Pol II-transcribed marker genes integrated within the
rDNA of Saccharomyces cerevisiae are repressed by a phenomenon
known as 'rDNA silencing'. SIR2 is the only Silent Information
Regulator gene required for rDNA silencing, suggesting that it is
related to, but different from silencing associated with telomeres and
the HM loci. SIR4 deletion paradoxically strengthened rDNA
silencing, suggesting SIR4 may be a regulatory gene. We have now
further characterized the modulation of rDNA silencing by SIR4
and found that its rDNA effects are mediated through a limiting pool of
Sir2p. rDNA silencing was eliminated by SIR4 overexpression, but
restored by co-overexpression with SIR2 . Manipulations of
SIR4 shown by others to promote cellular longevity and
redistribute Sir proteins from telomeres to the nucleolus, strengthened
rDNA silencing in our reporter strains. These manipulations include
SIR4-42 and overexpression of the extreme C-terminus of Sir4p.
Furthermore, mutations in several genes which lengthen telomeres, weaken
rDNA silencing. We conclude that SIR4 regulates rDNA silencing
strength by modulating the distribution of Sir2p between the rDNA and
competing telomeres. To identify non- SIR genes which participate
in rDNA silencing, we have performed a genetic screen for mutants which
alter the strength of rDNA silencing. The mutants were separated into
two groups, loss of rDNA silencing ( lrs ) and increased rDNA
silencing ( irs ). Representatives from both groups will be
presented.
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