Abstract
Gene expression control is a fundamental determinant of cellular life with transcription being the most important step. The spatial nuclear arrangement of the transcription process driven by RNA polymerases II and III is nonrandomly organized in foci, which is believed to add another regulatory layer on gene expression control. RNA polymerase I transcription takes place within a specialized organelle, the nucleolus. Transcription of ribosomal RNA directly responds to metabolic requirements, which in turn is reflected in the architecture of nucleoli. It differs from that of the other polymerases with respect to the gene template organization, transcription rate, and epigenetic expression control, whereas other features are shared like the formation of DNA loops bringing genes and components of the transcription machinery in close proximity. In recent years, significant advances have been made in the understanding of the structural prerequisites of nuclear transcription, of the arrangement in the nuclear volume, and of the dynamics of these entities. Here, we compare ribosomal RNA and mRNA transcription side by side and review the current understanding focusing on structural aspects of transcription foci, of their constituents, and of the dynamical behavior of these components with respect to foci formation, disassembly, and cell cycle.
Highlights
IntroductionProper regulation of gene expression is of utmost importance to ensure maintenance of cell integrity, coordinated cellular differentiation, and adequate responses to external and internal cues
Proper regulation of gene expression is of utmost importance to ensure maintenance of cell integrity, coordinated cellular differentiation, and adequate responses to external and internal cues.Nuclear transcription is exerted by three RNA polymerases, RNA polymerase I (Pol I) exclusively dedicated to transcription of ribosomal RNA, RNA polymerase II (Pol II) transcribes mRNA and noncoding RNAs, and RNA polymerase III (Pol III) produces tRNAs and 5S rRNA.With the introduction of labeled nucleosides that become incorporated into nascent RNA, it was realized that nuclear transcription does not occur randomly distributed throughout the nuclear volume but rather is found in discrete regions within the nuclear space (Haeusler and Engelke 2006; Jackson et al 1993; Wansink et al 1993)
It was shown that genes tend to share transcription factories, that co-regulated genes often occupy one and the same transcription factory, and that specialized transcription factories exist for certain genes
Summary
Proper regulation of gene expression is of utmost importance to ensure maintenance of cell integrity, coordinated cellular differentiation, and adequate responses to external and internal cues. The authors could demonstrate that upon stimulation of transcription, the lifetime of transcription factories increased significantly This is consistent with an earlier observation on the highly transcribed globin genes in erythroid cells where the authors found that these genes were active in a near-continuous expression mode, whereas other genes studied displayed marked on–off periods (Osborne et al 2004). The insert did not contain the Pol I promoter sequence and did not form functional nucleoli, Pol I was recruited by UBF This group went on to produce “neo-NORs” (Grob et al 2014) which contain fully functional ectopic rDNA repeats which actively produce ribosomes and some of these NORs fuse with endogenous ones, suggesting the existence of a “NOR-territory” (Grob et al 2014). In the case of nucleoli, biophysical forces and self-assembly have been postulated as contributing factors for nucleolus formation and maintenance [reviewed in (Hancock 2014; Lam and TrinkleMulcahy 2015)]
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