Pheromones and Receptors in Mating Yeast
(A) Haploid yeast cells of the a type express the genes for the a-factor and a-factor receptor. Haploid yeast cells of the a type express the genes for the a-factor and a-factor receptor. When a-factor binds to the a-factor receptor of the a cell, and the a-factor binds the a-factor receptor on the a cell, the two haploid yeast become competent and form a mating pair.
(B) The two mating factors are small peptides of 12 to 13 amino acids. The a-factor has a farnesyl group attached to the last amino acid, cysteine, via its sulfhydryl group.
pheromones or mating factors and their corresponding receptors (Fig. 19.20). Thus a MATa cell produces a-specific pheromone or a-factor and the receptor for a-factor, whereas a MATa cell produces a-factor and receptor for a-factor. The pheromones bind to the receptors on the cells of opposite mating type.
Although both versions of the MAT locus express two genes, the two mating types do not behave in a strictly symmetrical manner. The two proteins from the MATa locus control expression of both the a-specific and a-specific genes (Fig. 19.21).The MATa1p protein binds to Mcm1p protein forming an activator that switches on a-specific genes (including those for a-factor and a-factor receptor). MATa2p also binds to Mcm1p but forms a repressor that switches off a-specific genes (including those for a-factor and a-factor receptor). The MATa locus has a rather different role. The MATa1p protein binds to MATa2p forming a repressor that switches off haploid specific genes in diploid cells. The MATa2p protein has no known function.
Haploid cells of yeast frequently and spontaneously change their mating type. This is not due to mutation but to DNA rearrangement at the MAT locus (Fig. 19.22).
mating factor Chemical messenger or pheromone that indicates the mating type and promotes sexual conjugation pheromone Chemical messenger that moves between separate individual organisms
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