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 leve ls ranging from 0.383 mu Sv/h to 1.275 mSv/h by selecting appropriate concentrations of tritiated water in the growth medium. These radiatio n levels correspond to 3-10 000 times the natural background. Paramete rs such as growth kinetics, gene conversion frequency at background ra diation and after a challenging dose of acute gamma-radiation or alkyl ating 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. Howev er, a number of the cultures showed conversion frequencies above 20 pe r 10(6) viable cells. This stochastic phenomenon occurred more frequen tly in cells which were incubated at higher radiation levels and for l onger durations. This suggests that radiation is responsible for the p henomenon. When subculturing continued beyond 900 h, gene conversion f requencies reverted back to normal values in all cultures in spite of elevated background radiation levels, thus suggesting an adaptive resp onse. The generation time of the cells was 78 min in all cultures irre spective of the radiation level. The response of the cells cultured at elevated background radiation levels to subsequent challenging treatm ent with gamma-radiation or MNNG was identical to that of the control cultures. Our results suggest that in eukaryotic yeast, low-level radi ation may induce an adaptive response to chronic radiation, whereas no such response could be detected when the cells were challenged with a cute high-dose exposure or with MNNG.