The effect of actinomycin D on both acetate-starved and normal logarithmically growing Euglena gracilis has been examined. Upon replenishment of the acetate, the starved cells resumed logarithmic growth after a lag period during which they resynthesized the RNA and protein utilized during starvation. If actinomycin D was added with the acetate, the length of the lag period was not changed, but the cells resumed growth slowly at a rate depending both on the concentration of actinomycin D and on thee duration of the preceding period of starvation. During the lag period actinomycin-treated cells resynthesized the same amount of protein and almost the same amount of RNA as the controls. After growth resumed, protein accumulated but RNA did not accumulate and DNA synthesis was not prevented. The addition of actinomycin D to logarithmically growing cells results in an inhibited rate of growth and of RNA synthesis for these Euglena. The results, which are consistent with the view that actinomycin D inhibits the synthesis of DNA-dependent RNA, suggest that the protein synthetic system loses its programming slowly during starvation.
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