Ribosomal RNA (rRNA) genes arrange themselves in a tandem pattern in nucleolus and during the transcription of rRNA genes, the elongating nascent rRNA transcripts create a structure called Christmas tree. rRNA genes in the rDNA locus can be either active or silent depending on the epigenetic regulation of the chromatin structure. Yeast Sir2 (silent information regulator 2) protein containing complexes can repress the recombination in the FDNA locus and subsequently extend the replicative lifespan of the budding yeast. The mammalian rDNA locus is also under the epigenetic regulation by protein complexes, such as NoRC (nucleolar remodeling complex) and eNoSC (energy-dependent nucleolar silencing complex), involving histone deacetylases and methyltransferases. SIRT1, a NAD(+)-dependent histone deacetylase, is the key component in the eNoSC complex and hence energetic changes can regulate the activation of eNoSC complex and in this way mediate the epigenetic silencing of rRNA gene expression. The eNoSC complex links SIRT1-induced longevity regulation to the metabolic rate theory of aging. (C) 2008 Elsevier Inc. All rights reserved.