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The yeast Saccharomyces cerevisiae is a single-celled eukaryotic fungus that produces both haploid and diploid spores. It can reproduce asexually. Mating occurs in different types of strains is also commonly found in this yeast. Yeast displays simple sexual differentiation, so mating is observed between two different haploid cells &lsquo;a and &alpha;&rsquo; mating types. The mating type is determined by a single locus gene MAT which is responsible for the sexual behavior of these cells. Yeast also can switch the mating-type through a type of genetic recombination.
Haploid cells of yeast are capable of mating with the haploid cells of the opposite mating types (an &lsquo;a cell&rsquo; can only mate with an &lsquo;&alpha; cell&rsquo; and vice versa) and produce a diploid zygote. This diploid zygote, in stressful condition, undergo meiosis and produce four haploid spores (2 a and 2 &alpha;).&nbsp;
<h2 style="text-align: justify;">1. Let's talk about a and &alpha; spores -</h2>
Haploid cells of yeast produce a specific type of signal. These signals work as mating pheromones for the neighboring opposite cell type. &lsquo;a cells&rsquo; produce &lsquo;a factors&rsquo; which are received by &lsquo;&alpha; cells&rsquo; and &lsquo;&alpha; cells&rsquo; secrete &lsquo;&alpha; factors&rsquo; which are received by &lsquo;a cells&rsquo; by their surface receptors. These factors induce the opposite strain to move towards each other and fuse with each other. When the receptors receive their corresponding signals(a or &alpha; factors) the cell responds and thus a projection begins to form toward the signal source, this facilitates the fusion and finally results in the production of a diploid spore.&nbsp;
<img style="display: block; margin-left: auto; margin-right: auto;" src="https://vediphlie-aws.s3.ap-south-1.amazonaws.com/Yeast_life_cycle_through_a_haploid_and_a_diploid_phase_527646c2fb.PNG" alt="Yeast life cycle through a haploid and a diploid phase" />
Fig.: Yeast life cycle through a haploid and a diploid phase.
<h2 style="text-align: left;">2. Cell specificity in yeast -</h2>
Mating occurs only between different types of cells but not between cells of the same mating type. The difference between &lsquo;a and &alpha;&rsquo; is made due to the presence of different sets of genes that are actively expressed and suppressed in these two mating types. The different expression patterns of both mating types are due to the MAT locus which is located on chromosome III in yeast. &lsquo;MAT a&rsquo; and &lsquo;MAT &alpha;&rsquo; are two different alleles of MAT locus.
In haploid cells, there is only one allele of MAT is present (&lsquo;MAT a&rsquo; or &lsquo;MAT &alpha;&rsquo;) that determines their mating type. The signals which are received by the cell is responsible to start a signaling cascade. The &lsquo;&alpha; cell&rsquo; has &lsquo;MAT &alpha;&rsquo; locus, so in these cells two different proteins &alpha;1 and &alpha;2 expressed. &alpha;1 with MCM1 works as an activator of &alpha; specific proteins whereas &alpha;2 with MCM1 works to repress a specific gene. Haploid-specific genes are also expressed and some of them work as repressors of meiosis. So meiosis is blocked in haploid cells.
In 'a' type of cell &lsquo;MAT a&rsquo; is present. But both &alpha;1 and &alpha;2 proteins are absent. So in a cell, the MATa1 gene is expressed. Haploid-specific genes are also expressed and some of them work as repressors of meiosis. So meiosis is blocked in haploid cells. In a/&alpha; diploids possess both of the alleles of MAT locus (heterozygous for MAT locus). In these diploid cells, the expression of MAT&alpha;2 represses the &lsquo;a specific&rsquo; genes, and MAT&alpha;2 /MATa1 dimer is formed which represses the haploid-specific genes. Meiosis is not blocked in diploid cells. Diploid cells undergo meiosis and form haploid spores again. The &lsquo;a and &alpha;&rsquo; specific genes have loci for encoding mating factors, receptors, and proteins for cell fusion.&nbsp;
<img style="display: block; margin-left: auto; margin-right: auto;" src="https://vediphlie-aws.s3.ap-south-1.amazonaws.com/Regulation_of_cell_type_specificity_in_yeast_943e91a25b.PNG" alt="Regulation of cell-type specificity in yeast " />
Table: Regulation of cell-type specificity in yeast.&nbsp;
<h2 style="text-align: left;">3. Regulation of cell specificity in yeast -</h2>
The cell specification is a transcription control mechanism. Haploid cells express haploid-specific genes of their type. In a/&alpha; diploid cell, both &lsquo;a specific&rsquo; and &lsquo;&alpha; specific&rsquo; genes are silent and diploid-specific genes are expressed. General transcription factor MCM1 with cell-specific transcription factors &alpha;1, &alpha;2, and a1 encoded by MAT locus regulates the cell-type-specific transcription.
MCM is a member of MADS family protein. (MADS proteins are also involved in floral organs). MCM1 shows different activities in haploid &lsquo;a and &alpha;&rsquo; cells, it binds with &alpha;1 or &alpha;2 proteins.
<h2 style="text-align: justify;">4. Activation of a specific gene -</h2>
In the yeast chromosome III, yeast mating-type promoters have a specific sequence named P-BOX which possesses MCM binding site. MCM1 binds to the P-box dimeric format a specific upstream regulatory sequence (URS) and stimulates the transcription of a specific gene.
<h3 style="text-align: justify;">Activation of &lsquo;&alpha; specific&rsquo; genes: </h3>
The &lsquo;&alpha; specific&rsquo; genes have two adjacent DNA sequences P-BOX and Q-BOX in their promoter region. MCM1 binds to the P-BOX and &alpha;1 with MCM1 bind to the Q-BOX in &alpha; specific URS and starts transcription from the PQ site.
<h3 style="text-align: justify;">Gene repression: </h3>
In a specific URS two &alpha;2 binding sites are also present. The binding of MCM and &alpha;2 simultaneously repress the transcription of a specific gene. MCM directs the DNA binding domains of &alpha;2 dimer toward &alpha;2 binding sites on a specific URS. So MCM1 plays an important role to increase the affinity of the complex and thus play role in transcription regulation. If a number of &alpha;2 binding sites are present in the genome, it can regulate various protein expressions. In diploid cell &alpha;2 forms heterodimer with a1 which can repress both &alpha;1 and haploid-specific genes. The &lsquo;a specific&rsquo; genes are not expressed in &lsquo;&alpha; specific&rsquo; and diploid cells.
<h3 style="text-align: justify;">The signaling pathway that induces mating: </h3>
Cells follow the GPCR-FUS pathway. As mating factors bind to their receptors some proteins are expressed that cause cell cycle arrest in G1 and promote attachment and fusion of two haploid cells. Mating pheromones bind to their respective receptors and work, as ligands. Binding of the signals (&lsquo;a or &alpha;&rsquo;) to the receptor results in the activation of G protein by phosphorylation. Activated G protein dissociates from active G&beta;-G&gamma; subunits which phosphorylate the various &lsquo;Ste&rsquo; proteins like a cascade.&nbsp;
<img style="display: block; margin-left: auto; margin-right: auto;" src="https://vediphlie-aws.s3.ap-south-1.amazonaws.com/Switch_of_mating_type_6fe215493e.PNG" alt="Switch of mating type" />
Fig.: Switch of mating type.
<img style="display: block; margin-left: auto; margin-right: auto;" src="https://vediphlie-aws.s3.ap-south-1.amazonaws.com/Signal_transduction_12ab4eace5.PNG" alt=" Signal transduction" />
Activation of Ste4/Ste5&nbsp; &nbsp;⟶ &nbsp;Ste20&nbsp; ⟶ &nbsp;Ste11&nbsp; ⟶ &nbsp;Ste7
Ste 20, Ste 11, Ste 7 have MAP kinase activity. They phosphorylate the FUS3 protein. Phosphorylated FUS 3 starts the transcription of MAT locus.
FUS3 also activates the &lsquo;Ste 12&rsquo; protein which works as a transcription factor. Fus-3 with ste-12 binds to pheromone responsive element (PRE) and transcribes the proteins, involved in mating, cell arrest, cell pairing, and fusion.
Ste12 bind on PRE when MCM1 bind on P-BOX. The binding of Ste12 on PRE stimulates the function of MCM1 as an activator.
The signaling mechanism is the same in both mating types of cells but the difference is in the receptor. The 'a' cells bear the Ste receptor whereas &lsquo;&alpha; cells&rsquo; have the Ste3 receptor on their cell surface.
<h3 style="text-align: justify;">Mating switch:</h3>
In a colony of yeast where only one mating type is present (a type or &alpha; type), some cells switch their mating type and convert into opposite mating type (a to &alpha; or vice versa). This phenomenon is called mating switch yeast can find the opposite mating type by mating switch process.
So if there are haploid cells of a single mating type found in a yeast colony. Mating type switching will cause cells of both &lsquo;a and &alpha;&rsquo; mating types to be present in that population after some time.&nbsp;
<h2 style="text-align: justify;">5. Cassette model for the mating-type switch -</h2>
This conversion of mating-type become possible by a specific feature of yeast. The cell has two additional silent copies of the MAT locus in the chromosome. That is HML (Hidden MAT Left) which is a silent copy of MAT &alpha; and HMR (Hidden MAT Right) silent copy of MAT a locus. These copies are not transcribed. SIR proteins bind with both HML and HMR to form a heterochromatin scaffold and prevent the transcription of these regions. The HML and HMR are present near the heterochromatic region so cannot express themselves due to Position effect variegation. Only the active MAT locus is transcribed.
The cassette model of mating-type switch explains that switching is the result of a specific type of genetic recombination event which is initiated by cleavage of DNA sequence by HO endonuclease. HO gene is a haploidspecific gene (homothallic). Expression of this gene results in the production of HO endonuclease which cut the DNA at the MAT locus due to its specificity. As the endonuclease cut the DNA, the exonuclease recognizes the ends and cut all the MAT sequences. The gap generated is filled by the copying genetic information present at the HML or HMR. In this repair, a cell uses HML as a source of information.HML is just like MAT a locus. Thus HML locus is copied at the MAT locus. This converts the mating-type &lsquo;a to &alpha;&rsquo; type . &lsquo;&alpha; cells&rsquo; follow the same to convert in a type. So the expression of HO is crucial for the mating switch.&nbsp;
In yeast chromosome III Recombination Enhancers (RE) are present on the left arm of the chromosome. In 'a' strain cell MCM1 binds to the enhancer, causing recombination of HML. The cell repairs the gap using HML and switch the mating-type to &alpha;. Whereas in the case of &alpha; type &alpha;2 binds with RE thus RE suppressed and cell repair the gap by using HMR and convert &lsquo;&alpha;&rsquo; type to a type.&nbsp;
<img style="display: block; margin-left: auto; margin-right: auto;" src="https://vediphlie-aws.s3.ap-south-1.amazonaws.com/Mechanism_of_mating_switch_in_yeast_from_a_strain_to_a_strain_edaa42f81b.PNG" alt="Mechanism of mating switch in yeast from 'a' strain to 'a' strain" />
Fig. Mechanism of mating switch in yeast from '&alpha;' strain to 'a' strain.
&nbsp;

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In the Mating Switch topic of Developmental Biology, we will cover Regulation of cell specificity in yeast, Activation of a specific gene, Cassette model for the mating-type switch etc. 

Mating Switch - Cassette model for the Mating-type Switch

Mating occurs in different types of strains is also commonly found in this yeast. Yeast displays simple sexual differentiation, so mating is observed between two different haploid cells ‘a and α’ mating types. In the Mating Switch topic of Developmental Biology, we will cover a and α spores, Cell specificity in yeast, Regulation of cell specificity in yeast, Activation of a specific gene, Cassette model for the mating-type switch etc.  

In the Mating Switch topic of Developmental Biology, we will cover Regulation of cell specificity in yeast, Activation of a specific gene, Cassette model for the mating-type switch etc.  

Mating Switch 

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