INHIBITION OF SYNCHRONIZED CELL DIVISION IN TETRAHYMENA BY ACTINOMYCIN D.

Abstract

The effects of actinomycin D on RNA and protein synthesis have been investigated in Tetrahymena pyriformis. The extent of inhibition of total RNA synthesis and the time before inhibition occurs are both dependent on the concentration of the drug over the range 0.5–10 μg/ml. In contrast to reports on mammalian systems, actinomycin D at low doses is not a selective inhibitor of rRNA synthesis in Tetrahymena; at all doses tested, poly(A+) RNA and poly(A−) RNA syntheses were reduced to similar extents.Finally, the action of actinomycin D was studied in conjunction with γ-radiation which alone causes a transient disaggregation of the polysomes. Actinomycin D partially inhibits the reassociation of polysomes in irradiated cells, but after recovery, the polysome content of the irradiated and drug-treated cells is greater than that of cells treated only with actinomycin D. The results support both a direct action of actinomycin D on translation and secondary effects resulting from inhibition of transcription.The effect of actinomycin D on protein synthesis was determined by two independent methods. 1.1. Investigation of drug-induced alterations in polysome content showed that as the dose increases (a) the lag period before observable polysome dissociation decreases; (b) the rate of polysome dissociation increases; and (c) the final extent of polysome loss increases.2.2. By measuring the relative rates of incorporation of [35S]methionine into N-terminal and internal positions of nascent peptides, it was determined that actinomycin D inhibits initiation and elongation to similar extents.