A comparison between rates of cell death and DNA damage during irradiation of Saccharomyces cerevisiae in N2 and N2O.

Yeast and several other organisms are more sensitive to the lethal effects of ionizing irradiation if exposed in the presence of N2O as compared to N2. It has been suggested that this increased sensitivity is due to the cooperative effects of OH and H2O2 generated external to the cell wall. Using diploid yeast, wild type for radiation resistance, we have compared the rates of cell death due to gamma irradiation in N2 and N2O with the rates of DNA damage measured by gene conversion of trp- to trp+ (a recombinational repair event). We find that DNA damage as measured by gene conversion increases at a faster rate, per unit dose, during irradiation in N2O as compared to N2, just as lethality was higher in N2O. When DNA damage was compared in N2 and N2O at equal levels of survival, however, there was no significant difference between the two irradiation conditions. Therefore, increased lethality during irradiation in N2O seems to be directly due to increased DNA damage. If the observed increased lethality results from external OH and H2O2, the effect of these highly reactive species is expressed by increased internal damage at the level of DNA.