The epigenetics of the nucleolus: Structure and function of active and silent ribosomal RNA genes

Abstract

To produce an elevated number of ribosomes, cells have to achieve synthesis of large amounts of ribosomal RNA (rRNA). To do this, a unique and efficient transcription system evolved by using a specific and efficient RNA polymerase (RNA polymerase I, Pol I) and by amplifying the number of rRNA genes to hundreds or even thousands of copies per genome. Concomitant with this process, a third mechanism has evolved to keep a large percentage of those rRNA genes in a silent state. In this review, I summarize current knowledge about structure and function of active and silent rRNA genes in eukaryotes. I explore what is known about the composition of these two classes of rRNA genes with a particular focus on chromatin structure and epigenetics. I discuss also the mechanisms that account for establishment and inheritance of active and silent rRNA genes through cell generation. Finally, I discuss emerging themes and highlights of the role of silent rRNA genes: whether they represent a reservoir for the cells to draw on in case of elevated ribosome demands or if their role goes beyond the ribosome factory. © 2011 Springer Science+Business Media, LLC.