Nucleolus and chromatin

Abstract

The nucleolus as site of ribosome biogenesis holds a pivotal role in cell metabolism. It is composed of ribosomal DNA (rDNA), which is present as tandem arrays located in nucleolus organizer regions (NORs). In interphase cells, rDNA can be found inside and adjacent to nucleoli and the location is indicative for transcriptional activity of ribosomal genes—inactive rDNA (outside) versus active one (inside). Moreover, the nucleolus itself acts as a spatial organizer of non-nucleolar chromatin. Microscopy-based approaches offer the possibility to explore the spatially distinct localization of the different DNA populations in relation to the nucleolar structure. Recent technical developments in microscopy and preparatory methods may further our understanding of the functional architecture of nucleoli. This review will attempt to summarize the current understanding of mammalian nucleolar chromatin organization as seen from a microscopist’s perspective.

In focus in HCB

“…The review by Schöfer and Weipoltshammer (2018) on Nucleolus and chromatin summarizes the current understanding of mammalian nucleolar chromatin organization as seen mainly from a microscopist's perspective. It starts with a historical synopsis which is followed by an in-depth analysis of the structure of human rDNA, the morphology and positioning of the nucleolus and perinucleolar chromatin, and aspects of the organization and epigenetic regulation of rRNA transcription.…”

Multiple epigenetic layers accompany the spatial distribution of ribosomal genes in Arabidopsis

“…Some studies suggest that this region is less associated with nucleosomes or even devoid of them compared to the inactive rDNA genes or intergenic spacers (IGS) (Zentner et al, 2011). Others demonstrated a nucleosome shift at the promoter site regulating rDNA transcription (Li et al, 2006;Zhao et al, 2016) and the presence of non-nucleosomal complexes, summarised in (Schofer & Weipoltshammer, 2018).…”

Promotion of hyperthermic-induced rDNA hypercondensation inSaccharomyces cerevisiae

“…Alterations in rDNA structure and function have implications far beyond the canonical roles of the nucleolus in rDNA transcription and ribosome biogenesis (Tiku and Antebi, 2018; Schöfer and Weipoltshammer, 2018; Kobayashi and Sasaki, 2017). For instance, rDNA is the most highly represented gene in any eukaryote and also the most heavily transcribed locus (accounting for over 60% of the entire RNA pool) (Tomson et al, 2006).…”

“…Clinically, disruption of rDNA function in humans results in neurodegeneration, tumorigenesis and severe developmental defects that include Treacher-Collins Syndrome, Blackfan Anemia, CHARGE Syndrome and several others (Danilova and Gazda, 2015; Udugama et al, 2018; Wang and Lemos, 2017; Xu et al, 2014; Xu et al, 2017; Hallgren et al, 2014). Given that a rather surprisingly small percentage of nucleolar proteins function in ribosome biogenesis (Tiku and Antebi, 2018; Schöfer and Weipoltshammer, 2018), it becomes critical to explore the regulatory mechanisms through which rDNA responds to the many challenges imposed on the cell to ensure proper development and cell cycle regulation.…”