Cell Cycle Inhibitor Whi5 Records Environmental Information to Coordinate Growth and Division in Yeast.

Proliferating cells need to evaluate the environment to determine the optimal timing for cell cycle entry. However, how this is achieved is not well understood. Here, we show that, in budding yeast, the G1 inhibitor Whi5 is a key environmental indicator and plays a crucial role in coordinating cell growth and division. We found that, under a variety of nutrient and stress conditions, Whi5 amount in G1 is proportional to the cell's doubling time in the environment, which in turn influences the timing for the next cell cycle entry. In addition, the coordination between division and environment is further fine-tuned in G1 by environmentally dependent growth rate, G1 cyclin-Cdk1 contribution, and Whi5 threshold at the start. Our results show that the cell stores the past environmental information in Whi5, which works together with other mechanisms sensing the current environmental condition to achieve an adaptive cellular decision making process.