Ultraviolet light-induced termination of RNA synthesis during Dictyotelium discoideum spore germination

Abstract

Unlike most microorganisms, the dormant spores of the cellular slime mold Dictyostelium discoideum are more sensitive to ultraviolet light than the vegetative cells. When activated by the usual heat-shocking technique, irradiated spores swell normally but do not release myxamoebae. The irradiated spores exhibit greater than 90% inhibition of [ 3H]uracil incorporation when compared to control germinating spores. These data support recent work by H. L. Ennis (1981, Antimicrob. Ag. Chemother 19: 657–665) which shows that RNA synthesis is not required for the activation, the postactivation lag, or the spore swelling stages of germination, but RNA synthesis is required for the emergence stage of germination. We present evidence that uv irradiation immediately stops RNA synthesis, and that irradiated spores accumulate prematurely terminated transcripts. Poly(U) hybridization studies indicate that the amount of poly(A) decreases drastically during germination in uv-irradiated spores while the amount of poly(A) increases by about twofold in control germinating spores. When ultraviolet light is used to terminate RNA synthesis, the decay of poly(A) + RNA synthesized just prior to emergence can be measured. The decay appears complex, resolving into a rapidly decaying component with a half-life of about 15 minutes and a stable component with a much longer half-life. In vivo pulse labeling of germinating spores with [ 3H]leucine demonstrates a cessation of detectable protein synthesis 15 minutes after irradiation with ultraviolet light.