Figure 5.

Fkh1 and Fkh2 directly mediate Ace2 binding to cell-separation genes (A) Ace2 binding is mediated by Fkh1 and Fkh2. Shown are normalized sum of signal on promoters for Swi5 and Ace2 in WT S. cerevisiae compared to fkh1Δ, fkh2Δ, and fkh1Δfkh2Δ. Color represents correlation of top 100 promoters to WT. (B) Cis- and trans-effects on Ace2 binding. Scatter plot of relative Ace2 binding differences between species (x-axis) and between hybrid alleles (y-axis). Color represents the differences in Ace2 binding between S. cerevisiae WT to fkh1Δfkh2Δ. Note that cell-separation genes show a strong trans-effect and are strongly affected by fkh1Δfkh2Δ. (C) S. paradoxus’ FKH2 is sufficient to restore Ace2 binding in S. cerevisiae. Scatter plots of normalized sum of signal on promoters of Ace2 in S. cerevisiae WT vs. S. cerevisiae fkh1Δ expressing S. paradoxus FKH2. Red dots represent cell-separation genes. (D) Scheme for proposed mechanism driving cell cycle variation: in S. cerevisiae, higher expression of Fkh1 and Fkh2 leads to higher expression of target genes and faster G2-M transition. Fkh1 (and Fkh2, to lower extent) binds Ace2 target genes and recruits Ace2, leading to higher expression of target genes, G1 daughter-cell delay and cell separation. In S. paradoxus, lower expression of Fkh1 and Fkh2 leads to reduced expression of target genes (including Fkh1 itself and Ace2) and longer G2. Downstream, Fkh1 exhibits reduced binding to Ace2 targets (trans-effect) leading to reduced binding of Ace2, lower expression of target genes, shorter G1-phase and reduced separation.