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The small subunit processome in ribosome biogenesis—progress and prospects

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Abstract The small subunit (SSU) processome is a 2.2‐MDa ribonucleoprotein complex involved in the processing, assembly, and maturation of the SSU of eukaryotic ribosomes. The identities of many of the factors involved in SSU biogenesis have been elucidated over the past 40 years. However, as our understanding increases, so do the number of questions about the nature of this complicated process. Cataloging the components is the first step toward understanding the molecular workings of a system. This review will focus on how identifying components of ribosome biogenesis has led to the knowledge of how these factors, protein and RNA alike, associate with one another into subcomplexes, with a concentration on the small ribosomal subunit. We will also explore how this knowledge of subcomplex assembly has informed our understanding of the workings of the ribosome synthesis system as a whole. WIREs RNA 2011 2 1–21 DOI: 10.1002/wrna.57 This article is categorized under: RNA Interactions with Proteins and Other Molecules > RNA–Protein Complexes Translation > Ribosome Biogenesis RNA Processing > rRNA Processing

Saccharomyces cerevisiae pre‐rRNA processing scheme. In yeast, the 35S polycistronic precursor rRNA is processed to the mature 18S, 5.8(S+L), and 25S rRNAs. The colored boxes signify pre‐ribosomal particles: Blue, small subunit processome; purple, pre‐40S particles; red, pre‐60S particles. See text for details on processing steps. ETS, external transcribed spacer; ITS, internal transcribed spacer.

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Miller chromatin spreads, so‐called ‘Christmas trees’, can be used to visualize pre‐rRNA transcription and steps of pre‐rRNA processing. This diagram represents a Miller spread and indicates how the small subunit processome is assembled via subcomplexes into the pre‐rRNA transcript. See text for details. One repeat of the rDNA tandem chromosomal repeats is shown above the Miller chromatin spread diagram to indicate the relative positions of the 35S pre‐rRNA.

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X‐ray crystal structures can be docked into electron microscopy reconstruction volumes to give pseudo‐atomic resolution of large complexes. Shown here is a docking of the crystal structures of Pyrococcus furiosus Nop5‐fibrillarin [PDB 2nnw]146 and Methanococcus jannaschii L7Ae [1xbi]147 in the isodensity map of the M. jannaschii di‐sRNP [EMBD accession code EMD‐1636].144

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The U3 small nucleolar (sno)RNA base pairs with the pre‐18S rRNA to direct cleavages at A0, A1, and A2. The U3 snoRNA has been proposed to act as a chaperone for the formation of the conserved 5′ end pseudoknot by base‐pairing with the 18S rRNA. Colored lines correspond to different RNAs. Blue, U3 snoRNA; purple, mature 18S rRNA sequences; red, 5′ external transcribed spacer (ETS) of the pre‐rRNA that is cleaved during pre‐rRNA processing. See text for details.

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RNA Processing > rRNA Processing
RNA Interactions with Proteins and Other Molecules > RNA–Protein Complexes
Translation > Ribosome Biogenesis

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