G1/S Transcription Factor Copy Number Is a Growth-Dependent Determinant of Cell Cycle Commitment in Yeast

To understand how commitment to cell division in late G1 phase (Start) is controlled by growth and nutrients in budding yeast, we determined the absolute concentrations of the G1/S transcription factors SBF (composed of Swi4 and Swi6) and MBF (composed of Mbp1 and Swi6), the transcriptional repressor Whi5, and the G1 cyclins, Cln1 and Cln2, in single live yeast cells using scanning number and brightness (sN&B) microscopy. In rich medium, Whi5, Mbp1, and Swi6 concentrations were independent of cell size, whereas Swi4 concentration doubled in G1 phase, leading to a size-dependent decrease in the Whi5/Swi4 ratio. In small cells, SBF and MBF copy numbers were insufficient to saturate target G1/S promoters, but this restriction diminished as cells grew in size. In poor medium, SBF and MBF subunits, as well as Cln1, were elevated, consistent with a smaller cell size at Start. A mathematical model constrained by sN&B data suggested that size- and nutrient-dependent occupancy of G1/S promoters by SBF/MBF helps set the cell size threshold for Start activation.