Repair of UVC-induced CPDs from the 3-kb <t>EcoRI</t> fragment of the Ad-encoded lacZ gene in human and rodent cells measured by loss of T4pdg-sensitive sites. ( A ) Southern blot analysis of the repair of UVC-induced CPDs in the Ad lacZ gene. Shown here is a representative blot. Lanes 1 and 2 contain untreated Ad <t>DNA,</t> while lanes 3 and 4 contain Ad DNA exposed to 240 J/m 2 UVC. The presence of ssDNA breaks in the 3-kb EcoRI lacZ fragment produce smaller ssDNA fragments that migrate further than the full-length fragment. These smaller fragments appear as a smear or tail below the defined 3-kb band. Smearing below the 3-kb band in samples that have not been treated with T4pdg (lanes 1 and 3) represent ssDNA breaks generated by other sources. Following UVC exposure, a large number of T4pdg-sensitive sites are generated (compare lanes 2 and 4). During repair incubation, NER removes CPDs resulting in the loss of T4pdg-sensitive sites and recovery of the full-length 3-kb lacZ fragment. As long as CPDs persist in the lacZ DNA, T4pdg will induce ssDNA breaks resulting in less full-length signal compared to the control. ( B ) Quantification of the percent removal of CPDs from the Ad-encoded lacZ gene in GM637F and CHO-AA8 cells. Each point on the graphs represents an arithmetic mean ± SE of the percent removal of UVC-induced T4pdg-sensitive sites from three independent experiments. A significant increase in the percent removal of UVC-induced T4pdg-sensitive sites was observed in GM637F at 24 h (indicated by an asterisk) and a significant difference in the percent removal of T4pdg-sensitive sites was observed between GM637F and CHO-AA8 24 h (indicated by a cross/plus sign).