Enforcement of a lifespan-sustaining distribution of Sir2 between telomeres, mating-type loci, and rDNA repeats by Rif1.

Telomere dysfunction is linked with genome instability and premature aging. Roles for sirtuin proteins at telomeres are thought to promote lifespan in yeast and mammals. However, replicative lifespan of the budding yeast Saccharomyces cerevisiae shortens upon deletion of Rif1, a protein that limits the recruitment of the sirtuin histone deacetylase Sir2 to telomeres. Here we show that Rif1 maintains replicative lifespan by ultimately stabilizing another age-related chromosomal domain harboring the ribosomal DNA (rDNA) repeats. Deletion of Rif1 increases Sir2 localization to telomeres and the silent mating-type loci, while releasing a pool of the histone deacetylase from the intergenic spacer 1 (IGS1) of rDNA. This is accompanied by a disruption of IGS1 silent chromatin assembly and increases in aberrant recombination within rDNA repeats. Lifespan defects linked with Rif1 deletion are abolished if rDNA repeats are forcibly stabilized via deletion of the replication fork-blocking protein Fob1. In addition, Sir2 overexpression prevents Rif1 deletion from disrupting Sir2 at IGS1 and shortening lifespan. Moreover, subjecting cells lacking Rif1 to caloric restriction increases IGS1 histone deacetylation and lifespan, while uncovering novel genetic interactions between RIF1 and SIR2. Our data indicate that Rif1 maintains lifespan-sustaining levels of Sir2 at rDNA by preventing excessive recruitment of the histone deacetylase to telomeric and silent mating-type loci. As sirtuin histone deacetylases, such as Sir2 or mammalian SIRT6, each operate at multiple age-related loci, we propose that factors limiting the localization of sirtuins to certain age-related loci can promote lifespan-sustaining roles of these sirtuins elsewhere in the genome.