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 cyclo butane pyrimidine dimers (CPD) in a large amplified genomic region in Chinese hamster ovary B11 cells. We found strand selective repair in D NA fragments within two active genes, DHFR and an unknown gene adjacen t to DHFR. These genes generate divergent transcripts from. the same p romoter region; their transcribed strands were virtually free of CPD w ithin 24 h after irradiation with 10 J/m(2) of ultraviolet light (254 nm), while their non-transcribed strands were poorly repaired. We also examined the repair of CPD in three DNA fragments within a 50 kb regi on downstream of DHFR, in which two origins of replication flanking a matrix attachment site have been characterized from independently deri ved cell lines with amplified DHFR domains; repair of CPD in this non- transcribed region was similarly poor in both DNA strands. Transcripti on-coupled repair of CPD in the DHFR gene exhibited the same proficien cy throughout the transcription unit: analysis of the efficiency of re moval of CPD over time revealed no differences between repair in the 5 ' and the 3' ends of the DHFR gene. Implications for mechanisms of tra nscription-coupled repair are discussed.