Differential DNA repair in transcriptionally active and inactive proto-oncogenes: c-abl and c-mos.

DNA repair was examined in the c-abl and c-mos proto-oncogenes in UV-irradiated mouse 3T3 fibroblasts using an optimized assay for measuring pyrimidine dimers in single-copy nucleotide sequences. We found similar initial dimer frequencies in the two genes. Within 24 hr, 85% of the dimers were removed from the 20 kb intragenic BamHl restriction fragment of the expressed c-abl gene, while only 22% of the dimers were removed from the 15 kb EcoRl fragment that spans the transcriptionally inactive c-mos locus. Quiescent and actively growing cells showed similar relative efficiencies of repair in the two regions. This is the first demonstration of differential DNA repair in two different genes. These findings have important implications for the mechanisms of proto-oncogene activation.