Cyclobutane pyrimidine dimers and bulky chemical DNA adducts are efficiently repaired in both strands of either a transcriptionally active or promoter-deleted APRT gene

Both prokaryotic and eukaryotic cells have the capacity to repair DNA damag e preferentially in the transcribed strand of actively expressed genes. How ever, we have found that several types of DNA damage, including cyclobutane pyrimidine dimers (CPDs) are repaired with equal efficiency in both the tr anscribed and nontranscribed strands of the adenine phosphoribosyltransfera se (APRT) gene in Chinese hamster ovary cells. We further found that, in tw o mutant cell lines in which the entire APRT promoter region has been delet ed, CPDs are still efficiently repaired in both strands of the promoterless APRT gene, even though neither strand appears to be transcribed. These res ults suggest that efficient repair of both strands at this locus does not r equire transcription of the APRT gene. We have also mapped CPD repair in ex on 3 of the APRT gene in each cell line at single nucleotide resolution. Ag ain, we found similar rates of CPD repair in both strands of the APRT gene domain in both APRT promoter-deletion mutants and their parental cell line. Our findings suggest that current models of transcription-coupled repair a nd global genomic repair may underestimate the importance of factors other than transcription in governing the efficiency of nucleotide excision repai r.