Model Organisms for Biomedical Research
Trans-NIH S.pombe Initiative  

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Schizosaccharomyces pombe, or fission yeast, is an African brewing yeast originally isolated from millet beer.  This species has a distinguished research history in studies of the cell cycle and mitosis, chromosome dynamics, and epigenetics.   More recently, it has been used to address other cell biological questions including, but not limited to, DNA repair, meiosis, cytokinesis and mRNA processing.  Fission yeast forms a rod-shaped cell that grows by elongation and divides by medial fission. With a doubling time of 2-4 hours and simple culture conditions, S. pombe is easily adapted to any molecular biology laboratory.  Investigators from non-genetic systems are frequent recruits to the fission yeast community, using S. pombe as a genetic adjunct in their laboratories, which has led to S. pombe being nicknamed the  “micro-mammal”.

Genome Characteristics:  
The genome is sequenced with approximately 4979 genes in about 14 Mb of total DNA, making it the smallest free-living eukaryote.   There are only three chromosomes in the haploid.  Importantly, fission yeast does not share synteny with budding yeast.  In fact, the Schizosasccharomyces genus is frequently classified as an archaeascomycete and is highly diverged from the familiar ascomycete lineage that includes Saccharomyces  or Neurospora.  The fission yeast genome does not have many duplicated genes, making functional genetics more approachable. Significantly, several groups of genes are conserved in S. pombe and humans, but missing in S. cerevisiae, including genes involved in heterochromatin and telomere function, some splicing genes, and components of the RNAi machinery.   Thus, fission yeast provides a model for study of these functions that is not available in other systems.  The planned sequencing of related species will facilitate comparative genomics approaches. 

Experimental Approaches:
Fission yeast is genetically very tractable, and can be maintained in the haploid or the diploid state.  Classical genetic methods including mutagenesis, selection, suppressor and synthetic lethal analysis, tetrad dissection and random spore analysis are all readily performed in this system.  S. pombe can be rapidly transformed with a wide selection of pombe-specific integrating or episomal plasmids.  Gene disruption (knock-outs) and integration (knock-ins) are straightforward.  The cells are very regular in size and shape, with well-distinguished nuclei, which facilitates cytological analysis.  Biochemistry is increasingly popular, and proteomics approaches using tandem-affinity purification and mass spectrometry are now common.  Commercial genomics tools are also available.

Fission yeast is a popular system worldwide and the community, while dispersed, is vibrant and interactive.  A number of fission-yeast specific meetings are held, and much communication occurs electronically through web sites and an e-mailing list.

Photo Credit:
Susan L. Forsburg,  PhD, University of Southern California

Neurospora - Click to enlarge
This image shows vegetatively growing fission yeast cells stained with DAPI to visualize the nucleus.  Cells maintain a constant diameter and grow in length through the cell cycle.  The stages of mitosis, where the nucleus divides, are clearly visible.. (Click on image to enlarge.)