Cell size modulates SBF and Whi5 chromatin association to regulate the Start of the budding yeast cell cycle

Cell growth and division are tightly coordinated with cell size. In budding yeast, increasing cell size promotes the G1/S transition, called Start, by activating the transcription factor SBF, which regulates cell cycle-dependent gene expression. Part of this regulation arises because the concentration of the SBF inhibitor Whi5 decreases as cells grow. However, cells lacking Whi5 can still maintain accurate size control when the SBF activator Cln3 is also removed, indicating that additional size control mechanisms exist. To understand how cell size is translated into the activity of SBF-regulated promoters, we quantified the chromatin-association dynamics of Whi5 and SBF in live cells using single-molecule fluorescence microscopy. We found that increasing cell size is associated with a reduced fraction of Whi5 molecules in a low-mobility state, consistent with chromatin association, and an increased fraction of SBF molecules in this state, together with a higher abundance of SBF relative to Whi5. Mobility trends under basal and Whi5 overexpression conditions indicate that Whi5 restricts SBF association with chromatin. The onset of CLN1/CLN2 expression, two SBF targets required for commitment to division, coincides with the cell size at which SBF and Whi5 mobility changes occur. Increased Whi5 mobility reflects changes in chromatin-association frequency rather than residence time, as Whi5 and SBF dwell times on chromatin remain approximately 10 s and are largely independent of cell size. Together, these results show how changes in SBF and Whi5 abundance and chromatin association transmit size information to the genome to regulate the Start transition in budding yeast.