Start, the G1/S transition of the cell cycle in budding yeast is controlled through the transcriptional activation of ≈200 genes in the G1/S regulon by heterodimeric transcription factor complexes SBF and MBF. SBF and MBF consist of unique DNA binding subunits, Swi4 and Mbp1 respectively and a common regulatory subunit Swi6 and target the SCB and MCB sites, respectively, in the promoters of the G1/S regulon. In early G1, the transcription factors Swi4, Mbp1, and Swi6 as well as the transcription repressor Whi5 are limiting with respect to their target promoters, and their copy-number accumulate throughout G1 in a growth-dependent manner to titrate their target sites and trigger Start. Interestingly, Swi4 copy-number has a positive differential accumulation with respect to growth as well as the accumulation of other G1/S transcription factors. Here, we report using quantitative fluorescence microscopy that a copy-number threshold of Swi4 triggers Start in budding yeast. Furthermore, we demonstrate that a threshold model in which Swi4 titrates SCB sites in G1/S promoters predicts the cell size of budding yeast in different nutrients, ploidy, and G1/S regulatory mutations. Even with sufficient G1/S transcription factors, cyclin-dependent, massive phosphorylation of the Whi5 repressor is required to trigger Start. We characterize the transient and low-level cyclin production that controls this event.
Ghimire et al. (Sun,) studied this question.