ISOLATION AND CHARACTERIZATION OF HIGH-OSMOLARITY-SENSITIVE MUTANTS OF FISSION YEAST

For the fission yeast Schizosaccharomyces pombe, adaptation to high-os molarity medium is mediated by a mitogen-activated protein (MAP) kinas e cascade, involving the Wis1 MAP kinase kinase and the Sty1 MAP kinas e. The MAP kinase pathway transduces an osmotic signal and accordingly regulates the expression of the downstream target gene (gpd1(+)) that encodes NADH-dependent glycerol-3-phosphate dehydrogenase, in order t o adaptively accumulate glycerol inside the cells as an osmoprotectant . We previously characterized a set of high-osmolarity-sensitive S. po mbe mutants, including, wis1, sty1, and gpd1. In this study, we attemp ted to further isolate novel osmolarity-sensitive mutants. For some of the mutants isolated, profiles of glycerol production in response to the osmolarity of the growth medium were indistinguishable from that o f the wild-type cells, suggesting that they are novel types. They were classified into three distinct types genetically and, thus, were desi gnated hos1, hos2, and hos3 (high osmolarity sensitive) mutants. One o f them, the hos1 mutant, was characterized in detail. The hos1 mutant was demonstrated to have a mutational lesion in the known ryh1(+) gene , which encodes a small GTP-binding protein. Disruption of the ryh1(+) gene results not only in osmosensitivity but also in temperature sens itivity for growth. It was also found that the Delta ryh1 mutant is se verely sterile. These results are discussed with special reference to the osmoadaptation of S. pombe.