Since antiquity, yeast has been domesticated unwittingly or purposefully for the conversion of grape juice into wine by a process called fermentation. In the eighteenth century, the French chemist Antoine Lavoisier (1734-1794) showed that sugar was transformed into ethanol during fermentation. Theodor Schwann (1810-1882) and Charles Cagniard-Latour (1977-1859) microscopically examined fermentation mixtures and advanced the view that the "force" that drove fermentation is "a mass of globules that reproduces by budding" and is a consequence of the growth of yeast—an idea that was quickly rejected by the influential German chemist Justus von Liebig, who maintained that the murkiness in fermenting liquid was not due to a living organism. Based on controlled experiments, chemical analyses of broth and microscopic examinations of the sediment from successful and "diseased" fermentation vats, the versatile French scientist Louis Pasteur (1822-1895) concluded that yeast cells did not spontaneously arise from fermenting liquid but from preexisting cells. He identified yeast as the causative agent of alcoholic fermentation. The brewer's yeast Saccharomyces cerevisiae has today become a supermodel—"an organism that reveals and integrates many diverse biological findings applying to most living things" (Davis, 2003). A number of investigators, distributed in approximately 700 laboratories around the world, have joined hands to make this fungus (though rather atypical) a model of all model organisms. Its advantages for the study of physiology and eukaryotic gene

'I wish to thank Morgen Kilbourn of CuraGen Corporation for generating figures.

Figure 6.1 Life cycle of Saccharomyces cerevisiae. Both haploid and diploid cells multiply by budding. Diploid (2n) cell undergoes meiosis to form four haploid (n) cells, which are enclosed in a cell called ascus.

functions are: (1) its unicellular nature, making it a eukaryotic counterpart of E. coli; (2) its amenability for mass culture in a simple minimal medium with a doubling time of about an hour; (3) the stability of its haploid and diploid phases (Figure 6.1); (4) the ease of generating and detecting mutants, including conditional-lethal mutations for study of indispensable gene functions; (5) availability of a large diversity of mutant stocks; (6) its growth under both anaerobic or aerobic conditions, making it ideal for study of mitochon-drial biogenesis; (7) its small genome, the smallest of any eukaryote; (8) the highly efficient cloning of genes by simple complementation of mutant genes; (9) the homologous integration of transforming DNA allowing disruption, deletion or replacement of a gene; and (10) its two-hybrid method to generate a protein-interaction map for a system biology modeling of multicellular organisms (Giot et al., 2003). This chapter gives some remarkable examples of using yeast in the study of biological processes in the eukaryotes and the likely further developments.

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