FUNCTIONAL NUCLEOTIDE EXCISION-REPAIR IS REQUIRED FOR THE PREFERENTIAL REMOVAL OF N-ETHYLPURINES FROM THE TRANSCRIBED STRAND OF THE DIHYDROFOLATE-REDUCTASE GENE OF CHINESE-HAMSTER OVARY CELLS

Transcription-coupled repair of DNA adducts is an essential factor tha t must be considered when one is elucidating biological endpoints resu lting from exposure to genotoxic agents. Alkylating agents comprise on e group of chemical compounds which modify DNA by reacting with oxygen and nitrogen atoms in the bases of the double helix. To discern the r ole of transcription-coupled DNA repair of N-ethylpurines present in d iscrete genetic domains, Chinese hamster ovary cells were exposed to N -ethyl-N-nitrosourea, and the clearance of the damage from the dihydro folate reductase gene was investigated. The results indicate that N-et hylpurines were removed from the dihydrofolate reductase gene of nucle otide excision repair-proficient Chinese hamster ovary cells; furtherm ore, when repair rates in the individual strands were determined, a st atistically significant bias in the removal of ethyl-induced, alkali-l abile sites was observed, with clearance occurring 30% faster from the transcribed strand than from its nontranscribed counterpart at early times after exposure, In contrast, removal of N-ethylpurines was obser ved in the dihydrofolate reductase locus in cells that lacked nucleoti de excision repair, but both strands were repaired at the same rate, i ndicating that transcription-coupled clearance of these lesions requir es the presence of active nucleotide excision repair.