EFFECTS OF K-SHELL X-RAY ABSORPTION OF INTRACELLULAR PHOSPHORUS ON YEAST-CELLS

The effects of K-shell absorption of phosphorus atoms on yeast cells were investigated using synchroton X-rays that were tuned to the resonance absorption peak (2153 eV). Three types of cellular effect (cell inactivation, induction of gene conversion at the trp-5 locus, and cell membrane impairment (changes in the permeability)) were measured. It was demonstrated that the enhancement factor was 1.4 at the resonance peak regarding both lethality and the induction of gene conversion in reference of off-peak irradiation (2146 and 2160 eV). No difference was found between the two off-peak irradiation energies. No cell membrane impairment was detected, irrespective of the X-ray photon energies employed within the fluence range tested. These results strongly suggest that K-shell X-ray absorption in the resonance mode by cellular phosphorus atoms causes significantly more cellular effects than the off-resonance mode of absorption, probably via some specific changes induced in the phosphates of the DNA strand. Calculations using the number of phosphorus atoms in a defined size of the trp locus (2127 base pairs) on the DNA and the absorption cross-section of the resonance mode of phosphorus showed that gene conversion is inducible at a rate of 0.13 per X-ray photon absorption per locus. These results are discussed regarding the modes of K-shell photoabsorption.