Previous evidence in a lower eukaryote indicated the lack of an oxygen effect for ionizing radiation induction of radioresistance [P. E. Bryant, Nature (London) 261, 588-590 (1976)], suggesting that the signal for induction may be different from that in prokaryotes, where DNA damage by a variety of agents has been shown to induce SOS. type repair and radioresistance. We show here that prior exposure to a sublethal dose of γ radiation caused induction of radioresistance in Saccharomyces cerevisiae, and that the rate of induction was proportional to the initial dose. Irradiation in oxygen produced a higher rate of increase in resistance, per unit dose, than irradiation in nitrogen, with an oxygen enhancement ratio of 2.8. The maximum level of resistance reached in response to saturating doses was the same regardless of the presence or absence of O2 during the initial irradiation. At less than saturating doses, however, the maximum increase in resistance was higher, per unit dose, following irradiation in the presence of O2 than in its absence. Oxic doses which gave the maximum possible increase in the level of resistance were considerably less than those required to produce the maximum rate of increase in resistance. These results suggest a difference in the level at which DNA damage saturates these processes. These results also indicate that the use of DNA damage as a signal for induction of radiation resistance has probably been conserved during the evolution of prokaryotes to lower eukaryotes.

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