Project: Research project

Project Details


The mating type loci in Saccharomyces cerevisiae provide a striking
example of position effect regulation. Mating type regulatory genes
located at the ends of chromosome III of yeast in loci, designated HML
and HMR, are repressed by a novel regulatory system comprised of
"silencers" that mediate the action of products of four unlinked genes,
SIR1 through SIR4, to render DNA in the vicinity of the silencers
generally refractory to transcription. We have shown that DNA spanning
the silent cassettes is packaged in nucleosomes that are hypoacetylated
relative to those in active portions of the genome and that this
hypoacetylation is catalyzed by the products of the SIR genes themselves.
In addition, we have defined the salient features of the silencers
located at HML and have shown that they act in tandem probably to direct
chromatin condensation across the silent cassettes. We propose (1) to
use hypoacetylation as a tag to define the extent and organization of
silenced chromatin and its relationship to the structure and orientation
of the silencers; (2) to examine the role of histones H1 and H5 homologs
in silencing and assess the facility with which chromatin from silenced
regions can undergo transition into a condensed state; (3) to identify,
clone and characterize novel genes involved in silencing.

We also propose to examine the molecular mechanism underlying
directionality of mating type interconversion in yeast. Activation of
information within the two silent mating cassettes occurs by directed
transposition to an expressor locus MAT. Selection of the donor cassette
during any one transposition is not random but is dictated by the mating
allele present at MAT. This yields a precisely patterned order for
mating type interconversion. We plan to examine the mechanism underlying
this directionality of mating type interconversion by identifying cis and
trans components required for selection of the donor locus, through (1)
characterization of trans-acting mutants we previously isolated that have
lost the normal bias in selecting mating type cassettes; and (2)
interchanging HML and HMR cassettes along with varying extents of their
flanking sequences in order to identify sites used by the cell to
distinguish the two loci.
Effective start/end date1/1/9712/31/97


  • National Institute of General Medical Sciences

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