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Saccharomyces cerevisiae is a species of yeast. It is perhaps the most useful yeast, having been instrumental to winemaking, baking and brewing since ancient times. It is believed that it was originally isolated from the skin of grapes (one can see the yeast as a component of the thin white film on the skins of some dark-colored fruits such as plums; it exists among the waxes of the cuticle). It is one of the most intensively studied eukaryotic model organisms in molecular and cell biology, much like Escherichia coli as the model bacterium. It is the microorganism behind the most common type of fermentation. S. cerevisiae cells are round to ovoid, 5–10 micrometres in diameter. It reproduces by a division process known as budding.

Many proteins important in human biology were first discovered by studying their homologs in yeast; these proteins include cell cycle proteins, signaling proteins, and protein-processing enzymes.

Saccharomyces cerevisiae is currently the only yeast cell that is known to have Berkeley bodies present, which are involved in particular secretory pathways.

Antibodies against S. cerevisiae are found in 60–70% of patients with Crohn's disease and 10–15% of patients with ulcerative colitis (and 8% of healthy controls).^[1]

Nomenclature

"Saccharomyces" derives from Latinized Greek and means "sugar mold" or "sugar fungus", saccharo- being the combining form "sugar-" and myces being "fungus". Cerevisiae comes from Latin and means "of beer". Other names for the organism are:

• S. cerevisiae short form of the scientific name
• Brewer's yeast, though other species are also used in brewing
• Ale yeast
• Top-fermenting yeast
• Baker's yeast
• Budding yeast

This species is also the main source of nutritional yeast and yeast extract.

Biology

Ecology

In nature, yeast cells are found primarily on ripe fruits such as grapes (before maturation, grapes are almost free of yeasts). Since S. cerevisiae is not airborne, it requires a vector to move. In fact, queens of social wasps overwintering as adults (Vespa crabro and Polistes spp.) can harbor yeast cells from autumn to spring and transmit them to their progeny.^[2]

Life cycle

There are two forms in which yeast cells can survive and grow: haploid and diploid. The haploid cells undergo a simple life cycle of mitosis and growth, and under conditions of high stress will, in general, die. The diploid cells (the preferential 'form' of yeast) similarly undergo a simple life cycle of mitosis and growth, but under conditions of stress can undergo sporulation, entering meiosis and producing four haploid spores, which can proceed on to mate. With adequate nutrient, yeast cells can double in 100 minutes.^[3][4] Mean replicative lifespan is about 26 cell divisions.^[5][6]

Nutritional requirements

All strains of S. cerevisiae can grow aerobically on glucose, maltose, and trehalose and fail to grow on lactose and cellobiose. However, growth on other sugars is variable. Galactose and fructose are shown to be two of the best fermenting sugars. The ability of yeasts to use different sugars can differ depending on whether they are grown aerobically or anaerobically. Some strains cannot grow anaerobically on sucrose and trehalose.

All strains can use ammonia and urea as the sole nitrogen source, but cannot use nitrate, since they lack the ability to reduce them to ammonium ions. They can also use most amino acids, small peptides, and nitrogen bases as a nitrogen source. Histidine, glycine, cystine, and lysine are, however, not readily used. S. cerevisiae does not excrete proteases, so extracellular protein cannot be metabolized.

Yeasts also have a requirement for phosphorus, which is assimilated as a dihydrogen phosphate ion, and sulfur, which can be assimilated as a sulfate ion or as organic sulfur compounds such as the amino acids methionine and cysteine. Some metals, like magnesium, iron, calcium, and zinc, are also required for good growth of the yeast.

Mating

Yeast has two mating types, a and α (alpha), which show primitive aspects of sex differentiation, and are, hence, of great interest.^[7] For more information on the biological importance of these two cell types, where they come from (from a molecular biology point of view), and details of the process of mating type switching, see Mating of yeast.

Cell cycle

Growth in yeast is synchronised with the growth of the bud, which reaches the size of the mature cell by the time it separates from the parent cell. In rapidly growing yeast cultures, all the cells can be seen to have buds, since bud formation occupies the whole cell cycle. Both mother and daughter cells can initiate bud formation before cell separation has occurred. In yeast cultures growing more slowly, cells lacking buds can be seen, and bud formation only occupies a part of the cell cycle. The cell cycle in yeast normally consists of the following stages – G1, S, G2, and M – which are the normal stages of the cell cycle.

Yeast in biological research

A model organism

Saccharomyces cerevisiae
Numbered ticks are 10 micrometres apart.

When researchers look for an organism to use in their studies, they look for several traits. Among these are size, generation time, accessibility, manipulation, genetics, conservation of mechanisms, and potential economic benefit. The yeast species S. pombe and S. cerevisiae are both well studied; these two species diverged approximately 600 to 300 million years ago, and are significant tools in the study of DNA damage and repair mechanisms.^[8]

S. cerevisiae has developed as a model organism because it scores favorably on a number of these criteria.

• As a single celled organism S. cerevisiae is small with a short generation time (doubling time 1.25–2 hours^[9] at 30 °C or 86 °F) and can be easily cultured. These are all positive characteristics in that they allow for the swift production and maintenance of multiple specimen lines at low cost.
• S. cerevisiae can be transformed allowing for either the addition of new genes or deletion through homologous recombination. Furthermore, the ability to grow S. cerevisiae as a haploid simplifies the creation of gene knockouts strains.
• As a eukaryote, S. cerevisiae shares the complex internal cell structure of plants and animals without the high percentage of non-coding DNA that can confound research in higher eukaryotes.
• S. cerevisiae research is a strong economic driver, at least initially, as a result of its established use in industry.

Genome sequencing

S. cerevisiae was the first eukaryotic genome that was completely sequenced.^[10] The genome sequence was released in the public domain on April 24, 1996. Since then, regular updates have been maintained at the Saccharomyces Genome Database. This database is a highly annotated and cross-referenced database for yeast researchers. Another important S. cerevisiae database is maintained by the Munich Information Center for Protein Sequences (MIPS). The genome is composed of about 12,156,677 base pairs and 6,275 genes, compactly organized on 16 chromosomes. Only about 5,800 of these are believed to be true functional genes. Yeast is estimated to share at least 31% of its genome with that of humans.^[11] Yeast genes are classified using gene symbols (such as sch9) or systematic names. Each of the 16 chromosomes of yeast are represented by letters A-P, and the gene is further classified by which side of the centromere, the location and which strand of double-stranded DNA where it is present.^{[citation needed]}

Other tools in yeast research

The availability of the S. cerevisiae genome sequence and a set of deletion mutants covering 90% of the yeast genome^[12] has further enhanced the power of S. cerevisiae as a model for understanding the regulation of eukaryotic cells. A project underway to analyze the genetic interactions of all double deletion mutants through synthetic genetic array analysis will take this research one step further. The goal is to form a functional map of the cell's processes. As of 2010 the yeast interactome is most comprehensive yet to be constructed, containing "the interaction profiles for ~75% of all genes in the Budding yeast". This model was made from 54 million two-gene comparisons with 170,000 gene interactions and "grouping genes with similar interaction patterns together".^[13] This research is aimed at helping scientist better understand which gene(s) are responsible for how the cell behaves. This is known as the relationship between genotype and phenotype.

Approaches that can be applied in many different fields of biological and medicinal science have been developed by yeast scientists. These include yeast two-hybrid for studying protein interactions and tetrad analysis.

Astrobiology

Among other microorganisms, a sample of living S. cerevisiae was included in the Living Interplanetary Flight Experiment, which would have completed a three-year interplanetary round-trip in a small capsule aboard the Russian Fobos-Grunt spacecraft, launched in late 2011.^[14][15] The goal was to test whether selected organisms could survive a few years in deep space by flying them through interplanetary space. The experiment would have tested one aspect of transpermia, the hypothesis that life could survive space travel, if protected inside rocks blasted by impact off one planet to land on another.^[14][15][16] Fobos-Grunt's mission ended unsuccessfully however when it failed to escape low Earth orbit. The spacecraft along with its instruments fell into the Pacific Ocean in an uncontrolled re-entry on January 15, 2012.

Yeast in commercial applications

Brewing

Saccharomyces cerevisiae is used in brewing beer, when it is sometimes called a top-fermenting or top-cropping yeast. It is so called because during the fermentation process its hydrophobic surface causes the flocs to adhere to CO₂ and rise to the top of the fermentation vessel. Top-fermenting yeasts are fermented at higher temperatures than lager yeasts, and the resulting beers have a different flavor than the same beverage fermented with a lager yeast. "Fruity esters" may be formed if the yeast undergoes temperatures near 21 °C (70 °F), or if the fermentation temperature of the beverage fluctuates during the process. Lager yeast normally ferments at a temperature of approximately 5 °C (41 °F), where Saccharomyces cerevisiae becomes dormant. Lager yeast can be fermented at a higher temperature to create a beer style known as "steam beer".

Uses in aquaria

Owing to the high cost of commercial CO₂ cylinder systems, CO₂ injection by yeast is one of the most popular DIY approaches followed by aquaculturists for providing CO₂ to underwater aquatic plants. The yeast culture is, in general, maintained in plastic bottles, and typical systems provide one bubble every 3–7 seconds. Various approaches have been devised to allow proper absorption of the gas into the water.

Yeast strains

The following yeast strains have been used in numerous research projects, e.g. for yeast two-hybrid screens. Note that many yeast strains come as pairs of haploid a and alpha strains (indicated by MATa or MATα) which can be mated to form diploid strains. Strains are usually characterized by their genetic differences to the sequenced "standard" strain S288C. For instance, strain AH109 has its gal4 gene deleted (indicated by a Greek Δ) and a mutation in its trp gene.

AH109

Genotype^[17]: MATa, trp 1-901, leu2-3, 112, ura3-52, his3- 200, Δgal4, Δgal80, LYS2: GAL1UAS-GAL1TATA-HIS3, GAL2UAS- GAL2TATA-ADE2, URA3: MEL1UAS-MEL1TATA-lacZ)

PJ69-4alpha

Genotype^[18]: MATα trp1-901 leu2-3,112 ura3-52 his3-200 gal4(deleted) gal80(deleted) LYS2::GAL1-HIS3 GAL2-ADE2 met2::GAL7-lacZ

PJ69-4a

Genotype ^[18]: MATa trp1-901 leu2-3,112 ura3-52 his3-200 gal4(deleted) gal80(deleted) LYS2::GAL1-HIS3 GAL2-ADE2 met2::GAL7-lacZ

Y187

Y187 has been sold commercially by Clontech^[19] since at least 2000 and is used as a partner with strain AH109 in mating assays.^[20]

Genotype^[21]: MATα, ura3-52, his3-200, ade2-101, trp1-901, leu2-3, 112, gal4Δ, met–, gal80Δ, URA3::GAL1_UAS-GAL1_TATA-lacZ.

More strains can be found on the SGD Wiki.^[22]

See also

• Beer
• Saccharomyces pastorianus
• Schizosaccharomyces pombe
• Killer yeast
• Mannan Oligosaccharides (MOS)

References

10. David B. Jansma, REGULATlON AND VARlATlON OF SUBUNITS OF RNA POLYMERASE II IN Saccharomyces cerevisiae http://www.collectionscanada.gc.ca/obj/s4/f2/dsk1/tape7/PQDD_0003/NQ41179.pdf

Further reading

• Arroyo-López FN, Orlić S, Querol A, Barrio E (May 2009). "Effects of temperature, pH and sugar concentration on the growth parameters of Saccharomyces cerevisiae, S. kudriavzevii and their interspecific hybrid". Int. J. Food Microbiol. 131 (2-3): 120–7. doi:10.1016/j.ijfoodmicro.2009.01.035. PMID 19246112. http://bib.irb.hr/datoteka/389483.Arroyo-Lopez_et_al.pdf.