Normal reconstruction of DNA supercoiling and chromatin structure in cockayne syndrome cells during repair of damage from ultraviolet light.

The chromatin of human cells undergoes structural rearrangements during excision repair of ultraviolet damage in DNA that were detected by transient relaxation of DNA supercoiling and increased staphylococcal nuclease digestibility of repaired sites. Inhibition of polymerization and/or ligation of repaired regions with inhibitors of DNA polymerase alpha (cytosine arabinoside and aphidicolin) resulted in the accumulation of single-strand breaks, delayed reconstruction of DNA supercoiling, and maintenance of the staphylococcal nuclease digestibility. These observations suggest that reconstruction of the native chromatin state requires completion of repaired regions with covalent ligation into the DNA strands. Although previous claims have been made that a late stage associated with ligation of repaired regions may be defective in cells from patients with Cockayne syndrome, complete reconstruction of the native chromatin occurred in cells from three unrelated patients after ultraviolet irradiation. No abnormality in repair was therefore detected in Cockayne syndrome cells. The hypersensitivity of cell survival and semiconservative DNA replication to damage by ultraviolet light in this human disorder must therefore be regarded as features of a primary defect in DNA metabolism unrelated to DNA repair.