Model organisms: Yeast
28/8/02. By Richard Twyman
Baker's yeast is one of the simplest eukaryotic organisms but many essential cellular processes are conserved between yeast and humans.
Baker's yeast (Saccharomyces cerevisie) is a single celled organism used in the bread-making industry. It would appear initially to have little in common with human beings. However, an important feature of yeast cells is that they are eukaryotic - they have a nucleus containing chromosomes just like our cells. Furthermore, S. cerevisiae cells divide in a similar manner to our own cells, and there are many other basic biological properties that are shared.
The yeast genome is just over 12 million base pairs in length and contains about 6000 genes. Perhaps surprisingly, about 20 per cent of human disease genes have counterparts in yeast (see Comparative genomics ). This suggests that such diseases result from the disruption of very basic cellular processes, such as DNA repair, cell division or the control of gene expression .
It also means that yeast can be exploited to look at functional relationships involving these genes, and to test new drugs. A yeast mutant that has lost the functional equivalent of a human disease gene can be screened with thousands of potential drugs in order to identify compounds that restore normal function to the yeast cell. These compounds, or molecules like them, might also be useful in humans.
The yeast genome was completed in 1997 and other projects have been initiated to determine the functions of all 6000 genes. The Saccharomyces Genome Deletion Project, for example, aims to produce mutant strains of yeast in which each of the 6000 genes is mutated. Mutation is achieved using the same technique as that used to produce knockout mice , although the gene targeting process is much more efficient in yeast cells.
Another large project involves the yeast two-hybrid system and mass spectrometry to catalogue all the different protein interactions that occur in yeast cells. Interacting proteins often function in conserved complexes or pathways. A pathway found in yeast might therefore exist in humans, and characterising the interacting proteins in yeast might help to identify the corresponding proteins in humans. The identification of interacting proteins is useful because they may provide alternative drug targets. For example, the product of a human disease gene might be an unsuitable drug target, perhaps due to extensive polymorphism. However, interacting proteins in the same pathway might show less variability and would be better targets for drug development.