Ectopic
recombination usually occurs frequently in S. cerevisiae meiosis,
sometimes as frequently as allelic recombination. This suggests that
single, initiating chromosome breaks always trigger an efficient,
genome-wide homology search mechanism resulting in homologous
recombination. We have shown that the rDNA locus ( RDN1 ) is an
unusually poor participant in inter chromosomal ectopic meiotic
recombination, due to sequestration from this homology search.
SIR2 , which represses intra chromosomal ectopic
recombination within RDN1 , also represses inter chromosomal
ectopic recombination involving RDN1 . We designed a system to
identify additional regions of the yeast genome that might be
sequestered from ectopic meiotic recombination. A recombination
substrate cassette carrying TRP1 and his3-621 was
constructed. This cassette is proficient at ectopic meiotic
recombination when inserted at ARG4 , yet displays sequestration
behavior at RDN1 . TRP1 his3-621 is proficient in ectopic
meiotic recombination when inserted at CUP1 , demonstrating that
sequestration is not a general property of naturally-occurring direct
repeats. TRP1-his3-621 was also introduced at many loci by
restriction enzyme-mediated illegitimate recombination. Several loci,
including RDN1 , were identified in a screen for poor participants
in ectopic meiotic recombination. We are characterizing the sites of
these insertions to determine the properties of recombinational
sequestration. Research sponsored by the National Cancer Institute,
DHHS, under contract with ABL.
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