Budding yeast (Saccharomyces cerevisiae) 

For thousands of years, the singled-celled fungus Saccharomyces cerevisiae has played an integral part in human culture due to its widespread use in bread-, beer-, and wine-making. Chemical analysis of archaeological artifacts has shown that yeasts were used as early as 8,000 to 9,000 years ago in China to ferment beverages made of rice, honey, and fruit.  In the past century, S. cerevisiae has become a principle model organism used by scientists to understand biological principles.  Many of the fundamental biological processes, such as cell-division and its regulation, DNA replication and repair, meiosis, gene transcription, protein translation, and the secretory pathway, are astonishingly similar between humans and yeast.  In many basic processes, it is possible to substitute a yeast protein with its human counterpart and obtain a fully functional yeast cell.  The ease of growing and genetically manipulating S. cerevisiae in the lab has made it an ideal organism for understanding these processes.  For instance, the ability to easily generate mutants and induce sexual reproduction has been used to map observable traits to specific regions of the genome for decades.

S. cerevisiae became the first eukaryotic organism to have its genome fully sequenced in 1996.  Since then, the number of molecular tools created to study S. cerevisiae has exploded.  Scientists have developed massively parallel techniques, manipulating hundreds of independent samples at a time with robots, graduate students, and other “high-throughput” handling devices, to understand the function of every gene in the genome.  For instance, the sub-cellular localization and expression levels of every gene in the genome have been determined by tagging each gene with a fluorescent marker that can be visualized under the microscope.  Thousands of individual gene knockout strains have been generated to determine whether or not a cell without the gene has specific defects; these strains have in turn been used to generate double and triple gene knockouts, elucidating genetic interactions.  Each gene product has also been biochemically purified, allowing scientists to determine physical interactions between individual products.

Recently, the genomes of several yeast species very closely related to S. cerevisiae have been sequenced, allowing scientists to glean new huge quantities of new information through careful genomic comparisons.  This approach to understanding S. cerevisiae has helped to underscore the importance and roles of many non-gene-encoding regions of the genome, including those involved in regulating gene expression.  It have also revealed events in S. cerevisiae’s evolutionary past, such as a whole-genome duplication that occurred close to 100 million years ago.

It is fair to say that humans know more about the inner workings of S. cerevisiae than any other eukaryote.  As is typical in science, however, our knowledge of S. cerevisiae has raised even more questions than it has answered. For instance, scientists are now striving to understand precisely how the individual molecular components of S. cerevisiae interact to give rise to a functioning organism.  Integrating our knowledge of individual components into a cohesive picture will pave the way to projects such as engineering yeasts and other microorganisms with desirable traits, such as the ability to degrade toxic byproducts of human activity.