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
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
Describe the appearance, habitat, etc. of the organism, and
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 mammals, soils from all regions of the world and even in aquatic environments. (3). Most often,
[talk about fermentation here.]
why it is important enough to have its genome sequenced. Although it is evident that the human genome will specify many proteins that are not found in the yeast proteome, it is reasonable to suggest that the majority of the yeast proteins have human homologs. If so, these human proteins could be classified on the basis of their structural or functional equivalence to members of the yeast proteome. (Yeast Genome)
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). However, isolation of the species did not occur until 1938, when Emil Mrak isolate it from rotten figs found in Merced, California. (2). It was discovered that yeast from that sample can produce two types of sterile haploid spores, now known as a and α, that reproduce when they come together. Robert Mortimer took advantage of this and 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 in 1996 (4).
[Outline: S. cerevisiae is a useful eukaryotic organism used to study the gene/protein homologues to humans. It is small, easy to grow, doubles population in 3-4 days.]
Genome structure
Describe the size and content of the genome. How many chromosomes? Circular or linear? Other interesting features? What is known about its sequence? Does it have any plasmids? Are they important to the organism's lifestyle?
A most conspicious feature of the yeast genome, compared with that of multicellular organisms, is its compactness. On average, open reading frames (ORFs) occupy 72% of the yeast genetic [excluding ribosomal DNA (rDNA), leaving little space for noncoding DNA anti for all other structural and functional elements. (1)
The genome of the yeast Saccharomyces cerevisiae has been completely sequenced through a worldwide collaboration. The sequence of 12,068 kilobases defines 5885 potential protein-encoding genes, approximately 140 genes specifying ribosomal RNA, 40 genes for small nuclear RNA molecules, and 275 transfer RNA genes. In addition, the complete sequence provides information about the higher order organization of yeast's 16 chromosomes and allows some insight into their evolutionary history. (2)
The four smallest chromosomes (I, III, VI, and IX) exhibit average recombination frequencies some 1.3 to to 1.8 times greater than the average for the genome as a whole. Kaback (30) has suggested that high levels of recombination have been selected for on these very small chromosomes to ensure at least one crossover per meiosis, and so permit them to segregate correctly. (Yeast Genome)
The yeast cell devotes 11% of its proteome to metabolism; 3% to energy production and storage; 3% to DNA replication, repair, and recombination; 7% to transcription; and 6% to translation. A total of 430 proteins are involved in intracellular trafficking or protein targeting, and 250 proteins have structural roles.
Cell structure and metabolism
Describe any interesting features and/or cell structures; how it gains energy; what important molecules it produces.
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
Describe any interactions with other organisms (included eukaryotes), contributions to the environment, effect on environment, etc.
Pathology
How does this organism cause disease? Human, animal, plant hosts? Virulence factors, as well as patient symptoms. http://jcm.asm.org/cgi/content/full/39/2/551
Application to Biotechnology
Does this organism produce any useful compounds or enzymes? What are they and how are they used?
Current Research
Enter summaries of the most recent research here--at least three required
References
Edited by Isabella Ballesta, student of Rachel Larsen
