Regulation of Mating Type in Yeast
Rapid progress has been made in learning the basic structure and properties of a great many regulated genes and gene families of eukaryotic cells. Much of the progress is attributable to the power of genetic engineering, which has made the task of cloning and sequencing many eukaryotic genes relatively straightforward. Once a gene has been cloned, its own sequences, and sequences governing its regulation can be determined. In the most amenable cases, sequences of the gene and of its promoter and regulatory elements, and any sequences the gene product interacts with, may all be deleted or altered. In some cases the DNA can be reintroduced into the original organism to examine the consequences of the alterations.
After the effects of DNA modifications on the expression of a gene have been explored, the effect on the organism of the altered regulation can be investigated and the biochemical mechanisms underlying the regulation can be more easily studied. In a few systems the depth of our understanding of eukaryotic gene regulation approaches or exceeds that of prokaryotic genes. One example of a well-studied eukaryotic system is the mating-type genes of baker’s yeast Saccharomyces cerevisiae.
A haploid yeast cell contains both a- and α-mating type genetic information. Although normally only one or the other is expressed, a cell can switch from one to the other. This appears to be a more complicated example of differentiation than that considered, but one that is still relatively simple. Nonetheless, it appears to contain many of the elements found in the much more complicated differentiation systems of higher eukaryotes. One of the virtues of this system is that both genetic and biochemical approaches may be applied to it.