The transient division block in the D9ts mutant of Tetrahymena is inducible at every cell cycle stage.

The temperature-sensitive mutant D9 of Tetrahymena thermophila doubles its size at restrictive temperature. It does so by complete cessation of cell division for a limited time. After resumption of proliferation, division rate and specific growth rate are the same as at the permissive temperature, thereby maintaining the new cell size. In this study a detailed analysis of the process of controlling the new cell size is presented, by probing the temperature sensitivity of cell cycle phases. It will be shown that high temperature affects the size-controlling system immediately upon shift in temperature. Temperature pulses are effective at every stage in the cycle and are executed at the time of expected division. After return to the permissive temperature, cells gradually recover from the temperature pulse as seen by a decrease in division delay. Preparation for the next division is unaffected by the temperature pulse. It occurs at the same time as in untreated controls. The results allow us to describe some features of the division initiating system.