Effect of UV light on DNA chain growth and replicon initiation in xeroderma pigmentosum variant cells

The effects of UV light on DNA replication were examined in wild-type and xeroderma pigmentosum (XP) variant cells using DNA fiber autoradiography. Replication segments were significantly shorter in UV-exposed XP variant cells than they were in UV-exposed wild-type human cells immediately after exposure. This is consistent with data obtained by others using alkaline sucrose gradients suggesting that there is more blockage of DNA fork progression at UV-induced lesions in XP variant cells. With time, (2.5-5.0 h) the lengths of replication segments increased in both cell lines, suggesting that some type of bypass was occurring in XP variant cells or that excision repair was removing the blocking lesions. To determine if the post-replication defect in XP variant cells involved a failure to activate alternative sites of replicon initiation, high/low specific activity labeling was performed. The results obtained indicated that XP variant cells were able to activate alternative sites of replicon initiation. Therefore the unique phenotype of the XP variant cells is probably due to some other defect.