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Report No.

Reparability of lethal lesions produced by phosphorus photoabsorption in yeast cells

Usami, Noriko*; Yokoya, Akinari; Ishizaka, Shozo*; Kobayashi, Katsumi*

The characteristics of DNA lesions produced by the phosphorus K-shell absorption in yeast cells were studied using monochromatized soft X-rays tuned to the phosphorus K-edge peak (2153 eV) and below the peak energy (2147 eV). The repaired fractions of DNA double-strand breaks (dsb) were measured relatively by using both a mutant, (${it rad 54-3}$), which shows the temperature-sensitive dsb repair-deficient, and a wild-type strain. The repaired fraction of lesion in ${it rad 54-3}$, which corresponds to the relative yield of dsb reparable by the ${it RAD 54}$ pathway, was not affected by the phosphorus photoabsorption. Repair of the produced lesions in the wild-type cells was also measured by comparing the surviving fraction of the immediately plated cells to that of those cells plated after holding in a non-nutrient medium for 80 hrs. The recovery of the surviving fraction after the holding treatment was dependent upon the soft X-ray energy. These results suggest that irrepairable lesions are produced by the inner-shell photoabsorption of phosphorus in DNA, although its yield is small.



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