The vegetative cells and spores of the cellular slime mold Dictyostelium discoideum NC-4 are very resistant to60 Co gamma rays, with a 10% survival dose ( D10) of 300 krad in air. Using ultraviolet light or nitrosoguanidine as a mutagenic agent and replica plating with gamma irradiation as a selective agent, we have isolated several stable daughter strains that are more sensitive to gamma rays. A comparison of some of the radiation responses of the parent (NC-4; wild type), γs-18 (${\rm D}_{10}=75\ {\rm krad}$), and γs-13 (${\rm D}_{10}=4\ {\rm krad}$) strains has been made. All three strains show the same gamma-ray-induced division delay of about 1 cell generation time for 54 krad. Strains wt (wild type) and γs-18 show split-dose recovery of colony-forming ability but γs-13 does not. For wt, this recovery is temperature-dependent with a maximum at 23°C, the most favorable growth temperature. Some recovery still occurs at 5-10°C, although growth does not. Strains wt and γs-18 are more sensitive when irradiated in oxygen or air compared with nitrogen. The sensitivity of γs-13 is the same in air, oxygen, or nitrogen. Postirradiation treatment with 1 mg caffeine/ml before plating sensitizes wt and γs-18 but increases the survival of γs-13. Strain γs-13 is more sensitive to killing by nitrosoguanidine than is wt or γs-18. These results suggest that γs-13 is more sensitive than wt because it is less capable of repairing gamma-ray and other damage and that γs-18 is intermediate in this ability.

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