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LECTURE2Bdunn - Regulation of Eukaryotic Gene expression II...

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Regulation of Eukaryotic Gene expression II The role of chromatin structure in the gene control Readings: Ch 11 pages 401-412
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Genetic discovery of chromatin mediated gene silencing: The bizarre case of the Yeast Mating Type Switch Haploid yeast either α or a mating type Haploid mating type can switch α to a , or a to α For mating fusion, only α to a allowed, a cannot mate with a , nor α with α . Diploid α /a does not mate at all. α cells secrete α factor that binds to specific receptor on a cells to signal mating phenotype a cells secrete a factor that binds to specific receptor on α cells. Control of mating type is at the Mating Type Locus MAT
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Control of yeast mating type: MAT, HML, HMR on ChrIII α a α or a Silent Silent Expressed Gene conversion
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Mating-type switching is controlled by recombination of DNA cassettes Figure 11-22 α mating type
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Both α and a cassettes have functional promoter sequences, yet they only drive gene expression when at the MAT locus, but are not active at the HML or HMR sites. What is blocking gene expression at HML and HMR? Experimental approach to this question was genetics. Yeast cells were treated with a mutagenic agent and then screened for mutants that had activated expression at HML and HMR.
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Genetic screen to identify genes that when mutant fail to silence HML and HMR These genes call SIRs, silent information regulators Four SIR genes identified, called SIR1-4 SIR2 gene encodes histone deacetylase enzyme SIR3 gene encodes protein that binds deacetylated tails of H3 and H4 SIR4 gene encodes protein also binds deacetylated tails of H3 and H4 and associates with other silencer proteins SIR1 gene encodes protein that promotes assembly of other SIR proteins at the silencer sites
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