LOCALIZATION OF SIR2P - THE NUCLEOLUS AS A COMPARTMENT FOR SILENT INFORMATION REGULATORS

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.