Saccharomyces cerevisiae
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| Saccharomyces cerevisiae Meyen ex E.C. Hansen |
Saccharomyces cerevisiae is a species of budding yeast. It is perhaps the most useful yeast owing to its use since ancient times in baking and brewing. It is believed that it was originally isolated from the skins 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 prokaryote. It is the microorganism behind the most common type of fermentation. Saccharomyces 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. The petite mutation in S. cerevisiae is of particular interest.
"Saccharomyces" derives from Greek, and means "sugar mold". "cerevisiae" comes from Latin, and means "of beer". Other names for the organism are:
- Brewer's yeast (the apostrophe may be after the s or missing), though other species are also used in brewing
- Ale yeast
- Top-fermenting yeast
- Baker's yeast (the apostrophe may be after the s or missing)
- Budding yeast
This species is also the main source of nutritional yeast and yeast extract.
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Life cycle
There are two forms in which yeast cells can survive and grow, haploid and diploid. The haploid cells undergo a simple lifecycle of mitosis and growth, and under conditions of high stress will generally simply die. The diploid cells (the preferential 'form' of yeast) similarly undergo a simple lifecycle of mitosis and growth, but under conditions of stress can undergo sporulation, entering meiosis and producing a variety of haploid spores, which can go on to mate (conjugate), reforming the diploid.
Mating
Yeast has two mating types, a and α, which show primitive aspects of sex differentiation, and are hence of great interest. 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 the main article.
In Biological Research
A Model Organism
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.
Saccharomyces cerevisiae's 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 1.5-2 hours @ 30oC) 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 had a strong economic driver, at least initially, as a result of its established use in industry (e.g. beer, bread and wine fermentation).
Genome Sequencing
S. cerevisiae was the first eukaryotic genome that was completely sequenced. The yeast genome database [1] is highly annotated and remains a very important tool for developing basic knowledge about the function and organization of eukaryotic cell genetics and physiology. Another important S. cerevisiae database is maintained by the Munich Information Center for Protein Sequences (MIPS). The genome is composed of about 13,000,000 base pairs and 6,275 genes, compactly organised on 16 chromosomes. Only about 5,800 of these are believed to be true functional genes. It is estimated that yeast shares about 23% of its genome with that of humans.
Other Tools in Yeast Research
The availability of the S. cerevisiae genome sequence and the complete set of deletion mutants [1] 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 farther.
Approaches have been developed by yeast scientists which can be applied in many different fields of biological and medicinal science. These include Yeast two-hybrid for studying protein interactions and tetrad analysis.
Top-fermenting yeast
Saccharomyces cerevisiae is known as a top-fermenting yeast, 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. It is one of the major types of yeast used in the brewing of beer, along with Saccharomyces pastorianus. Some beers that use top-fermenting yeast are called ales, and for that reason these yeasts are also sometimes called "ale yeast". Top-fermenting yeasts are often fermented at higher temperatures than lager yeasts and the resulting beers are normally "fruitier."
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 generally 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.
Virus L-A
Saccharomyces cerevisiae Virus L-A is a double-stranded RNA (dsRNA) virus of Saccharomyces cerevisiae with similarity to animal double-stranded RNA viruses.[2]
See also
References
- ↑ http://www-sequence.stanford.edu/group/yeast_deletion_project/deletions3.html
- ↑ Wickner et al (2008). "The Yeast dsRNA Virus L-A Resembles Mammalian dsRNA Virus Cores". Segmented Double-stranded RNA Viruses: Structure and Molecular Biology. Caister Academic Press. ISBN 978-1-904455-21-9.
External links
- Saccharomyces Genome Database
- Cell cycle and metabolic cycle regulated transcription in yeast
- Yeast Resource Center Public Data Repository
- Munich Information Center for Protein Sequences - abstract of the paper describing the genome
- What are Yeast
- BioPIXIE
- BioGRID: A General Repository for Saccharomyces cerevisiae Interactions
- YEASTRACT (Yeast Search for Transcriptional Regulators And Consensus Tracking)
CO2 injection by Yeast for Planted Aquaria
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