Repair in a constitutively expressed gene remains proficient in quiescent B lymphocytes. (A) Representative experiment (n = 10 donors) showing repair of CPD lesions in B lymphocytes in the transcribed (TS) and nontranscribed (NTS) strands of the dihydrofolate reductase gene. Cells were harvested at various times after irradiation with 10 J/m2 UV light. For each time point, 50 μg of genomic DNA was digested with KpnI, half of the sample was treated (+) or not treated (−) with the CPD-specific nicking enzyme T4 endonuclease V (Endo V), resolved in a denaturing agarose gel, transferred to a nylon membrane, and hybridized with 32P-labeled strand-specific RNA probes. (B) Quantification of 2 independent experiments, including the aforementioned one. WT and XP-E B lymphoblasts are included for reference. The amount of lesion-free fragment was quantified with Molecular Analyst software, and lesion frequency was determined with the Poisson expression. Error bars represent SEM.
