Net1, a Sir2-associated nucleolar protein required for rDNA silencing and nucleolar integrity

The Sir2 protein mediates gene silencing and repression of recombination at the rDNA repeats in budding yeast. Here we show that Sir2 executes these f unctions as a component of a nucleolar complex designated RENT (regulator o f nucleolar silencing and telophase exit). Net1, a core subunit of this com plex, preferentially cross-links to the rDNA repeats, but not to silent DNA regions near telomeres or to active genes, and tethers the RENT complex to rDNA. Net1 is furthermore required for rDNA silencing and nucleolar integr ity. During interphase, Net1 and Sir2 colocalize to a subdomain within the nucleolus, but at the end of mitosis a fraction of Sir2 leaves the nucleolu s and disperses as fool throughout the nucleus, suggesting that the structu re of rDNA silent chromatin changes during the cell cycle. Our findings sug gest that a protein complex shown to regulate exit from mitosis is also inv olved in gene silencing.