Abstract
The nucleolus is the largest substructure in the nucleus, where ribosome biogenesis takes place, and forms around the nucleolar organizer regions (NORs) that comprise ribosomal RNA (rRNA) genes. Each cell contains hundreds of rRNA genes, which are organized in three distinct chromatin and transcriptional states—silent, inactive and active. Increasing evidence indicates that the role of the nucleolus and rRNA genes goes beyond the control of ribosome biogenesis. Recent results highlighted the nucleolus as a compartment for the location and regulation of repressive genomic domains and, together with the nuclear lamina, represents the hub for the organization of the inactive heterochromatin. In this review, we aim to describe the crosstalk between the nucleolus and the rest of the genome and how distinct rRNA gene chromatin states affect nucleolus structure and are implicated in genome stability, genome architecture, and cell fate decision.
Highlights
The nucleolus is the largest substructure in the nucleus, where ribosome biogenesis takes place, a process that is responsible for the assembly of the translational machinery, the ribosome, and is tightly regulated according to cell state
RRNA genes are generally present in high copy numbers and arranged in arrays of tandem repeats among different chromosomes at regions called nucleolar organizer regions (NORs). ribosomal RNA (rRNA) genes play a crucial role in ribosome biogenesis, a highly coordinated process regulated by a myriad of factors. rRNA gene transcription generates 45S/47S pre-rRNA that is modified and processed to form 28S, 18S and
We described the distinct rRNA gene classes and their role in nucleolar transcription with a special focus on the mechanisms regulating the formation of silent rRNA repeats and their impact in genome stability and architecture
Summary
The nucleolus is the largest substructure in the nucleus, where ribosome biogenesis takes place, a process that is responsible for the assembly of the translational machinery, the ribosome, and is tightly regulated according to cell state. RRNA genes can be subdivided in three major classes according to the transcriptional state and chromatin and epigenetic features: silent, inactive (or pseudogenes), and active genes (Figure 1). The structure of inactive genes can be mediated by the nucleosome remodeling and non-transcribed repeats that lack promoter DNA methylation, are nucleosome-packed at the coding deacetylase (NuRD) complex, the energy-dependent nucleolar silencing complex (eNOSC) or other yet region, and are not bound by UBF or NoRC. Genes dowith not the gene body, spacer promoter and enhancer repeats, allowing the formation of the interact with UBF and belong to the nucleosome-packed rDNA chromatin as in the case of silent rRNA pre-initiation complex (PIC) with subsequent RNA Pol I loading and rRNA transcription [33]. Detailed information about the structure and regulation of active and inactive genes can be found in recent reviews [9,58]
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