Continuous protein synthesis is required to maintain the probability of entry into S phase.

"Normal fibroblast" lines such as 3T3 cells arrest in the G0/G1 compartment of the cell cycle when starved of serum. Following readdition of serum and after a lag of 14 hr, the cells enter S phase with first-order kinetics. Cell cycle progress after stimulation is thus consistent with the existence of a single, rate-limiting random event (or transition) in G1 as proposed by Smith and Martin (1973). The addition of low concentrations of cycloheximide (33-100 ng/ml) at any time after the end of the lag phase brings about a rapid reduction (within 1-2 hr) of the rate constant for entry into S phase by an amount that is proportional to the inhibition of leucine incorporation. This suggests that the transition probability depends upon the continuous synthesis of a protein with a short half-life, or on some other unstable substance whose concentration is geared to the rate of translation. More importantly at present, the results indicate that the rate-limiting transition occurs within 2 hr of the start of DNA synthesis. When the same low concentrations of cycloheximide are added at the time of serum stimulation, they also lead to a marked elongation of the lag phase which again is related to the inhibitor concentration. This result is surprising since the lag is independent of serum concentration which itself influences the rate of protein synthesis.