Schizosaccharomyces pombe
Schizosaccharomyces pombe, also called "fission yeast", is a species of yeast used in traditional brewing and 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.
These cells maintain their shape by growing exclusively through the cell tips and divide by medial fission to produce two daughter cells of equal size, which makes them a powerful tool in cell cycle research.
Fission yeast was isolated in 1893 by Paul Lindner from East African millet beer. The species name pombe is the Swahili word for beer. It was first developed as an experimental model in the 1950s: by Urs Leupold for studying genetics, and by Murdoch Mitchison for studying the cell cycle.
Paul Nurse, a fission yeast researcher, 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 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. S. pombe researchers are supported by the PomBase MOD. This has fully unlocked the power of this organism, with many genes orthologous to human genes identified — 70% to date, including many genes involved in human disease. In 2006, sub-cellular localization of almost all the proteins in S. pombe was published using green fluorescent protein as a molecular tag.
Schizosaccharomyces pombe has also become an important organism in studying the cellular responses to DNA damage and the process of DNA replication.
Approximately 160 natural strains of S. pombe have been isolated. These have been collected from a variety of locations including Europe, North and South America, and Asia. The majority of these strains have been collected from cultivated fruits such as apples and grapes, or from the various alcoholic beverages, such as Brazilian Cachaça. S. pombe is also known to be present in fermented tea, kombucha. It is not clear at present whether S. pombe is the major fermenter or a contaminant in such brews. The natural ecology of Schizosaccharomyces yeasts is not well-studied.
History
Schizosaccharomyces pombe was first discovered in 1893 when a group working in a Brewery Association Laboratory in Germany was looking at sediment found in millet beer imported from East Africa that gave it an acidic taste. The term schizo, meaning "split" or "fission", had previously been used to describe other Schizosaccharomycetes. The addition of the word pombe was due to its isolation from East African beer, as pombe means "beer" in Swahili. The standard S. pombe strains were isolated by Urs Leupold in 1946 and 1947 from a culture that he obtained from the yeast collection in Delft, The Netherlands. It was deposited there by A. Osterwalder under the name S. pombe var. liquefaciens, after he isolated it in 1924 from French wine at the Federal Experimental Station of Vini- and Horticulture in Wädenswil, Switzerland. The culture used by Urs Leupold contained cells with the mating types h90, h-, and h+. Subsequent to this, there have been two large efforts to isolate S. pombe from fruit, nectar, or fermentations: one by Florenzano et al. in the vineyards of western Sicily, and the other by Gomes et al. in four regions of southeast Brazil.Ecology
The fission yeast S. pombe belongs to the division Ascomycota, which represents the largest and most diverse group of fungi. Free-living ascomycetes are commonly found in tree exudates, on plant roots and in surrounding soil, on ripe and rotting fruits, and in association with insect vectors that transport them between substrates. Many of these associations are symbiotic or saprophytic, although numerous ascomycetes represent important plant pathogens that target myriad plant species, including commercial crops. Among the ascomycetous yeast genera, the fission yeast Schizosaccharomyces is unique because of the deposition of α--glucan or pseudonigeran in the cell wall in addition to the better known β-glucans and the virtual lack of chitin. Species of this genus also differ in mannan composition, which shows terminal d-galactose sugars in the side-chains of their mannans. S. pombe undergo aerobic fermentation in the presence of excess sugar. S. pombe can degrade L-malic acid, one of the dominant organic acids in wine, which makes them diverse among other Saccharomyces strains.Comparison with budding yeast (''Saccharomyces cerevisiae'')
The yeast species Schizosaccharomyces pombe and Saccharomyces 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 5,600 open reading frames; S. pombe has approximately 5,070 open reading frames.
- Despite similar gene numbers, S. cerevisiae has only about 250 introns, while S. pombe has nearly 5,000.
- S. cerevisiae has 16 chromosomes, S. pombe has 3.
- S. cerevisiae is often diploid while S. pombe is usually haploid.
- S. pombe has a shelterin-like telomere complex while S. cerevisiae does not.
- S. cerevisiae is in the G1 phase of the cell cycle for an extended period, while S. pombe remains in the G2 phase of the cell cycle for an extended period.
- Both species share genes with higher eukaryotes that they do not share with each other. S. pombe has RNAi machinery genes like those in vertebrates, while this is missing from S. cerevisiae. S. cerevisiae also has greatly simplified heterochromatin compared to S. pombe. Conversely, S. cerevisiae has well-developed peroxisomes, while S. pombe does not.
- S. cerevisiae has small point centromere of 125 bp, and sequence-defined replication origins of about the same size. On the converse, S. pombe has large, repetitive centromeres more similar to mammalian centromeres, and degenerate replication origins of at least 1kb.
S. pombe pathways and cellular processes
Life cycle
The fission yeast is a single-celled fungus with simple, fully characterized genome and a rapid growth rate. It has long been used in brewing, baking, and molecular genetics. S. pombe is a rod-shaped cell, approximately 3 μm 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 reproduction are chromosome duplication, which takes place in S phase, followed by chromosome segregation and nuclear division and cell division, which are collectively called M phase. G1 is the gap between M and S phases, and G2 is the gap between S and M phases. In the fission yeast, the G2 phase is particularly extended, and cytokinesis does not happen until a new S 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 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
The general features of cytokinesis are shown here. The site of cell division is determined before anaphase. The anaphase spindle is then positioned so that the segregated chromosomes are on opposite sides of the predetermined cleavage plane.Size control
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 is responsive to growth when the G2/M control is lost. Furthermore, cells in poor nutrient conditions grow slowly and therefore take longer to double in size and divide. Low nutrient levels also reset the growth threshold so that cell progresses through the cell cycle at a smaller size. Upon exposure to stressful conditions S. pombe cells undergo aging as measured by increased cell division time and increased probability of cell death. 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.
The Pom1 protein kinase 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. a, In small cells, the Pom1 gradient reaches most of the cortical nodes. Pom1 inhibits Cdr2, preventing Cdr2 and Cdr1 from inhibiting Wee1, and allowing Wee1 to phosphorylate Cdk1, thus inactivating cyclin-dependent kinase activity and preventing entry into mitosis. b, In long cells, the Pom1 gradient does not reach the cortical nodes, 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.