Actinomycin D in Tetrahymena. Non-specific inhibition of RNA synthesis and primary and secondary effects on protein synthesis.

The effects of actinomycin D on RNA and protein synthesis have been investigated in Tetrahvmena twiformis. The extent of inhibition of total RNA svnthesis and the time before inhibition occurs & both dependent on the concentration of the drug over the range OS-10 pg/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 y-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. I. 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. By measuring the relative rates of incorporation of [Wlmethionine into N-terminal and internal positions of nascent peptides, it was determined that actinomycin D inhibits initiation and elongation to similar extents.