Nucleosome disruption by DNA ligase III-XRCC1 promotes efficient base excision repair.
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Each day, approximately 20,000 oxidative lesions form in the DNA of every nucleated human cell. The base excision repair (BER) enzymes that repair these lesions must function in a chromatin milieu. We have determined that the DNA glycosylase hNTH1, apurinic endonuclease (APE), and DNA polymerase ? (Pol ?), which catalyze the first three steps in BER, are able to process their substrates in both 601- and 5S ribosomal DNA (rDNA)-based nucleosomes. hNTH1 formed a discrete ternary complex that was displaced by the addition of APE, suggesting an orderly handoff of substrates from one enzyme to the next. In contrast, DNA ligase III?-XRCC1, which completes BER, was appreciably active only at concentrations that led to nucleosome disruption. Ligase III?-XRCC1 was also able to bind and disrupt nucleosomes containing a single base gap and, because of this property, enhanced both its own activity and that of Pol ? on nucleosome substrates. Collectively, these findings provide insights into rate-limiting steps that govern BER in chromatin and reveal a unique role for ligase III?-XRCC1 in enhancing the efficiency of the final two steps in the BER of lesions in nucleosomes.