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Cotranscriptional events in eukaryotic ribosome synthesis

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Eukaryotic ribosomes are synthesized in a complex, multistep pathway. This begins with transcription of the rDNA genes by a specialized RNA polymerase, accompanied by the cotranscriptional binding of large numbers of ribosome synthesis factors, small nucleolar RNAs and ribosomal proteins. Cleavage of the nascent transcript releases the early pre‐40S and pre‐60S particles, which acquire export competence in the nucleoplasm prior to translocation through the nuclear pore complexes and final maturation to functional ribosomal subunits in the cytoplasm. This review will focus on the many and complex interactions occurring during pre‐rRNA synthesis, particularly in budding yeast in which the pathway is best understood. WIREs RNA 2015, 6:129–139. doi: 10.1002/wrna.1263 This article is categorized under: Translation > Ribosome Biogenesis RNA Processing > rRNA Processing
Structure of the yeast rDNA and pre‐rRNA. (a) Structure of the rDNA locus, which consists of a single tandem array of 150–200 repeats of 9.8 kb rDNA units located on chromosome XII. Within each repeat the 35S pre‐rRNA is transcribed by RNA Pol I, whereas the 5S rRNA is transcribed in the opposite direction by RNA Pol III. The Pol I transcription units are separated by intergenic spacer (IGS) regions that are transcribed by RNA Pol II into noncoding RNAs (ncRNAs). (b and c) Schematic of transcription by Pol I. The nascent transcripts are cotranscriptionally packaged by the small subunit (SSU) processome, and a less well characterized, large subunit (LSU) packaging complex.
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Cotranscriptional events in ribosome synthesis. Schematic showing the sequence of loading of the Utp complexes and other components of the large SSU processome, and the cotranscriptional cleavages at A2 and B0 that release the pre‐40S and pre‐60S particles, respectively. Insets indicate the multiple binding sites identified for (a) U3 snoRNA and (b) Rrp5, which are likely to contribute to pre‐rRNA compaction. See Table for composition of SSU processome subcomplexes.
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Pre‐rRNA processing pathway. (a) Schematic of the 35S pre‐rRNA showing the processing sites (a–e). (b) Simplified overview of yeast pre‐rRNA processing. RNA processing enzymes acting at specific steps are indicated in red.
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RNA Processing > rRNA Processing
Translation > Ribosome Biogenesis

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