Enhanced repair of pyrimidine dimers in coding and non-coding genomic sequences in CHO cells expressing a prokaryotic DNA repair gene

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Abstract
We have previously demonstrated that the active dihydrofolate reductase (DHFR) gene is efficiently repaired in Chinese hamster ovary (CHO) cells which remove only a small fraction o u.v.-induced pyrimidine dimers from the overall genome. Preferential DNA repair of essential genes may explain why the u.v. resistance of normal CHO cells is as high as that of fully repair-proficient normal human cells. In this report, we have studied the removal of pyrimidine dimers in a CHO cell line expressing the cloned denV gene from bacteriophage T4 which codes for the pyrimidine dimer specific enzyme T4 endonuclease V (T4 endo V). This cell line was derived from a u.v.-sensitive excision deficient mutant of a CHO wild type line by transformation with the denV gene, and partial restoration of u.v. resistance was achieved. We have examined an important aspect of the u.v. excision repair in these denV+ cells by studying the repair efficiencies in the active DHFR gene and in a non-coding sequence located downstream from it. In the u.v.-sensitive CHO mutant cell line from which the denV+ was derived, we detected no pyrimidine dimer removal from the gene or from the downstream sequence after irradiation of the cells with 20 J/m2 u.v. (254 nm) light. In the wild type CHO cells, .apprx. 50% of the pyrimidine dimers were removed from a sequence in the DHFR gene within 8 h, whereas none were removed from the downstream sequence in that period. This represents the normal pattern of preferential DNA repair of active genes, which we have described in previous communications. In the denV+ cells, .apprx. 70% of the pyrimidine dimers were removed from both the DHFR gene and from the downstream sequence; these cells thus repair both coding and noncoding regions of the genome and show no pattern of preferential repair. The endogenous activity that initiates excision repair in normal CHO cells is evidently much more restricted in its accessibility to DNA lesions in chromatin than is the activity in cells containing substantial amounts of the small T4 endo V enzyme.

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