Figure 2. Prion conversion as a molecular switch to respond to environmental stress. [mod⁻] yeast contains the Mod5 protein as a soluble and functional monomer while [MOD⁺] yeast contains the Mod5 protein in its aggregated non/less functional form (top, schematic illustration). Mod5-GFP exhibits nuclear and cytoplasmic diffusible localization in [mod⁻] yeast cells, while Mod5-GFP forms cytoplasmic foci in [MOD⁺] yeast cells (middle images). [mod⁻] yeast show the sensitivity to antifungal drugs such as fluconazole and ketoconazole whereas [MOD⁺] yeast are resistant against them (bottom images). Prion conversion between [mod^-] and [MOD⁺] occurs at very low frequency. However, in the presence of environmental stress such as antifungal drugs, [MOD⁺] yeast cells become dominant because of the newly acquired drug resistant phenotype. Once cells are released from the environmental stress, [mod⁻] yeast cells rapidly become dominant due to their growth advantage under normal conditions. Thus, the prion conversion of Mod5 acts as a “molecular switch” for enhanced survival under different conditions.