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Schizosaccharomyces pombe
Scientific classification
Kingdom: Fungi
Phylum: Ascomycota
Subphylum: Taphrinomycotina
Class: Schizosaccharomycetes
Order: Schizosaccharomycetales
Family: Schizosaccharomycetaceae
Genus: Schizosaccharomyces
Species: S. pombe
Binomial name
Schizosaccharomyces pombe
Lindner


Schizosaccharomyces pombe, also called "fission yeast", is a species of yeast. It is used as a model organism in molecular and cell biology. It is a unicellular eukaryote, whose cells are rod-shaped. Cells typically measure 3 to 4 micrometres in diameter and 7 to 14 micrometres in length. Its genome, which is approximately 14.1 million base pairs, is estimated to contain 4,970 protein-coding genes and at least 450 non-coding RNAs[1].

These cells maintain their shape by growing exclusively through the cell tips and divide by medial fission to produce two daughter cells of equal sizes, which makes them a powerful tool in cell cycle research.

Fission yeast was isolated in 1893 by Lindner from East African millet beer. The species name is derived from the Swahili word for beer (Pombe). It was first developed as an experimental model in the 1950s: by Urs Leupold for studying genetics[2][3], and by Murdoch Mitchison for studying the cell cycle[4][5].

The fission yeast researcher Paul Nurse successfully merged the independent schools of fission yeast genetics and cell cycle research. Together with Lee Hartwell and Tim Hunt, Nurse won the 2001 Nobel Prize in Physiology or Medicine for their work on cell cycle regulation.

The sequence of the S. pombe genome was published in 2002, by a consortium led by the Sanger Institute, becoming the sixth model eukaryotic organism whose genome has been fully sequenced. This has fully unlocked the power of this organism, with many genes homologous to human disease genes being identified.

In 2006, sub-cellular localization of all the proteins in S. pombe was published using green fluorescent protein as a molecular tag.

S. pombe has also become an important organism in studying the cellular responses to DNA damage and the process of DNA replication.

Contents

Comparison with budding yeast (Saccharomyces cerevisiae)

The yeast species S. pombe and S. cerevisiae are both extensively studied; these two species diverged approximately 300 to 600 million years before present, and are significant tools in molecular and cellular biology. Some of the technical discriminants between these two species are:

  • S. cerevisiae has approximately 5600 open reading frames; S. pombe has approximately 4970 open reading frames.
  • S. cerevisiae has 16 chromosomes, S. pombe has 3.
  • S. cerevisiae is often diploid while S. pombe is usually haploid.
  • S. cerevisiae is in the G1 phase of the cell cycle for an extended period (consequently, G1-S transition is tightly controlled) while S. pombe remains in the G2 phase of the cell cycle for an extended period (consequently, G2-M transition is under tight control).
  • Both species share genes with higher eukaryotes that they do not share with each other. S. pombe has heterochromatin and RNAi machinery genes like those in vertebrates, while these are missing from S. cerevisiae. Conversely, S. cerevisiae has well developed peroxisomes, while S. pombe does not.
  • S. cerevisiae has small point centromere of about 100 bp, and sequence-defined replication origins of about the same size. Conversely, S. pombe has large, repetitive centromeres (40–100 kb) more similar to mammalian centromeres, and degenerate replication origins of at least 1kb.

Life cycle of Schizosaccharomyces pombe (fission yeast)[6]

The fission yeast is a single-celled fungus with simple, fully characterized genome and a rapid growth rate. It has long since been used in brewing, baking and molecular genetics. S.Pombe is a rod-shaped cell, approximately 3µm in diameter, that grows entirely by elongation at the ends. After mitosis, division occurs by the formation of a septum, or cell plate, that cleaves the cell at its midpoint.

The central events of cell repoduction are chromosome duplication, which takes place in S (Synthetic) phase, followed by chromosome segregation and nuclear division (mitosis) and cell division (cytokinesis), which are collectively called M (Mitotic) phase.G1 is the gap between M and S phases, and G2 is the gap between S and M phases. In the budding yeast, the G2 phase is particularly extended, and cytokinesis (daughter-cell segregation) does not happen until a new S (Synthetic) phase is launched.

Fission yeast governs mitosis by mechanisms that are similar to those in multicellular animals. It normally proliferates in a haploid state. When starved, cells of opposite mating types (P and M) fuse to form a diploid zygote that immediately enters meiosis to generate four haploid spores. When conditions improve, these spores germinate to produce proliferating haploid cells.

Cytokinesis in fission yeast

Cytokinesis of the fission yeast.

The general features of cytokinesis are shown here. The site of cell division is determined before anaphase. The anaphase spindle (in green on the figure) is then positioned so that the segregated chromosomes are on opposite sides of the predetermined cleavage plane.

Size control in fission yeast

Cell-cycle length of the fission yeast depends on nutrient conditions.

In fission yeast, where growth governs progression through G2/M, a wee1 mutation causes entry into mitosis at an abnormally small size, resulting in a shorter G2. G1 is lengthened, suggesting that progression through Start (beginning of cell cycle) is responsive to growth when the G2/M control is lost. Furthermore, cells in poor nutrient conditions grow slowly and thereforetake longer to double in size and divide. Interestingly enough, low nutrient levels also reset the growth threshold so that cell progresses through the cell cycle at a smaller size. Finally, wee1 mutant fission yeast cells are smaller than wild-type cells, but take just as long to go through the cell cycle. This is possible because small yeast cells grow slower, that is, their added total mass per unit time is smaller than that of normal cells.


A spatial gradient is thought to coordinate cell size and mitotic entry in fission yeast[7][8] The Pom1 protein kinase (green) is localized to the cell cortex, with the highest concentration at the cell tips. The cell-cycle regulators Cdr2, Cdr1 and Wee1 are present in cortical nodes in the middle of the cell (blue and red dots). a, In small cells, the Pom1 gradient reaches most of the cortical nodes (blue dots). Pom1 inhibits Cdr2, preventing Cdr2 and Cdr1 from inhibiting Wee1, and allowing Wee1 to phosphorylate Cdk1, thus inactivating cyclin-dependent kinase (CDK) activity and preventing entry into mitosis. b, In long cells, the Pom1 gradient does not reach the cortical nodes (red dots), and therefore Cdr2 and Cdr1 remain active in the nodes. Cdr2 and Cdr1 inhibit Wee1, preventing phosphorylation of Cdk1 and thereby leading to activation of CDK and mitotic entry. (This simplified diagram omits several other regulators of CDK activity.)

External links

References

  1. ^ Wilhelm et al. Dynamic repertoire of a eukaryotic transcriptome surveyed at single-nucleotide resolution. Nature (2008) vol. 453 (7199) pp. 1239-43
  2. ^ Leupold U. (1950) Die Vererbung von Homothallie und Heterothallie bei Schizosaccharomyces pombe. CR Trav Lab Carlsberg Ser Physiol 24:381-480.
  3. ^ Leupold U. (1993) The origins of Schizosaccharomyces pombe genetics. In: Hall MN, Linder P. eds. The Early Days of Yeast Genetics. New York. Cold Spring Harbor Laboratory Press.. p 125-128.
  4. ^ Mitchison JM. (1957) The growth of single cells. I. Schizosaccharomyces pombe. Exp Cell Res 13:244-262.
  5. ^ Mitchison JM. (1990) My favourite cell: The fission yeast, Schizosaccharomyces pombe. Bioessays 4:189-191.
  6. ^ Cell Cycle. Principles of Control” by David O Morgan, Primers in Biology
  7. ^ A spatial gradient coordinates cell size and mitotic entry in fission yeast by James B. Moseley, Adeline Mayeux, Anne Paoletti & Paul Nurse, Nature, 11 June 2009
  8. ^ Cell cycle: Cell division brought down to size” by Kenneth E. Sawin, Nature 459, 782-783(11 June 2009)
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