Differential scaling of gene expression with cell size may explain size control in budding yeast.

Yeast cells must grow to a critical size before committing to division. It is unknown how size is measured. We find that as cells grow, mRNAs for some cell cycle activators scale faster than size, increasing in concentration, while mRNAs for some inhibitors scale slower than size, decreasing in concentration. Size-scaled gene expression could cause an increasing ratio of activators to inhibitors with size, triggering cell cycle entry. Consistent with this, expression of the CLN2 activator from the promoter of the WHI5 inhibitor, or vice versa, interfered with cell size homeostasis, yielding a broader distribution of cell sizes. We suggest that size homeostasis comes from differential scaling of gene expression with size. Such regulation of gene expression as a function of cell size could affect many cellular processes. Overall design: Arrest yeast cells in cell cycle by inhibiting CDK activity, allow them to grow large, and analyze transcript abundance by RNA-Seq as a function of size.