3-Methyladenine and 7-methylguanine exhibit no preferential removal from the transcribed strand of the dihydrofolate reductase gene in Chinese hamster ovary B11 cells.

The removal of cylclobutane pyrimidine dimers from cellular DNA occurs preferentially in actively transcribed genes of cells subjected to ultraviolet radiation. In contrast, reports concerning the transcription-dependent repair of N-methylpurines formed in cellular DNA following exposure to methylating agents are quite conflicting, with some studies suggesting that no biased clearance of these lesions occurs and others indicating that preferential removal of these adducts transpires in active genetic loci. Even in the cases where no preferential clearance was demonstrated, a slight but statistically insignificant biased removal of N-methylpurines from the transcribed strand of active genes was often evident. We proposed that these results might be due to the preferential clearance of only one of the two principal N-methylpurines formed, 3-methyladenine, or to the source of the methylating species to which the cells were exposed. Therefore, we investigated the clearance of 3-methyladenine and 7-methylguanine as individual lesions from the amplified dihydrofolate reductase gene of Chinese hamster ovary cells, and we examined the gene-specific removal of N-methylpurines formed by several different methylating agents as well. We observed no biased clearance of 3-methyladenine toward the transcribed strand of the locus being examined. This result indicates that any minor gene-specific preferential repair that has been observed previously for N-methylpurines in toto--which actually reflects the removal of the predominant methylated purine 7-methylguanine--is not due to biased clearance of the transcription-inhibiting 3-methyladenine lesion.(ABSTRACT TRUNCATED AT 250 WORDS)