Variation in the modality of a yeast signaling pathway is mediated by a single regulator

Palme, Julius; Wang, Jue; Springer, Michael

Bimodal gene expression by genetically identical cells is a pervasive feature of signaling networks. In the galactose-utilization (GAL) pathway of Saccharomyces cerevisiae, induction can be unimodal or bimodal depending on natural genetic variation and pre-induction conditions. Here, we find that this variation of modality is regulated by an interplay between two features of the pathway response, the fraction of cells that are in the induced subpopulation and their expression level. Combined, the variations in these features are sufficient to explain the observed effects of natural variation and pre-induction conditions on the modality of induction in both mechanistic and phenomenological models. Both natural variation and pre-induction conditions act by modulating the expression and function of the galactose sensor GAL3. The ability to alter modality may allow organisms to adapt their level of “bet hedging” to the conditions they experience, and thus help optimize fitness in complex, fluctuating natural environments.