Yeast

Yeast

Yeast are single-celled eukaryotic organisms related to fungi. The baker's yeast Saccahromyces cerevisiae and the distantly related Schizosaccharomyces pombe are favored model organisms for genetic research. The interest in yeast research stems from the fact that, as eukaryotic organisms, the sub-cellular organization of yeast is similar to that of cells of more complex organisms. Thus, understanding how a particular gene functions in yeast frequently correlates to how similar genes function in mammals, including humans.

Yeast Genetics

Yeast have many advantages as a genetic research tool. First, yeast are nonpathogenic (they do not cause diseases) and are therefore easy and safe to grow. Yeast can divide by simple fission (mitosis) or by budding and, like bacteria, they can be rapidly grown on solid agar plates or in liquid media . After just a few days in culture, a single yeast cell can produce millions of identical copies of itself, giving scientist a large supply of a genetically pure research tool.

Second, yeast grow as either haploids (having only one set of chromosomes) or diploids (with two chromosome sets). Thus, genetically recessive mutations can be readily identified by phenotypic (visually observable) changes in the haploid strain. In addition, complementation can be performed by simply mating two haploid strains, where one does not contain the mutation. The resulting diploid strain contains both the functional and nonfunctional version of a gene responsible for a phenotype. The addition of the functional gene complements for the defect caused by the nonfunctional gene in the haploid strain. Diploid strains can be induced to undergo meiosis, a process in which the cell divides and passes one-half of its chromosomes to each of the resulting cells. After two such divisions, reproductive structures called asci are produced that contain four haploid offspring, called ascospores. The asci can be dissected and each of the ascospores isolated. In this way, scientists can easily mate different yeast strains and obtain new haploid genotypes through sexual reproduction and meiosis.

Third, the genome of yeast is small, about 3.5 times larger than that of bacteria and 200 times smaller than that of mammals. The yeast genome is arranged in 16 linear chromosomes that range from 200 to 2,200 kilobases in length. Unlike mammals, the yeast genome is very compact, with only 12 million base pairs, very few introns , and very little spacer DNA between functional genes. As a result, in 1996 baker's yeast was the first eukaryotic organism to have its entire genome sequenced.

Genetic Transformation

Finally, one of the most useful properties of yeast for genetic studies is the ease with which DNA can be introduced into them, in a process called transformation. The introduced DNA can be maintained on self-replicating, circular strands of DNA called plasmids, or it can integrate into the yeast genome. Most importantly, integration usually occurs by a process called homologous recombination, whereby the introduced DNA replaces chromosomal DNA that contains the same sequence. This process permits scientists to readily mutate any yeast gene and replace the native gene in the cells with the mutated version. Since yeast can be grown as haploids, the phenotypic changes caused by the introduced gene can be readily identified. In addition, the function of a cloned piece of DNA (e.g., a gene) can be identified by transforming yeast in which the DNA is carried on a circular plasmid. The introduced gene may either functionally replace a defective gene or cause a phenotypic defect in the cells indicating a function for that gene.

The ability to complement yeast defects with cloned pieces of DNA has been extended to mammalian genes. Recognizing that some genes have similar sequences and functions in both mammals and yeast, scientists sometimes use yeast as a tool to identify the functions of mammalian genes. Not many mammalian genes can directly substitute for a yeast gene, however. More frequently, scientists study the yeast gene itself to understand how its protein functions in the cell. The knowledge gained can often lead to an understanding of how similar genes might function in mammals. Now that the yeast genome has been completely sequenced and the results have been deposited in a public databank for all to use, rapid progress is being made in identifying all yeast genes and their functions.

An important method for studying mammalian genes in yeast is called the two-hybrid system. This system is used to determine if two proteins functionally interact with each other. Both genes are cloned into yeast plasmids and transformed into the cells. A special detection system is used that is active only when both cloned proteins physically contact each other in the cell. When that happens, scientist can identify which proteins need to interact with each other in order to function.

Yeast are also being used in the laboratory and commercial production of important nonyeast proteins. Foreign genes are transformed into yeast and, after transcription and translation, the foreign proteins can be isolated. Because of the ease of growing large quantities of cells, yeast can produce a large amount of the protein. While similar protein production can be performed by bacteria, eukaryotic proteins often do not function when made in bacteria. This is because most eukaryotic proteins are normally altered after translation by the addition of short sugar chains, and these modifications are often required for proper function, but bacteria do not carry out these necessary post-translational modifications. Yeast, however, does permit these modifications, and is thus more likely to produce a functional protein.

see also Cell, Eukaryotic; Cell Cycle; Genome; Human Genome Project; Model Organisms; Plasmid; Post-translational Control; Transformation; Transgenic Animals.

Suzanne Bradshaw

Bibliography

Sherman, Fred. "Getting Started with Yeast." In Methods in Enzymology, vol. 194, Christine Guthrie and Gerald R. Fink, eds. New York: Academic Press, 1991.

Watson, James D., Michael Gilman, Jan Witowski, and Mark Zoller. Recombinant DNA. New York: Scientific American Books, 1992.