Transcription‐coupled repair removes both cyclobutane pyrimidine dimers and 6‐4 photoproducts with equal efficiency and in a sequential way from transcribed DNA in xeroderma pigmentosum group C fibroblasts.

We investigated the contribution of the global and the transcription‐coupled nucleotide excision repair pathway to the removal of structurally different DNA lesions. The repair kinetics of UV‐induced cyclobutane pyrimidine dimers (CPDs) and pyrimidine (6‐4) pyrimidone photoproducts (6‐4PPs) were determined in an active and inactive gene in normal human fibroblasts and in xeroderma pigmentosum group C (XP‐C) fibroblasts. Previously we have shown that in normal human cells exposed to a UV dose of 10 J/m2 repair of CPDs takes place via two pathways: global repair and transcription‐coupled repair, the latter being responsible for accelerated repair of CPDs in the transcribed strand of active genes. So far, no clear evidence for transcription‐coupled repair of 6‐4PPs has been presented. Here we demonstrate that 6‐4PPs really form a target for transcription‐coupled repair. In XP‐C cells, exposed to 30 J/m2 and only capable of performing transcription‐coupled repair, CPDs as well as 6‐4PPs are removed selectively and with similar kinetics from the transcribed strand of the adenosine deaminase (ADA) gene. The non‐transcribed strand of the ADA gene and the inactive 754 gene are hardly repaired. In contrast to XP‐C cells, normal cells exposed to 30 J/m2 lack strand‐specific repair of both 6‐4PPs and CPDs, suggesting that transcription‐coupled repair is overruled by global repair, probably due to severe inhibition of transcription at this high UV dose. The much more rapid repair of 6‐4PPs compared with CPDs in normal cells may be related to higher affinity of the global repair system for the former lesion. In XP‐C cells the similarity of the rate of repair of both 6‐4PPs and CPDs in the transcribed strand at 30 J/m2 indicates that transcription‐coupled repair of photolesions takes place in a sequential way. Our results strongly suggest that the significance of transcription‐coupled repair for removal of lesions depends on the type of lesion and on the dose employed.