Synchronized populations of diploid yeast were exposed to uv light and their survival thereafter upon immediate plating (IP) and after liquid holding (LH) treatment in the presence and absence of caffeine was examined. These cells were found to be most resistant to uv light during early budding ( G2 period). The effect of LH is positive throughout the cell cycle and is dependent on the radiosensitivity of the cells upon IP. Caffeine-sensitive and caffeine-insensitive components of dark repair were demonstrated. The uv light responses of logarithmic growing and synchronized populations were compared with X-ray responses of the same strain analyzed under the same conditions in a previous work (1). The pattern of variation in sensitivity throughout the cell cycle and the ability to recover are qualitatively similar. The caffeine-insensitive component of repair has a similar efficiency in uv and X-irradiated G1 cells, increasing in uv-irradiated cells after the beginning of the DNA synthetic period and during the G2 period. The findings observed under IP conditions are consistent with a model already proposed (1) which requires the repair efficiency to oscillate during the cell cycle. The results suggest that repair pathways for lethal uv damage may share some common steps with those for X-ray damage.

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