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Three genetic loci on chromosome III genetically control the mating type of Saccharomyces cerevisiaeoUnder normal conditions, Saccharomyces cerevisiaewill exist in haploid form and will be one of two possible mating types: HMLα and HMRaThe HMLα and HMRa loci must be silenced otherwise the cells will be diploid a/α and cannot mate oEverytime a mother yeast cell buds, the daughter cell will take on a completely different mating type = mating type switching oUnder bad growing conditions for the yeast, they form diploids and have to mate –they can only mate with the other mating type = reason why mate switching takes place oThe mating type is expressed in the mating type locus on a chromosome in the yeast oEvery yeast has the capacity to be either a or αoHappens on the same chromosome but at distal ends (there are silenced) oWhen mating type switching takes place, regions have to be recombined and one of them is expressed oSome mutant yeast have both mating types suggesting that something is faulty in silencing oMutants that allow for the desilencing of these loci are present in the histones oCondense chromatin so enzymes that interact with DNA can no longer access the DNA because they are physically impeded by the higher order chromatin structures conferred by these regions Transcriptional repression depends on silencer sequences. This silencer works outside the context of mating and can even block expression of tRNA genes (RNA Pol III)
Genetic experiments indicate that histones (H3 & H4) affect repression, while regions around telomeres behave similarly Genetic screens identified factors required for repression of the silent mating type loci RAP1 oBinds to DNA in the region of the silencer oAlso binds to repetitive sequence in telomeres SIR1 oSilent Information Regulator oCooperates with RAP1 and is important for binding the silencer region in the silent mating type loci oChange chromatin configuration around those regions SIR2, 3, 4 oBind to hypoacetylated histone tails (H3, H4) and recruits SIR2 oSIR 3 and SIR 4 interacts with RAP 1 oSIR 2 interacts with SIR 4 oSIR 2 has enzymatic activity that allows it to deacetylate histone tails oOnce it sees RAP 1 scaffold, SIR 2 and SIR 4 will join that complex and deacetylate histone tails oForms large complexes with telomeric DNA oPositive charges on histones, negative charges on DNA Transcriptional repressors may act through histone deacetylation complexes (HDACs) Positive charge of N-terminal histone tail interacts electrostatically with the DNA phosphate groups Transcriptional pre-initiation complex cannot form on TATA boxes within regions of condensed chromatin (hypoacetylated) Acetylation neutralizes the electrostatic interaction and permits complex formation Sin3 is conserved and in a large complex with rpd3 Rpd3p is required for repressing some genes in yeast. It has substantial homology to a histone deacetylase and demonstrates deacetylase activity o