A screen for mutations that
increase the Ty1 transposition rate revealed that simultaneous
inactivation of a histone transcriptional regulator (Hir3) and a CAF-I
subunit (Cac3) significantly increased the Ty1 transposition rate,
caused sensitivity to MMS and reduced telomeric silencing and growth
rate. Other combinations of hir and cac mutations caused
similar effects, but single hir or cac mutations did not.
Ordinarily, Ty1 elements prefer to integrate into 5', rather than
downstream, regions of PolII transcribed genes. We studied this promoter
bias using a simple genetic test, verified by PCR, to identify Ty1
integration-site positions within a MET3-URA3 fusion.
Transcriptional activation and repression of the MET3-URA3 fusion
had no effect on Ty1 target-site preference in CAC HIR strains.
However, dramatic effects on target-site distribution were noted in a
cac3 hir3 double mutant whether the fusion was on or off, and in
hir3 single mutants only when the fusion was on. Simultaneous
disruption of CAC3 HIR3 also altered transposition rate and
target-site specificity of Ty1 elements into CAN1 . Effects of
cac3 hir3 on nucleosome ladders will be described. Since
mutations in RAD6 also caused alterations in Ty1 transposition
and silencing, other rad and sir mutations were tested for
effects on Ty1 transposition rate and target-site specificity, but no
dramatic effects were observed. The data imply that changes in chromatin
structure can affect retrotransposition. Since Ty1-like elements and
CAF-I are both found in higher eukaryotes, these results in yeast may
have a wider relevance. Supported by NIH GM50365.
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