Mating Type Conversion in Saccharomyces cerevisiae
Yeast possessing the ho allele switch from mating type MATa, to MATα or vice versa at a frequency of about 10-6 per doubling time, whereas yeast possessing the HO allele, the form most commonly found in nature, switch mating types as frequently as once each generation.
As in the study of many biological phenomena, the study of mutants revealed hidden details of the mating-type system. Saccharomyces mat-ing-type mutants showed an astonishing fact. If an ho strain which is MATα-, making it defective in mating, is switched to MATa and thenswitched back to MATα, the original defect, the MATα- mutation, disappears! The original MATα- allele cannot be found, no matter how many additional times the a-α conversion is performed.
New MATα- mutations can be isolated in the healed strain, and these too heal upon mating-type switching. The only reasonable conclusion is the one first postulated by
Figure 16.2 Reference and expression copies of yeast mating-type genes. Inthis example α mating type information is being copied from HMLα to MAT to switch the a mating type cell to α mating type.
Oshima and Takano–that the cells contain nonexpressed reference copies of the MATa and MATα information on the chromosome in addition to the expressible forms of MATa or MATα information. The reference sites are named HML and HMR, and the expression site is named MAT (Fig. 16.2). Usually, but not always, HML contains α mating-type information, and HMR contains a type information. As explained below, direct physical data have confirmed this conclusion.
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