Mating Type Genes in S cerevisiae

Many fungi use some form of homothallic mating-type switching to produce diploids from haploid meiotic segregants. In budding yeast MATalMATa diploids are created by conjugation of haploid MATa and MATa cells. The MATa

Fig. 1. Arrangement of mating-type genes in S. cerevisiae. The MAT locus is approximately 2400 bp. Mating-type-specific Ya and Ya sequences are about 650 and 750 bp, respectively, and are surrounded by 700-bp W and X regions, a 230-bp Z1 region and a 90-bp Z2 region. The orientation of MATa and MATa transcripts are shown. The donor sequences HMLa and HMRa are unexpressed and maintained in a silent, heterochromatic state by action of the Sir2 histone deacetylase and accessory Sirl, Sir3 and Sir4 proteins, which are first recruited to E and I silencer sites surrounding the donors. MAT switching is initiated by HO endonuclease cleavage in Zl, cutting 7 and 3 bp on the top and bottom strands from the Y/ Z1 border, creating 4-bp 3' overhanging ends. The donor loci cannot be cut. Donor preference is governed by the recombination enhancer (RE), which controls recombination efficiency along the entire left arm of chromosome III

Fig. 1. Arrangement of mating-type genes in S. cerevisiae. The MAT locus is approximately 2400 bp. Mating-type-specific Ya and Ya sequences are about 650 and 750 bp, respectively, and are surrounded by 700-bp W and X regions, a 230-bp Z1 region and a 90-bp Z2 region. The orientation of MATa and MATa transcripts are shown. The donor sequences HMLa and HMRa are unexpressed and maintained in a silent, heterochromatic state by action of the Sir2 histone deacetylase and accessory Sirl, Sir3 and Sir4 proteins, which are first recruited to E and I silencer sites surrounding the donors. MAT switching is initiated by HO endonuclease cleavage in Zl, cutting 7 and 3 bp on the top and bottom strands from the Y/ Z1 border, creating 4-bp 3' overhanging ends. The donor loci cannot be cut. Donor preference is governed by the recombination enhancer (RE), which controls recombination efficiency along the entire left arm of chromosome III

and MATa alleles are located in the middle of chromosome III (Fig. 1) and differ from each other in a 643-bp (Ya) or 748-bp (Ya) region that contains different DNA sequences encoding regulators of mating type (reviewed by Haber 2002). MATa encodes two genes, MATa 1 and MATa2. MATa 1 encodes a positive transcriptional regulator of a-specific genes (e.g. the pheromone a-factor, and an a-pheromone receptor, Ste3). MATal encodes a repressor with two modes of action. In MATa cells, MATal pairs with the general transcriptional regulator, Mcml, to repress a-specific genes (e.g. those encoding the pheromone, a-factor, and the a-factor receptor, Ste2). In MATalMATa diploid cells, MATal and MATal proteins form a repressor that turns off hap-loid-specific genes, including the pheromone signal transduction pathway, the nonhomologous end-joining (NHEJ) pathway of DNA repair and a repressor of meiosis.

Most S. cerevisiae strains carry two additional copies of mating-type genes, HMLa and HMRa, at distant locations on the same chromosome as the MAT locus (Fig. 1). In some strains the specific mating-type sequences at these loci are reversed (i.e. HMLa or HMRa). These genes are, however, not expressed, as both HML and HMR are surrounded by silencer sequences (designated E and I) that recruit the specialized histone deacetylase, Sir2, and other silencing factors (Sirl, Sir3 and Sir4) to create a heterochromatic domain that is not transcribed (Rusche et al. 2003). These silent loci are the key to the ability of HO endonuclease to promote the switching of the MAT locus.

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