Saccharomyces_A

Rna polymerases transcription factors and the ho

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Unformatted text preview: yeast contribute in a complex way to the configuration of the chromatin structure in the vicinity of each storage locus. RNA polymerases, transcription factors, and the HO endonuclease cannot gain access to these regions. As a result, the HO endonuclease cannot make double-stranded cuts at the storage loci. The silencing effect is general: Genes from other chromosomes that are normally active become transcriptionally silent when inserted into a storage locus. Are there gene products that function to keep these storage loci silent? Mutational analyses show that the answer is yes. Loss-of-function mutations isolated in four 11 SIR (silent information regulator) genes cancel repression and result in the transcriptional activation of the 1, 2, or a1 genes located at HML or HMR. In haploid cells, transcription of the storage loci results in a phenotype resembling that of an a/ diploid because both MATa and MAT alleles are expressed. Moreover, in sir strains, the HO enzyme cleaves the chromosome at both MAT and the storage loci. Depending on which site is...
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