of chromatin on DNA repair complex assembly and function

By Matt Wargo • December 8th, 2011

Citation: Odell, I.D., Barbour, J.E., Murphy, D.L., Della-Maria, J.A., Sweasy, J.B., Tomkinson, A.E., Wallace, S.S., Pederson, D.S. 2011. Nucleosome Disruption by DNA Ligase III-XRCC1 Promotes Efficient Base Excision Repair. Mol Cell Biol. 31(22):4623-32

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Authors Associated with MMG:
Ian Odell – recently earned his Ph.D. in MMG. At present, Ian is finishing clinical rotations for his M.D. in 2012, and interviewing for residency training programs.
Joy-El Barbour – conducted undergraduate research and worked as a technician in the MMG Department. She is now a Ph.D. candidate at U.C.Berkeley.
Joann Sweasy – Adjunct professor in MMG and a Professor in the Department of
Therapeutic Radiology at Yale.
Susan Wallace – Professor and Chair of MMG.
David Pederson – Associate Professor and Director of the Graduate Program in

The eukaryotic enzymes that repair DNA damage must function in presence of chromatin. To study the effects of chromatin on DNA repair, the authors reconstituted nucleosomes containing oxidatively damaged DNA. They then reconstituted the entire four-step DNA repair reaction, using purified, recombinant enzymes. The authors demonstrated that the DNA glycosylase hNTH1 can recognize and excise damaged bases from nucleosomes without irreversibly disrupting the nucleosome. The same proved true for apurinic endonuclease APE (which was observed to displace hNTH1 from its product) and, to a lesser extent, the DNA repair polymerase Pol Beta. However, Ligase IIIa, which catalyzes the final step in repair, disrupted nucleosomes when added along with the scaffolding protein XRCC1. Importantly, the Ligase IIIa-XRCC1 heterodimer proved able to disrupt nucleosomes containing either Pol Beta or ligase IIIa substrates. Because Pol Beta, ligase IIIa and XRCC1 form a trimeric complex, these results suggest that, in vivo, the first two steps in the repair of oxidative damage occur in nucleosomes, but that nucleosome disruption facilitates the final two steps in repair.

Impact and Significance:
Each day, approximately 20,000 oxidative lesions form in the DNA of every nucleated human cell. The authors’ findings provide insights into rate-limiting steps that govern DNA damage repair in chromatin and reveal a unique role for ligase IIIα-XRCC1 in enhancing the repair efficiency in the presence of nucleosomes.

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