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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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- Spring '08