EVIDENCE FROM PHOTOREACTIVATION KINETICS FOR MULTIPLE DNA PHOTOLYASES IN YEAST

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Abstract
Abstract— We have analyzed the kinetics of photoreactivation of ultraviolet‐light‐irradiated cells of two strains of the yeast Saccharomyces cerevisiae. The analysis yields plots of h(t), the number of photorepairable lethal hits remaining in the cell as a function of the time t of exposure to a continuous source of photoreactivating light. In the case of one of the strains studied, an excision‐deficient haploid strain, it is found that the time‐dependence of h(t) is given by the sum of three decreasing exponential functions of t for stationary phase cultures grown at 23 or 30°C. When the culture is grown at 37°C. the fastest component is absent and the intermediate component is reduced in importance relative to the remaining slow component. In the case of the other strain, a diploid of wild‐type radiation resistance, h(t) is found to contain just one decreasing exponential for cultures grown at 23, 30, and 37°C. The rate constant depends on the growth temperature of the cells, decreasing with increasing temperature. The results are interpreted as evidence for multiple DNA photolyases in the sensitive haploid strain.
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