In wild-type budding yeast strains, the proteins encoded by SIR3, SIR4
and RAP1 co-localize with telomeric DNA in a limited number of foci i
n interphase nuclei. Immunostaining of Sir2p shows that in addition to
a punctate staining that coincides with Rap1 foci, Sir2p localizes to
a subdomain of the nucleolus. The presence of Sir2p at both the space
r of the rDNA repeat and at telomeres is confirmed by formaldehyde cro
ss-linking and immunoprecipitation with anti-Sir2p antibodies. In stra
ins lacking Sir4p, Sir3p becomes concentrated in the nucleolus, by a p
athway requiring SIR2 and UTH4, a gene that regulates life span in yea
st. The unexpected nucleolar localization of Sir2p and Sir3p correlate
s with observed effects of sir mutations on rDNA stability and yeast l
ongevity, defining a new site of action for silent information regulat
ory factors.