Question

10. In Saccharomyces cerevisiae, cell type is determined by which genes are present at the MAT locus. Describe the function of the a1, 1 and 2 proteins in each of the three cell types (a-haploids, -haploids and diploids).

Answer 1

The yeast ,Saccharomyces cerevisiae is known to be a single-celled eukaryote having a diploid as well as a haploid mode of existence. The mating of yeast only occurs between haploids, which can be either the a or α (alpha) mating type and thus leads to display lsimple sexual differentiation. Mating type is determined by a single locus, MAT, which in turn governs the sexual behaviour of both haploid and diploid cells. Through a form of genetic recombination, haploid yeast can switch mating type as often as every cell cycle.

The different sets of transcriptional repression and activation which characterize a and α cells are caused by the presence of one of two alleles of a locus called MAT: MATa or MATα located on chromosome III. The MAT locus is usually divided into five regions (W, X, Y, Z1, and Z2) based on the sequences shared among the two mating types. The difference always lies in the Y region (Ya and Yα), which contains most of the genes and promoters.

The MATa allele of MAT encodes a gene called a1, which in haploids direct the transcription of the a-specific transcriptional program (such as expressing STE2 and repressing STE3) that defines an a cell. The α1 and α2 genes are encoded by MATα allele of MAT, which in haploids used for directing the transcription of the α-specific transcriptional program (such as expressing STE3, repressing STE2) which causes the cell to be an α cell. S. cerevisiae has an a2 gene with no apparent function that shares much of its sequence with α2; however, other yeasts like Candida albicans do have a functional and distinct MATa2 gene.

a and α (alpha) cells:

a cells is know for producing ‘a-factor’, a mating pheromone, used for signallng due to the presence of an a cell to neighbouring α cells. a cells respond to α-factor, the α cell mating pheromone, by growing a projection (known as a shmoo, due to its distinctive shape resembling the Al Capp cartoon character Shmoo) towards the source of α-factor. Similarly, α cells produce α-factor, and respond to a-factor by growing a projection towards the source of the pheromone. The response of haploid cells only to the mating pheromones of the opposite mating type allows mating between a and α cells, but not between cells of the same mating type.

These phenotypic differences between a and α cells are due to a different set of genes being actively transcribed and repressed in cells of the two mating types. a cells activate genes which produce a-factor and produce a cell surface receptor (Ste2) which binds to α-factor and triggers signaling within the cell. a cells also repress the genes associated with being an α cell. Similarly, α cells activate genes which produce α-factor and produce a cell surface receptor (Ste3) which binds and responds to a-factor, and α cells repress the genes associated with being an a cell.