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.
MGC, Department of Radiation Genetics, Leiden University, The Netherlands.
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van Hoffen
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Venema
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Meschini
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van Zeeland
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Mullenders
D002478:Cells, Cultured; D004247:DNA / Q000235:genetics* / Q000528:radiation effects; D004260:DNA Repair; D004706:Endodeoxyribonucleases / Q000378:metabolism; D029968:Escherichia coli Proteins; D005347:Fibroblasts / Q000378:metabolism; D006801:Humans; D010777:Photochemistry; D011740:Pyrimidine Dimers; D014158:Transcription, Genetic; D014466:Ultraviolet Rays; D014983:Xeroderma Pigmentosum / Q000235:genetics* / Q000473:pathology
D029968:Escherichia coli Proteins; D011740:Pyrimidine Dimers; D004247:DNA; D004706:Endodeoxyribonucleases; C036385:endodeoxyribonuclease uvrABC