Osmoregulation of fission yeast: cloning of two distinct genes encoding glycerol-3-phosphate dehydrogenase, one of which is responsible for osmotolerance for growth.

Many types of microorganisms, including both prokaryotes and eukaryotes, have developed mechanisms to adapt to severe osmotic stress. In this study, we isolated multicopy suppressor genes for a Schizosaccharomyces pombe mutant, which exhibited the clear phenotype of being osmosensitive for growth (Osms) on agar plates containing high concentrations of either non-ionic or ionic osmotic solutes. Two genes were thus identified, and each was suggested to encode an NADH-dependent glycerol-3-phosphate dehydrogenase (GPD), which is required for glycerol synthesis. The nucleotide sequences, determined for these genes (named gpd1+ and gpd2+, respectively), revealed that S. pombe has two distinct GPD isozymes. They are only 60% identical to each other in their amino acid sequences. One such isozyme, GPD1, was shown to be directly involved in osmoregulation, based on the following observations. (i) Expression of gpd1+ was regulated at the mRNA level in response to osmotic upshift. (ii) It was demonstrated that wild-type cells markedly accumulated internal glycerol under high-osmolarity growth conditions. (iii) delta gpd1 mutants, however, failed to do so even in a high-osmolarity medium, and thus exhibited an Osms phenotype. On the other hand, the gpd2+ gene was constitutively expressed at a particular low level, regardless of the osmolarity of the medium.