What is Saccharomyces cerevisiae? |

The Organism

Saccharomyces cerevisiae (S. cerevisiae, or baker’s yeast) has been used for millennia as a leavening agent in bread products and for fermentation in beer and wine. Yeast is a simple, one-celled eukaryote with six thousand genes on sixteen chromosomes. It was the first eukaryote to have its entire DNA sequenced.

Yeast can produce offspring using two different methods, a sexual life cycle and an asexual life cycle. In the asexual life cycle, the yeast cell produces the next generation by a process called budding. All genetic components of the mother cell are duplicated, and a small “bud” begins to grow from the mother cell. The bud continues to grow until it is nearly the size of the mother cell. The DNA and other duplicated cellular components are then partitioned into the new bud. The cells undergo cytokinesis and are now separate entities able to grow and continue reproducing independently of one another.

To produce offspring that are not clones of the mother cell, yeast use a sexual life cycle. A yeast cell exists stably as either a diploid or a haploid organism, but only the haploid organism is able to mate and exchange genetic information. Haploid yeast can contain either the MATa or MATalpha gene. These genes produce soluble factors that distinguish them as one of two mating types. An “a” cell (MATa) and an “alpha” cell (MATalpha) mate by sequentially fusing their cell walls, their cytoplasms, and finally their nuclei. This diploid cell now contains two copies of each chromosome that can undergo recombination during meiosis. When all environmental signals are ideal, the diploid yeast will undergo meiosis, allowing exchange and recombination of genetic information brought to the diploid by both haploid cells. The result of meiosis is an ascus that contains four recombinant spores that will grow into haploid yeast cells when environmental conditions are ideal.

A Model Organism

Researchers choose yeast as a model organism to study specific areas of interest for many different reasons. Saccharomyces cerevisiae is nonpathogenic to humans, allowing manipulation in a laboratory with little or no containment required. At a temperature of 30 degrees Celsius (86 degrees Fahrenheit), the yeast population can double in ninety minutes, allowing many experiments to be completed in one day. Among the primary reasons for selection of yeast as a model system is that they offer the possibility of studying the genes and proteins that are required for basic growth functions and cell division. Yeast use many of the same genes and proteins to govern the same processes that animal and plant cells use for growth and division. Each single cell has to take in nutrients, grow, and pass along information to its progeny. In many ways, yeast can be considered a simplified version of a plant or animal cell, in that it lacks all the genes that provide the determinants that are expressed as differences between plants and animals. Another important reason for using yeast is that yeast is amenable to investigation using both genetic and biochemical approaches. This allows for correlation of findings from both approaches and a better understanding of a specific process or activity.

Yeast is also ideal for use as a model system due to at least four well-established techniques and procedures. First, genetics in yeast takes advantage of well-established auxotrophic markers. These markers are usually mutations in biosynthetic pathways that are used to synthesize required cellular components such as amino acids and nucleotides. By using these marker genes, researchers can follow genes and their associated chromosomes from one generation to the next.

Second, yeast is readily transformed by plasmids that function as artificial chromosomes. All that is needed is an auxotrophic marker to follow the plasmid through succeeding generations, a yeast origin of replication to allow replication of the plasmid DNA, and a region into which the gene of interest can be inserted in the plasmid DNA. This allows the researcher to move genes easily from yeast strain to yeast strain and quickly examine the effect of the gene in combination with many other genes.

Third, yeast is easily mutated by chemicals and can be grown in a small space, which allows the researcher quickly to identify mutations in genes that result in a specific phenotype. For example, to define all the genes in the adenine biosynthetic pathway, a researcher would mutate a yeast strain with one of many available mutagenic chemicals, resulting in changes within the DNA. The mutated yeast strains would then be checked to see if the strain was able to grow on media lacking adenine. All of the strains mutant for growth on adenine would be collected and could identify a number of genes involved in the adenine biosynthetic pathway. Further research could establish whether each of these mutations in the yeast identified one gene or many genes.

Fourth, yeast is the model system of choice when examining and identifying proteins that interact with one another in the cell. This technique is called the two-hybrid system.

Two-Hybrid System

The two-hybrid system takes advantage of scientists’ understanding of transcription at the GAL1 gene in yeast. The promoter region of GAL1 contains a binding site for the Gal4p transcription factor. When the cell is grown on the sugar galactose, Gal4p binds to the promoter of GAL1 and activates transcription of the GAL1 gene. Gal4p can be essentially divided into two functional regions: one region that binds to DNA and another region that activates transcription.

The two-hybrid system uses the GAL1-Gal4p transcription system to identify previously unknown proteins that interact with a protein of interest. The system consists of a reporter gene under the control of the GAL1 promoter and two plasmids that produce fusions with the Gal4p transcription factor. The first plasmid contains a gene of interest fused to a DNA-binding domain. This plasmid expresses a protein that is able to bind to the DNA-binding site in the GAL1 promoter of the reporter gene. This plasmid is unable to activate transcription of the reporter gene, since the Gal4p fragment does not contain the information to activate transcription. The second plasmid is provided from a collection of plasmids that consist of unknown or random genes fused to the transcription activation domain of Gal4p. This plasmid by itself is unable to bind to the DNA-binding site in the GAL1 promoter and thus is unable to activate transcription of the reporter gene. If both plasmids contain genes whose protein products physically interact in the cell, the complex is able to bind to the DNA-binding region of the
GAL1 promoter, and since the activation domain of Gal4p is also present in this complex, activation of the reporter gene will occur. The production of the reporter gene serves as a signal that both of the gene products interact in the cell. The yeast strain containing the active reporter gene is then selected and further examined to determine the unknown DNA that resides on the second plasmid by sequence analysis.

Cell Cycle Mutants

The isolation, characterization, and identification of conditional mutations in Saccharomyces cerevisiae has led to great advances in our understanding of the genes involved in the cell cycle. Because cell division is an essential process, null mutations in cell cycle genes are lethal. Thus, an approach to isolate temperature-sensitive mutants, cells that grow at room temperature but not at 36 degrees Celsius (96.8 degrees Fahrenheit), was taken. The observation that the formation of the yeast bud occurs at the beginning of the cell cycle and the bud continues to grow through cell cycle progression facilitated analysis of defects in cell division cycle (CDC). At the permissive temperature (room temperature), cell cultures are asynchronous; however, when the culture is switched to the restrictive temperature, cells with mutations in genes affecting cell cycle progression become synchronously arrested, which can be visualized microscopically. Temperature-sensitive mutants with defects in budding, DNA synthesis, nuclear division cytokinesis, and cell division were analyzed. This approach led to the isolation and characterization of the cell division cycle mutants, each of which undergoes growth arrest at specific points in the cell cycle and essentially represents all the key regulators of cell cycle progression. A fundamental observation that arose from this work was that cell cycle progression is controlled by cell-cycle checkpoints, whereby progression of the cell cycle is dependent upon the successful completion of upstream events. These checkpoints maintain cellular integrity by causing the cell cycle to arrest and initiate repair processes before errors are passed on to daughter cells. One of the genes thought to be most important that was identified using this approach is CDC28, which, like all of the CDCs, has homologues in all eukaryotes, including humans. Cdc28p, a cyclin-dependent kinase, initiates two pathways that lead to cell division. The identification of the CDC genes in yeast and mammalian homologues has led to important insights into defects in cell-cycle checkpoints that ultimately lead to cancer.

Research and Implications

The years of work on yeast as a model system have provided many insights into how genes and their protein products interact to coordinate the many cellular mechanisms that take place in all cells from simple yeast to complicated humans. It is impossible to exhaustively list the different areas of research currently being examined or completely list the new understandings that have come to light through the use of the S. cerevisiae model system. Every major area of cellular research has at one time or another used yeast to ask some of the more difficult questions that could not be asked in other systems. Work in yeast has aided identification of genes and elucidated the mechanism of many different areas of research, including cell-cycle regulation, mechanisms of signal transduction, the process of secretion, DNA replication, transcription of DNA, translation of messenger RNA into proteins, biosynthetic pathways of amino acids and other basic building blocks of cells, and regulation and progression of cells through mitosis and meiosis. Despite all these advances, there is still much to learn from yeast, and it will continue to provide information for years to come.

Outside of its value in genetic research, S. cerevisiae continues to be an important resource in other ways. As implied by its common name of baker's yeast, it is still used as the chief leavening agent for the production of bread products. A different strain of the yeast is also an important factor in the fermentation process used in winemaking and beer brewing. As the craft beer market has expanded, greater attention has been paid to the specific processes used in producing different types and flavors of beer; S. cerevisiae belongs to the group of yeasts often referred to as top fermenting. In 2013 the government of Oregon named S. cerevisiae as the state's official microbe in recognition of its role in the history of brewing and the state's close association with the craft brew movement.

Key terms



ascus

:

the cellular structure that results from meiosis in yeast, containing four recombinant spores that are fully capable of growing into haploid yeast cells




budding

:

the asexual method of duplication used by yeast to create a clone of the original cell




diploid cell

:

a cell that contains two copies of each chromosome




haploid cell

:

a cell that contains one copy of each chromosome




mating type

:

one of two types of yeast cell, depending on a soluble factor that each cell secretes





model organism


:

an organism well suited for genetic research because it has a well-known genetic history, a short life cycle, and genetic variation between individuals in the population





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