Saccharomyces cerevisiae
A Microbial Biorealm page on the genus Saccharomyces cerevisiae
Classification
Higher order taxa
Domain: Eukarya
Kingdom: Fungi
Subkingdom: Dikarya
Phylum: Ascomycota
Subphylum: Saccharomycotina
Class: Saccharomycetes
Order: Saccharomycetales
Family: Saccharomycetaceae
Genus: Saccharomyces
Species: Cerevisiae
Species
| Taxonomy of Saccharomyces cerevisiae |
Major Strains of Saccharomyces cerevisiae
1. Natural
Saccharomyces boulardii: Formerly used as a probiotic used to treat diarrhea caused by bacteria. Clinical tests have demonstrated that this and a species of S. cerevisiae were genetically identical. (1)
Saccharomyces uvaruium: Used in fermenting lager-type beer. Due to a recent reclassification, this strain is now considered to be another wild-type strain (2)
2. Laboratory Strains While S. cerevisiae contains many different strains used in research, below are some of the strains most commonly used in laboratories. The choice of which strain to use depends on what part of the organism is being studied.
S288c: This strain was isolated in the 1950's by Robert K. Mortimer through genetic crosses. It was used as a parental strain when isolating mutants (1). S288c was the strain used when the genome of S. cerevisiae was fully sequenced in 1996. However, its low rate of sporulation and the lack of protein growth in the absence of nitrogen prompted scientists to pick alternative strains for their research.(2)
A634A: Used in cell cycle studies. It is also closely related to S288c due to a cross with S288c and another unknown strain. (10)
BY4716: Since this is nearly identical to S288c, it is often used as a reference or control stain. (7)
CEN.PK: In Europe, this is used as a secondary reference strain alongside S288c when studying the yeast genome. Additionally, it can grow well on several different carbon sources as well as under anaerobic conditions. It is used when studying rates of growth and product formation.(3)
∑1278b: What distinguishes this strain is that it contains genes unique for nitrogen metabolism. (8). It is best studied when nitrogen is limited; cells become elongated and undergo a unique budding pattern where cells remain physically attached to each other. This is known as pseudohyphal growth. (8)
SK1: Because this strain produces lots of spores, it is used in meiotic studies. (5)
W303: Closely related to S288c due to a cross between S288c and an unknown strain, (3), it is used in genetic and biochemical analysis. (4).
Description and significance
Saccharomyces cerevisiae is an eukaryotic microbe. More specifically, it is a globular-shaped, yellow-green yeast belonging to the Fungi kingdom, which includes multicellular organisms such as mushrooms and molds. Natural strains of the yeast have been found on the surfaces of plants, the gastrointestinal tracts and body surfaces of insects and warm-blooded animals, soils from all regions of the world and even in aquatic environments. (3). Most often it is found in areas where fermentation can occur, such as the on the surface of fruit, storage cellars and on the equipment used during the fermentation process (6)
S. cerevisiae is famously known for its role in food production. It is the critical component in the fermentation process that converts sugar into alcohol, an ingredient shared in beer, wine and distilled beverages. It is also used in the baking process as a leavening agent; yeast releasing gas into their environment results in the spongy-like texture of breads and cakes. Because of its role in fermentation, humans have known about and used Saccharomyces cerevisiae for a long time. Archaeologists have found evidence of a fermented beverage in China as early as 7000BC (1), and evidence of the yeast being used in fermentation was found in a wine jar dating back to 3150BC(2).
Isolation of the species did not occur until 1938, when Emil Mrak isolated it from rotten figs found in Merced, California. (2). Taking advantage of its unique reproductive cycle, Robert Mortimer performed genetic crosses that used the isolated fig strain and other yeast strains obtained through other researchers. As a result, he created a new strain called S288c(2), which was then used as a parental strain in order to isolate most of the mutant strains currently used in research. (3), Furthermore, this strain was then used to sequence the S. cerevisiae genome(4).
S. cerevisiae is also considered to be a "model organism" by scientists. Its big advantage is that it is both a unicellular and eukaryotic organism. As a eukaryote, a majority of the yeast genes and proteins have human homologs, (14) and a greater understanding of the yeast genome would also help scientists understand the human genome. Another advantage is its fast growth grate. On a normal yeast medium, it takes 90 minutes for the yeast population to double. (13), and colonies are usually visible 2-3 days after placing them on fresh medium. Since the complete genome sequence is now available, mutants unique to eukaryotic organisms can now be expressed in an eukaryote as opposed to studying a similar gene in prokaryotes.
Genome structure
On April 24, 1996, the complete yeast genome sequence was available to the public. The genome contains 12,068 kilobases contained in sixteen linear chromosomes. (1). Unlike prokaryotes, DNA is concentrated in the nucleus, and are grouped into chromosomes during DNA replication. 70% the genome is consisting of open reading frames (ORF's), DNA sequences that would code for a protein, and the average ORF length is about 1450 bp long (4). Relative to more complex eukaryotes like nematodes (6kb) and humans (30kb), the yeast genome is more compact(1). In the genome, 5,885 genes code for proteins, 275 code for tRNA, 40 code for snRNA's, and 140 genes on chromosome 12 code for ribosomal RNA. 4% of the genome is comprised of introns, which are pieces of mRNA cut by snRNA-protein complexes prior to translation(1). Out of all the genes that code for proteins, 11% of the protenome is devoted to metabolism, 3% to energy production and storage, 3% to DNA replication, 7% to transcription and 6% to translation. Nearly 430 proteins are involved in intracellular trafficking, and 250 proteins have structural roles.
Protein-coding genes have been documented in the genome, but so far very few of those genes have been identified. Furthermore, the genome also shows signs of two or more copies of a gene in different locations. The genes that code for citrate synthase, an enzyme that converts acetyl CoA and oxaloacetate to citrate, is located in three different chromosomes. Chromosome 3 encodes the enzyme in the peroxidase, chromosome 12 encodes the enzyme in the mitochondria and another copy of the gene is located in chromosome 16. (5). One reason for the redundancy in the genome could be that multiple copies of a yeast gene are required in order for it to survive in its natural habitat (5).
Now that the yeast genome is available, scientists then created new mutants that deleted exactly one ORF in the genome. Experiments were then conducted that had the mutants grow on various media, altered environmental conditions as well as if there were any changes in the morphology. 15% of the total genome is needed for growth in rich medium, since their deletion can retard their growth by 12-90% (3). At least 10 genes were needed for growth on galactose. Such genes could either code for ribosomal proteins or proteins involved in respiration (3). A lot of the mutants studied were crucial to homeostasis. At least three genes are needed for the signal transduction pathway leading to the production of glycerol, which is produced when growing on high osmolarity solutions like NaCl and sorbitol. (6). Mutants of 62 genes were unable to grow on high salt concentrations and 128 mutants were unable to grow at a high pH. A high amount of homeostasis genes are needed since yeast often grow on areas unsuitable for other organisms, such as fermented wine and the gastrointestinal cavities of insects. Finally, about 673 mutants caused slight to big changes in the yeast morphology. Such mutants were either more elongated, pointed, round, clumped or changed in size from the wild type (9).
The mitochondrial DNA sequence has been attempted, but it is incomplete and contains many errors. The mitochondrial genome is about 85,000 base pairs long and contains seven hypothetical ORF's. Further experiments will determine if any of the seven ORF's are expressed in the mitochondria. In addition to the ORF's, the genome contains genes for three subunits of complex IV used in the electron transport chain, and three subunits of ATP synthase. (3)
S. cerevisiae strain A364A also contains a 2um circle plasmid. It is 6,318 base pairs long and constitutes 3% of the yeast genome. (2). Although the sequence contains coding regions for three proteins, the exact identity or function of the proteins is unknown. Like nuclear chromosomes, the plasmid is comprised of chromatin and histones, and can condense itself during mitosis. Unlike bacterial plasmids, which replicate independently of the bacterial chromosome, it replicates only once during the S phase of the cell cycle, and is regulated by the same genes that regulate nuclear DNA replication (2). While there is no evidence that it can integrate into chromosomal DNA, the yeast plasmid is capable of acting as vector in yeast transformation. (2). Foreign DNA extracted from eukaryotes could now be inserted directly into an eukaryote.
Cell structure and metabolism
Describe any interesting features and/or cell structures; how it gains energy; what important molecules it produces.
The organism can exist in two states: haploid and diploid. Diploid yeasts can reproduce asexually by budding or undergo sporulation to produce haploid spores. Each spore can either have one of two mating types, a or α (10). When cells of the opposite mating type meet, they produce a diploid organism. Yeasts strains can differ in their haploid states. In homothallic strains, the haploid cells cannot change mating type or undergo mitosis, but in the heterothallic strains, haploid cells can change mating type as they undergo mitosis. Most wild strains of S. cerevisiae are of the homothalic type.(11).
Encountering harsh growth conditions and intense competition, S. cerevisiae has adapted to survive and dominate its environment by actively modifying the local conditions. Yeasts rapidly begin to ferment sugars to produce ethanol (Querol et al . 2003). Although fermentation is widely regarded as a losing strategy owing to the relatively low output of ATP, it is such a rapid process that optimization of glycolysis allows S. cerevisiae to produce a similar number of ATP molecules per second as produced in aerobic metabolism (Pfeiffer et al . 2001). As long as glucose is present, it is transformed into ethanol. The ability to produce ethanol during fermentation is not unique to Saccharomyces species. The uniqueness of Saccharomyces rather resides in its ability to produce and to tolerate high levels of ethanol, which may later be utilized as a source of energy once the glucose is depleted (Pretorius 2000; Thomson et al . 2005). The high level of ethanol produced, along with the anaerobic conditions, low pH, and the osmotic stress, eliminates other microorganisms. As a result, strains of S. cerevisiae are essentially the only organisms remaining alive at the end of a fermentation (Querol et al . 2003). (http://www.blackwell-synergy.com/doi/pdf/10.1111/j.1365-294X.2006.02778.x)
In glucose- grown batch cultures of Saccharomyces cerevisiae, on the other hand, ethanol is produced under aerobic conditions and the rate of alcoholic fermentation is barely influenced by a change to anaerobiosis (Fiechter et al. 1981). (http://www.springerlink.com/content/r57812680t1m3k46/fulltext.pdf)
http://www3.interscience.wiley.com/cgi-bin/fulltext/107582605/PDFSTART
Ecology
It has been difficult to observe and collect Saccharomyces cerevisiae outside areas of human contact, so not much research has been done on its interactions with its natural environment. Because it is rarely associated with any other environments other than areas that are close to sites of fermentation, people have wondered whether the yeast could ever be found in the wild. (1). So far, most interactions with its environment have been limited to fermentation. In 1871, Louis Pasteur discovered that grapes had to be crushed in order for fermentation to occur (2). The grape itself has been an ideal habitat for yeast due to its high sugar concentration and low pH, precluding the growth of rival species. (1). Despite this, not many intact grapes contain S. cerevisiae at any one time. In an estimate, only one intact grape berry has the yeast on its surface. (3).
While intact grapes have little to no yeast present on the skin, damaged grapes are more likely to contain the yeast as well as other organisms. Berries were damaged due to the weather, mold infections or birds feeding on the grapes. Additionally, insects may also appear more often if the berry is already damaged. (3) These insects would harbor the yeast in their bodies and deposit them unknowingly while feeding, and the yeast would divide upon exposure to the grape. While it is known that insects harbor microorganisms inside their bodies, it is unknown how yeast is introduced into the insect. (3).
Pathology
Saccharoyces cerevisiae is not normally considered to be a pathogen. In healthy people, disease resulting from S. cerevisiae colonizing in a particular area are very rare, but have been reported. While yeast that normally colonize in the GI tract are not the direct cause of any disease, hypersensitivity to antibodies produced against could prove an irritant for people with Crohn's disease, an autoimmune disorder. (2). 1% of all vaginal yeast infections occur due to S. cerevisiae in the vagina, but symptoms associated with it are identical to the symptoms caused due to another organism more commonly associated with yeast infections, Candida albicans. (1). The only people susceptible to serious problems are immunosuppressed individuals as well as those who have taken S. cerevisiae as an probiotic for diarrhea. For these individuals, the prevailing condition is fungemia (01). Caused by the presence of yeasts in the blood, its symptoms have been described as "flu-like".
Application to Biotechnology
1. Ethanol Production
One of the oldest applications of Saccharomyces cerevisiae in biotechnology is its role the creation of alcoholic beverages. In a process called fermentation, yeast feeds off sugars from their substrate and convert it to ethanol, giving these beverages their alcoholic content. Depending on the beverage, yeast is incorporated into the creation process in several ways.
Winemakers select their yeast based on several factors: type of grape, local climate, geographical area and the desired taste of the final product. (1). Yeast is then produced in the winery, then added to the crushed grapes when it is time for fermentation. Champagne is an exception where natural yeast strains are used, since yeast goes directly into the bottle instead of a huge vat. (1). More gas is trapped in the bottle, creating sparkling wine.
In brewing beer, two different types of yeasts are produced in the fermentation process, depending on the type of beer created. Top-fermenting yeasts, also known as ale yeasts, form foam on top of the wort, the liquid containing the sugars used to be converted into ethanol. The yeast stays at the top of the tank, and begins to ferment at warm temperatures. This process is used in the creation of ales, porters, stouts and wheat beers. (1). Bottom-fermenting yeasts, also known as lagers yeast, ferment at cooler temperatures, and the yeast settle at the bottom of the tank. (1). They are used in the production of most commercial beers sold in America.
Another process is used to create the beverages collectively known as spirits, such as vodka and tequilla. Yeast used in the fermentation of these beverages are isolated from beet or sugar cane. Selection criteria for these yeasts include high ethanol production, have high tolerance to ethanol concentration, and must be able to ferment various substrates specific to the beverage.(1).
2. Food Production
S. cerevisiae also acts as a leavening agent. During preparation, dried yeast cells are added with the rest of the ingredients. While baking, yeast reacts with its environment and releases gas. This gas is trapped, forming holes as it bakes. This contributes to the spongy-like texture of breads and cakes seen after baking. While dried yeast cells include a leavening agent, unleavened yeast could also be used to add flavor to the bread. (1).
Yeast used to brew beer are still useful after the fermentation process. After fermentation is finished, the leftover yeast is dried and can be sold in liquid, tablet or powdered form. It is an excellent source of B vitamins, various minerals and proteins, and can be taken as a nutritional supplement (1). Yeast still contains these nutrients even after being broken down by its own enzymes, and the resulting yeast extract can be used as a flavor enhancer ([1]). One of the components of the famous food paste Vegemite contains yeast extract. Finally, S. cerevisiae has also been shown to survive living in the gastrointestinal tract while eliminating the potentially pathogenic bacteria residing. Since it does not colonize the GI tract permanently, it is used as a probiotic.
Current Research
Lantana camara used as substrate for fuel ethanol production
Research has been conducted to find economically viable methods to produce ethanol, a possible alternate fuel source to petroleum since it can be made from renewable resources. Currently, starch-containing plants are used for ethanol production, but starch production is limited to the amount of plants grown. Cellulose, another complex sugar, is preferred over starch because it is the more abundant sugar, but cellulose is harder to break down than starch.
http://www.blackwell-synergy.com/doi/full/10.1111/j.1472-765X.2007.02116.x
Increased glycolytic flux due to whole-genome duplication
http://www.nature.com/msb/journal/v3/n1/full/msb4100170.html
Effects of Aneuploidy on Cellular Physiology and Cell Division in Haploid Yeast
References
Edited by Isabella Ballesta, student of Rachel Larsen
