Eukaryotes Genomes - SCHIZOSACCHAROMYCES POMBE

The fission yeast Schizosaccharomyces pombe is a unicellular eukaryote in the class of Ascomycetes. Fission yeasts are rod shaped. They measure approximately two to three microns in diameter and seven to 14 microns in length.

Schizosaccharomyces pombe was first isolated from East African millet beer more than a century ago, Although its origin dates back to quite a long time ago, it was not widely known before the 1890's. A group working in a Brewery Association Laboratory in Germany were looking at sediment found in millet beer imported from East Africa that gave it an unsavoury acidulous taste. It was identified as yeast and it became known as the fission yeast as it was noted that it only reproduces, besides sporulation, by means of fission, unlike the other currently identified yeast species. The name Schizosaccharomyces was assigned to highlight the essential difference that exists from a morphologic standpoint in relation to the genus Saccharomyces, and to also show the common characteristics such as spore formation and fermentation that existed between them. Schizo, meaning "different", had previously been used to describe other fission species such as the fission fungi, Schizomycetes, and so was used here for the same purpose. The addition of the word pombe was due to its isolation from beer, as pombe essentially means "from beer", in Swahili.

S. pombe was the sixth organism with a nucleus to have its genome sequenced, following humans, the nematode worm, the fruit fly, mustard weed, and budding yeast. Of these completed eukaryotic organisms, S. pombe has the smallest and most compact genome. It contains 13.8 million base pairs distributed among 3 chromosomes and 4,824 genes. This is the lowest number of protein-coding genes yet identified in a free-living eukaryotic cell. It is also substantially lower than the 5,600 genes found in Saccharomyces cerevisiae.

Schizosaccharomyces pombe shares many features with cells of more complicated eukaryotes and has long been used as a model for cell division in research in molecular genetics and cell biology.From gene sequence comparisons and phylogenetic analyses, it has been suggested that fission yeast diverged from budding yeast around 330-420 million years ago, and from Metazoa and plants around 1,000-1,200million yeas ago. Some gene sequences are as equally diverged between the two yeasts as they are from their human homologues, probably reflecting a more rapid evolution within fungal lineages than in the Metazoa.

Study of the Cell Cycle regulation is important to understand how cells grow and differentiate as well as how non-controlled growth results in cancerous cells. Combining biochemical methods for purifying molecules and characterising enzymes with genetic approaches has revealed that, under normal conditions, the growth of animal cells is precisely controlled and cancer occurs when mutations affect the restriction of cell division. Because of the constant barrage of environmental mutagens and by-products of normal metabolism (free radicals), eukaryotic cells have developed ways to sense damage and arrest cell division until DNA can be repaired. In this regard, checkpoints are crucial to maintain the integrity of the genome. S. pombe has been very useful to investigate cell cycle control. Fission yeast are rod shaped cells that grow by elongation. Following cytokinesis the ends of each daughter cell differ in age; the old end is inherited from the parent whereas a new end is created. Cell division requires that the cells have achieved a critical cell length before intranuclear mitosis occurs. Fission yeast are so named because after mitosis, cytokinesis follows by cell cleavage at a septum, or midpoint of the cell. S. pombe has been used for cell cycle investigations from the 1950s because of its growth by length extension. This property allows the position of a cell in the cell cycle to be estimated by measurement of its length. In the 1970s, Paul Nurse and colleagues began isolating and characterising cell cycle mutants (The 2001 Nobel Prize in Physiology or Medicine was awarded to Lee Hartwell, Paul Nurse, and Tim Hunt for their groundbreaking work on cell cycle regulation). Potential cell cycle fission yeast mutants were identified on the basis of lethality and formation of elongated cells caused by the continuous cellular growth without cell division.

Researchers have identified fifty genes of S. pombe associated with human diseases-including cystic fibrosis, hereditary deafness, and diabetes. Researchers state that the largest group of human disease-related genes are those implicated in cancer. There are 23 such genes, and they are involved in DNA damage and repair, checkpoint controls, and the cell cycle, all processes involved in maintaining genomic stability.

These findings will also allow researchers to find out more about the evolution of one-celled and multi-celled eukaryotic organisms compared to others such as bacteria, which do not have nucleated cells. Further analyses and comparisons should reveal which genes define eukaryotic cells and the transition from one-celled to multi-celled organisms

References:

http://www.sanger.ac.uk/Projects/S_pombe/
http://www.genomenewsnetwork.org/articles/03_02/s_pombe.shtml
http://www.agc.co.jp
http://www.biols.susx.ac.uk/home/Trevor_Pemberton/whatSp.html