This workshop was chaired by Dr. Anwar Nasim who started the session by delivering a few introductory remarks. He commented on the evolutionary divergence of these two yeasts, as highlighted by Russel and Nurse (Cell, 45: 78 1 782, 1986), and indicated how, in the past, budding yeast had always been regarded as the model eukaryote of choice. However, recently the fission yeast has seen more use as a model system (Fink and Botstein, Science (Washington, D.C.), 240: 1439 1442, 1988), especially since the identification of a human homolog of the Saccharomyces pombe cdc2 gene (Lee and Nurse, Nature (London), 327: 3 1 35, 1987; Draetta et al. Cell, 50: 3 19 325, 1987). Indeed interest in fission yeast has developed to the extent that in 1986 and again in 1988 a S. pombe satellite meeting was held in conjunction with the International Meeting on Yeast Genetics and Molecular Biology. Thus it seemed an opportune time to hold a workshop on comparative genetics of these two yeasts. Dr. Byron Johnson (National Research Council, Ottawa) initiated the talks with a comparison of the characteristics of asexual flocculation in both yeasts. Asexual flocs of Saccharomyces were dispersed in mannose syrup and by EDTA (10 mM), but not by galactose syrup. Asexual flocs of Schizosaccharomyces were dispersed in galactose syrup and by EDTA, but not by mannose syrup. The interpretation was that a mannose-specific lectin (Saccharomyces) or a galactosespecific lectin (Schizosaccharomyces), both stabilized by calcium ion, were the effectors of asexual flocculation. Sexual flocs of Schizosaccharomyces were not dispersed by those reagents, consistent with published reports of protein -protein interactions mediating sexual flocculation. Scanning electron micrographs show a heavy, scarobscuring layer of pronase-sensitive material on flocculent fission yeasts, but not on flocculent budding yeasts. This suggests the potential of S. pombe as a protein secretor, an idea .that is now just beginning to be investigated in .the biotechnology field. Dr. Susan Nadin-Davis (National Research Council, Ottawa) summarized her work investigating the molecular basis of SP-ras function and indicated how this contrasted with the established mode of action of SC-RAS genes. Although ras is clearly involved in nutritional sensing in both organism, SC-RAS genes effect a response via changes in CAMP levels and thus indirectly through alterations of protein phosphorylation (Toda et al., Cell, 40: 27-36, 1985). SP-ras effects its action through a CAMP-independent protein kinase encoded by the byr gene. The role of SP-ras in sexual differentiation and induction of specific transcripts required for this process was discussed in detail. Dr. Eva Streiblovii (Institute of Microbiology, Czechoslovak Academy of Sciences) described the unusual morphology of rasfission yeast cells using strains provided by Drs. Fukui and Yamamoto. Electron micrographs pictures clearly indicated the disruption of normal actin arrangements and an unusual switching in the cell division plane of rascells. These observations are intriguing since the ras-like gene SC-YPTI affects microtubule organization (Schmitt et al. , Cell, 47: 401 -4.12, 1986), but it clearly is not a functional homolog of SP-ras. In the final discussions it was concluded that the similarities and differences of these two yeasts were clearly indicative of a need to use both systems to assess the degree of conservation and divergence of cellular and molecular processes in eukaryotic cells.
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