Induction of gene conversion in yeast cells continuously cultured at high radiation background

The induction of genetic damage was investigated by culturing diploid yeast Saccharomyces cerevisiae D7 cells continuously at radiation levels ranging from 0.383 microSv/h to 1.275 mSv/h by selecting appropriate concentrations of tritiated water in the growth medium. These radiation levels correspond to 3-10,000 times the natural background. Parameters such as growth kinetics, gene conversion frequency at background radiation and after a challenging dose of acute gamma-radiation or alkylating agent N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) were assessed. The gene conversion frequency in most of the assays was in the range of 5-10 convertants per 10(6) cells, as in the case of controls. However, a number of the cultures showed conversion frequencies above 20 per 10(6) viable cells. This stochastic phenomenon occurred more frequently in cells which were incubated at higher radiation levels and for longer durations. This suggests that radiation is responsible for the phenomenon. When subculturing continued beyond 900 h, gene conversion frequencies reverted back to normal values in all cultures in spite of elevated background radiation levels, thus suggesting an adaptive response. The generation time of the cells was 78 min in all cultures irrespective of the radiation level. The response of the cells cultured at elevated background radiation levels to subsequent challenging treatment with gamma-radiation or MNNG was identical to that of the control cultures. Our results suggest that in eukaryotic yeast, low-level radiation may induce an adaptive response to chronic radiation, whereas no such response could be detected when the cells were challenged with acute high-dose exposure or with MNNG.