Cells of Tetrahymena pyriformis have been cold-synchronized using a repetitive cycle of six, two-hour cold shocks (9.5"C) alternating with decreasing periods (60-30 minutes.) at 28ยฐC. This system gives a maximum division index of 7040% occurring at 90 minutes from the end of the last synchronizing cold-treatment (EC). Examination of the division sensitivity of these cells to actinomycin D applied continuously at ten-minute intervals from EC reveals that division is essentially blocked until approximately 40 minutes past EC, after which a rapid decrease in sensitivity to the inhibitor occurs. Coinciding with this, period of high sensitivity is the occurrence of a peak of C1* uridine incorporation at 40 minutes past EC. Inhibition of this peak is correlated with an inhibition of division, whereas strong inhibition of RNA synthesis beyond 60 minutes past EC has little effect on division activity. The similarity of these findings with those of the heat-synchronized system is discussed with the suggestion that both heat-and cold-synchronizing treatments result in the synchronous resynthesis of a division-associated fraction of RNA.
Recent findings with actinomycin D have indicated that division in both heatsynchronized (Moner, '64; Nachtwey and Dickinson, '64; Lazarus, Levy and Scherbaum, '64) and cold-synchronized (Whitson and Padilla, '64) cells of Tetruhymenu pyriformis exhibits a strong sensitivity to this inhibitor. It has been concluded from these studies that RNA synthesis is critical to synchronous division in this organism. However, the period of synthesis specifically linked to the first synchronous division appears to be restricted, in time, relative to the onset of this division. In the heat-synchronized system of Tetruhymenu pyrifomzis (GL), Moner ('64), and Lazarus, Levy and Scherbaum ('64) have found that the first 30 minutes, following the end of the last heat treatment, represents a period of high sensitivity to actinomycin D for the first synchronous divi- sion, but that thereafter this sensitivity rapidly disappears. Nachtwey and Dickinson ('64), using higher concentrations of the inhibitor, have suggested that this period of sensitivity extends to as high as 49 minutes from the last heat treatment, although the question has been raised as to the possible direct effects of high actinomycin concentrations on RNA stability (Lazarus, Levy and Scherbaum, '64). Moner, ('65a), using radioautography, has