FREQUENCY OF ULTRAVIOLET RADIATION-INDUCED MUTATION AT THE HPRT LOCUSIN REPAIR-PROFICIENT MURINE FIBROBLASTS TRANSFECTED WITH THE DENV GENE OF BACTERIOPHAGE-T4

The frequency of spontaneous and ultraviolet radiation (UVR)-induced m utation at the hprt locus was determined in control and denV-transfect ed, repair-proficient murine fibroblasts. Control cells removed an ave rage of 25 % of pyrimidine dimers induced by exposure to 150 J/m2 UVR from an FS40 sunlamp within 24 h; under the same conditions of inducti on and repair, denV-transfected cells removed an average of 71 % of py rimidine dimers. Control cells were somewhat more resistant than denV- transfected cells to killing by UVR. The average frequency of spontane ous mutation at the hprt locus for control and denV-transfected cells was 3 and 15 6-thioguanine (6-TG)-resistant colonies per 10(6) Survivi ng cells, respectively; there was no statistically significant differe nce between control and denV-transfected cells. However, after exposur e to 75 or 150 J/m2 UVR, denV-transfected cells had a significantly lo wer frequency of mutation to 6-TG resistance. After exposure to a flue nce of 75 J/M2, the average frequency of UVR-induced mutation at the h prt locus was 166 mutant colonies per 10(6) surviving cells for contro l cells and 92 mutant colonies for denV-transfected cells; after 150 J /m2, control cells had 205 6-TG-resistant colonies per 10(6) cells, wh ile denV-transfected cells had 61 mutant colonies. These results demon strate that UVR-induced pyrimidine dimers are mutagenic photoproducts in mammalian cells.