8.5.         Cassette model for mating type switch:

This conversion of mating type become possible by a specific feature of yeast. The cell has two additional silent copies of MAT locus in a chromosome. That is HML (Hidden MAT Left) which is the silent copy of MAT α and HMR (Hidden MAT Right) silent copy of MAT a locus. These copies are not transcribed. SIR proteins bind with both HML and HMR form a heterochromatin scaffold and prevent the transcription of these regions. The HML and HMR are present near the heterochromatic region so cannot express itself 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 the specific type of genetic recombination event which is initiated by cleavage of DNA sequence by HO endonuclease. HO gene is a haploid-specific 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 endonuclease cut the DNA, the exonuclease recognizes the ends and cut all the MAT sequence. The gap generated 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 MAT locus. This converts the mating type ‘a to α’ type. ‘α cells’ follow the same to convert in a type. So the expression of HO is crucial for the mating switch.

In yeast chromosome III Recombination Enhancers (RE) present on the left arm of the chromosome. In 'a' strain cell MCM1 binds to the enhancer, cause recombination of HML. Cell repairs the gap using HML and switches mating type to α. Whereas in case of α type α2 binds with RE thus RE suppressed and cell repair the gap by using HMR and convert ‘α’ type to ‘a’ type.

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