Whi5 phosphorylation embedded in the G 1 /S network dynamically controls critical cell size and cell fate

In budding yeast, overcoming of a critical size to enter S phase and the mitosis/mating switch - two central cell fate events - take place in the G 1 phase of the cell cycle. Here we present a mathematical model of the basic molecular mechanism controlling the G 1 /S transition, whose major regulatory feature is multisite phosphorylation of nuclear Whi5. Cln3-Cdk1, whose nuclear amount is proportional to cell size, and then Cln1,2-Cdk1, randomly phosphorylate both decoy and functional Whi5 sites. Full phosphorylation of functional sites releases Whi5 inhibitory activity, activating G 1 /S transcription. Simulation analysis shows that this mechanism ensures coherent release of Whi5 inhibitory action and accounts for many experimentally observed properties of mitotically growing or conjugating G 1 cells. Cell cycle progression and transcriptional analyses of a Whi5 phosphomimetic mutant verify the model prediction that coherent transcription of the G 1 /S regulon and ensuing G 1 /S transition requires full phosphorylation of Whi5 functional sites.