Fig. 9

Model of CPD recognition in nucleosome cores. UV irradiation induces CPDs that are distributed evenly in chromatin and, hence, are formed also within nucleosome cores where the DNA is wrapped around histone octamers. Recognition of CPDs in nucleosome cores by DDB2 leads to recruitment of the histone methyltransferase ASH1L and, as a consequence, to methylation of the amino-terminal tail of histone H3 at position lysine 4. The presence of methylated H3 facilitates the stable docking of the XPC subunit to damaged sites in the nucleosome core, such that XPC protein is endowed with the ability to induce downstream GG-NER reactions, including DNA unwinding by TFIIH and dual DNA incision by the XPG and XPF-ERCC1 endonucleases. In the absence of ASH1L and H3K4 methylation, the loose interaction between DDB2 and XPC is not sufficient to initiate the GG-NER reaction at nucleosomal CPDs