THE 2 ISOENZYMES FOR YEAST NAD(-DEPENDENT GLYCEROL 3-PHOSPHATE DEHYDROGENASE ENCODED BY GPD1 AND GPD2 HAVE DISTINCT ROLES IN OSMOADAPTATIONAND REDOX REGULATION())

The two homologous genes GPD1 and GPD2 encode the isoenzymes of NAD-de pendent glycerol 3-phosphate dehydrogenase in the yeast Saccharomyces cerevisiae, Previous studies showed that GPD1 plays a role in osmoadap tation since its expression is induced by osmotic stress and gpd1 Delt a mutants are osmosensitive, Here we report that GPD2 has an entirely different physiological role, Expression of GPD2 is not affected by ch anges in external osmolarity, but is stimulated by anoxic conditions, Mutants lacking GPD2 show poor growth under anaerobic conditions, Muta nts deleted for both GPD1 and GPD2 do not produce detectable glycerol, are highly osmosensitive and fail to grow under anoxic conditions, Th is growth inhibition, which is accompanied by a strong intracellular a ccumulation of NADH, is relieved by external addition of acetaldehyde, an effective oxidizer of NADH, Thus, glycerol formation is strictly r equired as a redox sink for excess cytosolic NADH during anaerobic met abolism, The anaerobic induction of GPD2 is independent of the HOG pat hway which controls the osmotic induction of GPD1, Expression of GPD2 is also unaffected by ROX1 and ROX3, encoding putative regulators of h ypoxic and stress-controlled gene expression. In addition, GPD2 is ind uced under aerobic conditions by the addition of bisulfite which cause s NADH accumulation by inhibiting the final, reductive step in ethanol fermentation and this induction is reversed by addition of acetaldehy de, We conclude that expression of GPD2 is controlled by a novel, oxyg en-independent, signalling pathway which is required to regulate metab olism under anoxic conditions.