The effects of puromycin on synchronized Tetrahymena pyriformis were investigated at two different concentrations, 43 µg per ml and 430 µg per ml. The rate of incorporation of histidine-14C into hot TCA-insoluble material was reduced by 30% at the low concentration and by 80–90% at the high concentration. The rate of oxygen uptake was lowered by only 10–20% at both concentrations. Cell division was prevented at both concentrations, if the drug was added prior to a "transition point" at about 45 min after the end of the synchronizing treatment. Development of "anarchic field" oral primordia was arrested, while primordia in early stages of membranelle differentiation were resorbed. Resorption began shortly after addition of the drug, and proceeded most rapidly at the lower concentration. If the drug was added after the "transition point," cell division and oral primordium formation were completed with only slight delay at the low concentration, and with considerable delay (in some cases complete arrest) at the high concentration. The results thus indicate that protein synthesis is involved in the later as well as the earlier stages of development; what specially characterizes the earlier stages, prior to the "transition point," is a dramatic response to partial inhibition of protein synthesis. It is suggested that this response involves the activation or release of a latent intracellular degradative system which is specific for developing structures.

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