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Cells from a subset of patients with the DNArepair-defective disease xeroderma pigmentosum complementation group E (XP-E) are known to lack a DNA damagebinding (DDB) activity. Purified human DDB protein was injected into XP-E cells to test whether the DNA-repair defect in these cells is caused by a defect in DDB activity. Injected DDB protein stimulated DNA repair to normal levels in those strains that lack the DDB activity but did not stimulate repair in cells from other xeroderma pigmentosum groups or in XP-E cells that contain the activity. These results provide direct evidence that defective DDB activity causes the repair defect in a subset of XP-E patients, which in turn establishes a role for this activity in nucleotide-excision repair in vivo.Ddb+ and Ddb-to indicate the phenotypes of cell culture strains that contain or lack DDB activity, respectively. The designation Ddb+ is not intended to imply that the DDB protein present in some XP-E cells is absolutely like wild type but to imply only that DDB activity can be detected in normal amounts in extracts from these strains.] Moreover, demonstration of a correlation between a defect in DDB activity and the XP-E disease neither proves a causal relationship nor confirms a role for the DDB protein in DNA repair. To directly address these issues, we examined the ability of purified DDB protein to correct the DNA-repair defect in XP-E cells.

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