Fine structure mapping of DNA repair within a 100 kb genomic region in Chinese hamster ovary cells.

We have investigated at a high level of resolution the repair of cyclobutane pyrimidine dimers (CPD) in a large amplified genomic region in Chinese hamster ovary B11 cells. We found strand selective repair in DNA fragments within two active genes, DHFR and an unknown gene adjacent to DHFR. These genes generate divergent transcripts from the same promoter region; their transcribed strands were virtually free of CPD within 24 h after irradiation with 10 j/m2 of ultraviolet light (254nm), while their non-transcribed strands were poorly repaired. We also examined the repair of CPD in three DNA fragments within a 50 kb region downstream of DHFR, in which two origins of replication flanking a matrix attachment site have been characterized from independently derived cell lines with amplified DHFR domains; repair of CPD in this non-transcribed region was similarly poor in both DNA strands. Transcription-coupled repair of CPD in the DHFR gene exhibited the same proficiency throughout the transcription unit: analysis of the efficiency of removal of CPD over time revealed no differences between repair in the 5' and the 3' ends of the DHFR gene. Implications for mechanisms of transcription-coupled repair are discussed.