snoRNPs: Functions in Ribosome Biogenesis

Figures

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  Figure 1. Box C/D snoRNPs. (A) Box C/D snoRNPs catalyze the methylation of the 2′ hydroxyl of RNA. This is thought to reduce the hydrophilic nature of the nucleotides and allow rRNA to be buried inside of the ribosome. (B) Secondary structure of a typical Box C/D snoRNA indicating the location of Box C/C′ (blue), Box D/D′ (green), and hybridized rRNA (red). Location of methylation is denoted as 5′ bps from Box D/D’. (C) Assemblage of protein factors on the snoRNA illustrates that SNU13 binds the K-turns which positions FBL at the site of methylation.

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  Figure 2. Box H/ACA snoRNPs. (A) Isomerization uridine to pseudouridine is catalyzed by Box H/ACA snoRNAs. This generates additional hydrogen bonding capacity that aids in maintaining the ribosome structure. (B) Secondary structure of the Box H/ACA snoRNAs, indicating the location of the hinge (H) box (green), ACA box (blue), and hybridized rRNA (red). Pseudouridylation occurs in the “pseudouridylation pocket” at the base of the hairpins. (C) Assemblage of protein factors on the snoRNA.

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  Figure 3. SnoRNA processing pathways. SnoRNAs are either independently transcribed or intronically encoded. (A) Independently transcribed snoRNAs exist as mono- or polycistronic units that are transcribed by RNA polymerase II or III. A trimethylguanosine (m2,2,7G) cap can be added or the 5′ end can be cleaved off. The 3′ end processing is required for final maturation of both capped and uncapped snoRNAs. (B) Intronically encoded snoRNAs are transcribed by RNA polymerase II from the promoters of their host genes. The host gene can encode a functional protein or a noncoding transcript that is often degraded by nonsense-mediated decay due to the presence of a premature termination codon (PTC). Maturation of the snoRNA requires mRNA splicing and debranching of the snoRNA-containing intron followed by exonucleolytic trimming. We presented Box C/D snoRNAs, but Box H/ACA snoRNAs undergo the same maturation pathway.

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  Figure 4. Simplified pathway of rRNA processing. (A) Yeast and (B) human rRNAs are generated from long polycistronic precursor molecules that must be processed to generate the mature rRNAs. This represents a simplified rRNA processing pathway in each organism, based on [94]. Note that there are minor pathways that are omitted from this schematic.

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  Figure 5. U3 snoRNP. (A) Secondary structure of the U3 snoRNA with protein components. Note that SNU13 still binds k-turns, here formed by Box C′ (blue) and Box D (green) and Box C′ (blue) and Box B (Red). The altered structure of the U3 snoRNA results in FBL not being positioned for 2′-O- methylation. (B) The 5′ extension forms multiple base-pairs with the pre-rRNA. Two different stretches in the 5′ETS (green) and the 18S rRNA (blue) are bound. Dashed lines indicate looped-out stretches of rRNA. Numbers indicate position in the 47S rRNA (green) or 18S rRNA (blue). Note that this schematic does not fully represent looping out. Secondary structure modified from predictions on snoRNABase [95].