http://www.ck12.org Chapter 9. Understanding Biodiversity: Fungi
- Domain: Eukarya
- Kingdom: Fungi
- Phylum: Ascomycetes
- Class: Saccharomycetes or Hemiascomycetes
- Order: Saccharomycetales
- Family: Saccharomycetaceae
- Genus: Saccharomyces
- Species:S. cerevisiae
Habitat
S. cerevisiae’s natural habitat is on the surface of fruit, but it is best known for its role in the baking and brewing
industries.
Biology
Cell Biology
S. cerevisiaecan exist as both haploid and diploid cells. The haploid cells undergo a simple life cycle of growth
followed by mitosis. Under stressful conditions, the cells will die. The diploid cells also undergo growth and
mitosis, although under stressful conditions, they will undergo meiosis and produce haploid spores. Under favorable
conditions, the spores will thenmate,producing a diploid cell.
Saccharomycesare heterotrophes, obtaining their energy from glucose, utilizing both aerobic and anaerobic respira-
tion.
S. cerevisiaeis a model organism in molecular and cell biology studies. Many signalling pathways of importance in
many eukaryotic organisms were first discovered in this species. Many common cell and molecular characteristics
of eukaryotic cells were also identified in yeast.
Yeast two-hybrid screening is a molecular biology technique used to discover protein–protein interactions and
protein–DNA interactions by testing for physical interactions between two proteins or binding between a single
protein and DNA. This technique was developed in 1989, and was originally designed to detect protein–protein
interactions using the GAL4 transcriptional activator ofS. cerevisiae. GAL4 is a nuclear protein, acting as a
positive regulator of gene expression for galactose-induced genes. The basic idea behind the test is the activation of
downstream reporter gene(s) by the binding of a transcription factor onto an upstream activating sequence (UAS) on
the DNA.
Genetics
The genome ofS. cerevisiaewas the first eukaryotic genome that was completely sequenced. The sequence was
released to the public on April 24, 1996. The genome is composed of about 12,156,677 base pairs and 6,275 genes,
distributed on 16 chromosomes. Only about 5,800 of these genes are believed to be functional. This species is
estimated to share about 23% of its genome with that of humans.
One of the first groups of growth mutants isolated from yeast were those that required adenine, one of the bases in
DNA and RNA nucleotides and a component of ATP. There are several genes in yeast that are involved in adenine
synthesis, and a mutation in any one of these genes requires that adenine be added to the growth medium. The first