title: Preferential DNA repair of an active gene in human cells.
abstract: Removal of pyrimidine dimers was measured in defined sequences in human cells amplified for the dihydrofolate reductase (DHFR) gene. We quantitated repair in specific restriction fragments by using the dimer-specific bacteriophage T4 endonuclease V and analysis by Southern blotting. Within 4 hr after 5- or 10-J/m2 UV irradiation, more than 60% of the dimers had been removed from a 20-kilobase fragment that lies entirely within the transcription unit of the DHFR gene and from a 25-kilobase fragment located in the 5' flanking region of the gene. Repair in the overall genome was measured by analyzing cellular DNA treated with T4 endonuclease V in alkaline sucrose gradients. Sixty-nine percent of the dimers were removed from the genome overall within 24 hr after irradiation, but only 25% were removed within 4 hr and 38% were removed within 8 hr. These results demonstrate a strong preferential rate of removal of dimers from the 50-kilobase region that includes the transcriptionally active DHFR gene compared to that in total cellular DNA. We confirmed that DHFR-containing DNA is repaired more rapidly than bulk DNA by using an approach that provides a direct comparison between repair in specific sequences and repair in total cellular DNA. We also show that the DHFR-containing sequences are repaired more rapidly than the nontranscribed repetitive alpha DNA sequences. Our finding of preferential early repair in a transcriptionally active region in overall repair-proficient cells suggests that selective dimer removal from active sequences may be a general characteristic of mammalian DNA repair.
authors.forename: I
authors.initials: I
authors.lastname: Mellon
authors.forename: V A
authors.initials: VA
authors.lastname: Bohr
authors.forename: C A
authors.initials: CA
authors.lastname: Smith
authors.forename: P C
authors.initials: PC
authors.lastname: Hanawalt
mesh_terms: D002460:Cell Line; D004260:DNA Repair; D005784:Gene Amplification; D005786:Gene Expression Regulation; D006570:Heterochromatin / Q000502:physiology; D006801:Humans; D013762:Tetrahydrofolate Dehydrogenase / Q000235:genetics*; D014158:Transcription, Genetic; D014466:Ultraviolet Rays
chemical_list: D006570:Heterochromatin; D013762:Tetrahydrofolate Dehydrogenase