The periodic synthesis of thymidine kinase has been studied in the naturally synchronous acellular slime mold, Physarum polycephalum. Density-shift experiments indicate that the peak of enzyme activity which appears 30-45 min past metaphase represents actual synthesis of the enzyme protein. Synthesis can be prevented by prior exposure of the plasmodium to actinomycin D, cycloheximide, or γ-radiation. For each of these three agents, time periods exist in the mitotic cycle during which mitosis and the synthesis of thymidine kinase become insensitive to that agent. These time periods are not coincident: for enzyme synthesis the transition period for actinomycin D occurs up to 15 min earlier in the mitotic cycle than the period for 9500 R of γ-radiation which, in turn, occurs 5-15 min earlier than that for cycloheximide. It is therefore proposed that γ-radiation interferes with a metabolic event which takes place after actinomycin D-sensitive transcription but prior to the completion of cycloheximide-sensitive translation.

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