CHARACTERIZATION OF THE OSMOTIC-STRESS RESPONSE IN SACCHAROMYCES-CEREVISIAE - OSMOTIC-STRESS AND GLUCOSE REPRESSION REGULATE GLYCEROL-3-PHOSPHATE DEHYDROGENASE INDEPENDENTLY

Micro-organisms have developed systems to adapt to sudden changes in t he environment. Here we describe the response of the yeast Saccharomyc es cerevisiae to osmotic stress. A drop in the water activity (a(w)) o f the medium following the addition of NaCl led to an immediate shrink age of the cells. During the 2 h following the osmotic shock the cells partially restored their cell volume. This process depended on active protein synthesis. During the recovery period the cells accumulated g lycerol intracellularly as a compatible solute and very little glycero l was leaking out of the cell. We have investigated in more detail the enzymes of glycerol metabolism and found that only the cytoplasmic gl ycerol-3-phosphate dehydrogenase was strongly induced. The level of in duction was dependent on the yeast strain used and the degree of osmot ic stress. The synthesis of cytoplasmic glycerol-3-phosphate dehydroge nase is also regulated by glucose repression. Using mutants defective in glucose repression (hxk2 triangle), or derepression (snf1 triangle) , and with invertase as a marker enzyme, we show that glucose repressi on and the osmotic-stress response system regulate glycerol-3-phosphat e dehydrogenase synthesis independently. We infer that specific contro l mechanisms sense the osmotic situation of the cell and induce respon ses such as the production and retention of glycerol.