Brooks R F
Cell. 1977 Sep;12(1):311-7. doi: 10.1016/0092-8674(77)90209-4.
"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.
像3T3细胞这样的“正常成纤维细胞”系在血清饥饿时会停滞在细胞周期的G0/G1期。重新添加血清并经过14小时的延迟后，细胞以一级动力学进入S期。因此，刺激后的细胞周期进程与Smith和Martin（1973年）提出的G1期存在单一的、限速随机事件（或转变）相一致。在延迟期结束后的任何时间添加低浓度的环己酰亚胺（33 - 100 ng/ml），会使进入S期的速率常数在1 - 2小时内迅速降低，降低幅度与亮氨酸掺入抑制程度成正比。这表明转变概率取决于半衰期短的蛋白质的持续合成，或者取决于浓度与翻译速率相关的其他不稳定物质。目前更重要的是，结果表明限速转变发生在DNA合成开始后的2小时内。当在血清刺激时添加相同低浓度的环己酰亚胺，也会导致延迟期显著延长，这同样与抑制剂浓度有关。这个结果令人惊讶，因为延迟期与血清浓度无关，而血清浓度本身会影响蛋白质合成速率。
