Spontaneous rDNA copy number variation modulates Sir2 levels and epigenetic gene silencing.
We show that in budding yeast large rDNA deletions arise frequently and cause an increase in telomeric and mating-type gene silencing proportional to repeat loss. Paradoxically, this increase in silencing is correlated with a highly specific down-regulation of SIR2, which encodes a deacetylase enzyme required for silencing. These apparently conflicting observations suggest that a large nucleolar pool of Sir2 is released upon rDNA loss and made available for telomeric and HM silencing, as well as down-regulation of SIR2 itself. Indeed, we present evidence for a reduction in the fraction of Sir2 colocalizing with the nucleolar marker Nop1, and for SIR2 autoregulation. Despite a decrease in the fraction of nucleolar Sir2, and in overall Sir2 protein levels, short rDNA strains display normal rDNA silencing and a lifespan indistinguishable from wild type. These observations reveal an unexpectedly large clonal variation in rDNA cluster size and point to the existence of a novel regulatory circuit, sensitive to rDNA copy number, that balances nucleolar and nonnucleolar pools of Sir2 protein.
Michel
Agnès H
AH
Department of Molecular Biology and NCCR Program "Frontiers in Genetics", University of Geneva, Sciences III, 30, quai Ernest-Ansermet, CH-1211, Geneva 4, Switzerland.
Kornmann
Benoît
B
Dubrana
Karine
K
Shore
David
D
D004271:DNA, Fungal / Q000378:metabolism*; D004275:DNA, Ribosomal / Q000378:metabolism*; D015536:Down-Regulation / Q000502:physiology*; D044127:Epigenesis, Genetic; D018628:Gene Dosage; D020869:Gene Expression Profiling; D015966:Gene Expression Regulation, Fungal / Q000502:physiology; D020868:Gene Silencing / Q000502:physiology*; D006655:Histone Deacetylases / Q000235:genetics / Q000502:physiology*; D006706:Homeostasis / Q000235:genetics / Q000502:physiology; D009154:Mutation; D009687:Nuclear Proteins / Q000378:metabolism; D020411:Oligonucleotide Array Sequence Analysis; D020718:Ribonucleoproteins, Small Nucleolar / Q000378:metabolism; D029701:Saccharomyces cerevisiae Proteins / Q000378:metabolism; D038281:Silent Information Regulator Proteins, Saccharomyces cerevisiae / Q000235:genetics / Q000502:physiology*; D056565:Sirtuin 2; D037761:Sirtuins / Q000235:genetics / Q000502:physiology*; D013045:Species Specificity; D016615:Telomere / Q000502:physiology
D004271:DNA, Fungal; D004275:DNA, Ribosomal; C064793:NOP1 protein, S cerevisiae; D009687:Nuclear Proteins; D020718:Ribonucleoproteins, Small Nucleolar; D029701:Saccharomyces cerevisiae Proteins; D038281:Silent Information Regulator Proteins, Saccharomyces cerevisiae; C087778:SIR2 protein, S cerevisiae; D056565:Sirtuin 2; D037761:Sirtuins; D006655:Histone Deacetylases