Matingtype Switching

In addition to promoting DNA pairing, DNA repair, and genetic exchange, homologous recombination can also serve to change the DNA sequence at a specific chromosomal location. This type of recombination is sometimes used to regulate gene expression. For example, recombination controls the mating type of the budding yeast S. cerevisioe by switching which mating-type genes are present at a specific location that is being expressed in that organism's genome.

5. cerevisioe is a single-cell eukaryote that can exist as any of three different cell types {see Chapter 21). Haplnid S. ceievisiac cells can he either of two mating types, a or a. And, when an a and a celi come in close proximity ihey can fuse (that is, "mate") to form an a/tn diploid cell. The a/a cell may then go through meiosis to form two haploid a-cells and two haploid «-cells.

The mating-type genes encode transcriptional regulators. These regulators control expression of target genes whose products define each cell type. The mating-type genes expressed in a given cell are those found at the mating-type locus (MAT locus) in that cell (Figure 10-20). Thus, in a-cells the al gene is present at the MAT locus, whereas in a-cells, Ihe al and a2 genes are present at the MAT locus. In the diploid cell, both sets of mating-type control genes are expressed. The regulators encoded by the mating-type genes, together with others found in all three cell types, act in various combinations to ensure that the correct pattern of genes is expressed in each cell type (see Chapter 17).

Cells can switch their mating type by recombination as we now describe. In addition to the a or a genes present at the MAT locus in each cell, there is an additional copy of both the a and a genes present (but not expressed) elsewhere in the genome. These additional silent copies are found at loci called HMR and HML (Figure 10-20).

a ceil

HML silent focus

MAT expressed focus

HML silent focus

MAT expressed focus

HML a

MATa t

HML a

HML a a-type silent cassette

MATa t

HO endonuclease cleavage

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