Photoreactivation and Excision Repair of Thymine Dimers in Ultraviolet-Irradiated Cultured Fish Cells

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Abstract
Colony-formation assays showed that cultured fish [goldfish (Carassius auratus)] cells (CAF-MM1) are more sensitive to UV light than mammalian cells (mouse fibroblast L cells). To study the high sensitivity of the fish cells, direct assay of thymine dimers was made by 2-dimensional paper chromatography. The ratio of thymine dimers to thymine .**GRAPHIC**. immediately after UV irradiation was proportional to UV dose and was 0.04% per 10 J/m2 of UV. Similar values were also obtained in the mouse L cells used for comparison. When CAF-MM1 cells were incubated in the dark after UV irradiation, the value of .**GRAPHIC**. did not change for up to 48 h; i.e., excision of dimers could not be detected. Visible light illumination immediately after UV irradiation reduced the amount of thymine dimers and increased the cell survival of CAF-MM1 cells. The results indicated a close relation between photoreactivation of dimers and the fraction of cells surviving. About half the thymine dimers could be photoreactivated upon exposure to visible light immediately after UV irradiation, but only 18% after 24 h.