Different epigenetic layers engage in complex crosstalk to define the epigenetic state of mammalian rRNA genes

摘要

Eukaryotic cells contain several hundred ribosomal RNA (rRNA) genes (rDNA), a fraction of them being silenced by epigenetic mechanisms. The presence of two epigeneticatly distinct states of rRNA genes provides a unique opportunity to decipher the molecular mechanisms that establish the euchromatic, i.e. tran-scriptionally active, and the heterochromatic, i.e. transcriptionally silent, state of rDNA. This article summarizes our knowledge of the epigenetic mechanisms that control rDNA transcription and emphasizes how DNA methyItransferases and histone-modifying enzymes work in concert with chromatin-remodeling complexes and RNA-guided mechanisms to establish a specific chromatin structure that defines the tran-scriptional state of rRNA genes. These studies exemplify the mutual dependence and complex crosstalk among different epigenetic players in the alteration of the chromatin structure during the process of gene activation or silencing.