RNA synthesis and cell division in cold-synchronized cells of Tetrahymena pyriformis

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 28°C. This system gives a maximum division index of 70–80% 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 is sensitivity to the inhibitor occurs. Coinciding with this period of high sensitivity is the occurrence of a peak of C¹⁴ 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.