GPD1, WHICH ENCODES GLYCEROL-3-PHOSPHATE DEHYDROGENASE, IS ESSENTIAL FOR GROWTH UNDER OSMOTIC-STRESS IN SACCHAROMYCES-CEREVISIAE, AND ITS EXPRESSION IS REGULATED BY THE HIGH-OSMOLARITY GLYCEROL RESPONSE PATHWAY

The yeast Saccharomyces cerevisiae responds to osmotic stress, i.e., a n increase in osmolarity of the growth medium, by enhanced production and intracellular accumulation of glycerol as a compatible solute. We have cloned a gene encoding the key enzyme of glycerol synthesis, the NADH-dependent cytosolic glycerol-3-phosphate dehydrogenase, and we na med it GPD1. gpd1 Delta mutants produced very little glycerol, and the y were sensitive to osmotic stress. Thus, glycerol production is indee d essential for the growth of yeast cells during reduced water availab ility. hog1 Delta mutants lacking a protein kinase involved in osmostr ess-induced signal transduction (the high-osmolarity glycerol response [HOG] pathway) failed to increase glycerol-3-phosphate dehydrogenase activity and mRNA levels when osmotic stress was imposed. Thus, expres sion of GPD1 is regulated through the HOG pathway. However, there may be Hog1-independent mechanisms mediating osmostress-induced glycerol a ccumulation, since a hog1 Delta strain could still enhance its glycero l content, although less than the wild type. hog1 Delta mutants are mo re sensitive to osmotic stress than isogenic gpd1 Delta strains, and g pd1 Delta hog1 Delta double mutants are even more sensitive than eithe r single mutant. Thus, the HOG pathway most probably has additional ta rgets in the mechanism of adaptation to hypertonic medium.