Heat shock dependent fluctuations of RNase activity during the cell cycle of synchronized Tetrahymena.

The total cellular acid RNase activity per milliliter of culture increases sharply following each heat shock in the cell cycle of Tetrahymena pyriformis ST synchronized with heat shocks spaced one generation time apart. Thus, the RNase activity per 10(5) cells is 24.5 units immediately after the end of the sixth heat shock, increases to 39.0 units during the following 55 minutes and decreases to 24.2 units at the start of the seventh heat shock. No change in the RNase activity occurs during the heat shock period. In logarithmically growing cells the RNase activity per 10(5) cells is 15.4 units. The heart shock stimulates the increase in the RNase activity, since no rapid increase occurs during the free running division cycle but a rapid increase occurs after an additional heat shock given at different times during the cell cycle. Inhibition of the increase in RNase activity by cycloheximide suggests that concurrent protein synthesis is required for the stimulation of the RNase activity by the heat shock treatment.