snoRNAs: Biogenesis, Structure and Function


The small nucleolar (sno) ribonucleic acids (RNAs) are mainly involved in the modification and processing of ribosomal RNA precursors, however, some are used to modify small nuclear RNAs (snRNAs), messenger RNAs (mRNAs) and transfer RNAs (tRNAs). They function in the form of ribonucleoproteins (snoRNPs).

Keywords: nucleolus; RNA modification; RNA processing; RNA–protein interactions

Figure 1.

Structure of guide snoRNAs and mode of action. The schematic secondary structure of snoRNAs are presented on the left; the blue line represents the target RNA that gets modified. The conserved sequences are boxed and indicated in colour. Models for the selection of specific position to be modified are shown on the right. In (a), box C/D snoRNAs are characterized by conserved motifs C and D (green) that form a kink‐turn (K‐turn) or alternate boxes C′ and D′ (orange) that could also form a K‐turn; 2′‐O‐ribose methylation is performed on the rRNA residue that is base‐paired to the fifth position upstream from box D (or D′). In (b), H/ACA snoRNAs adopt a hairpin‐hinge‐hairpin‐tail structure where box H (red) is found in the hinge region and box ACA (red) is found three nucleotides upstream of the 3′‐ end; each hairpin usually contains an internal loop called pseudouridylation pocket where Ψ formation in rRNA occurs on the first unpaired U residue upstream from box H or ACA (usually at a distance of 14 to 16 nucleotides from the box).

Figure 2.

Pre‐rRNA modification and processing. The large pre‐rRNAs encode the small ribosomal subunit rRNA (18S) and the large ribosomal subunit rRNAs (5.8S and 25–28S). The coding sequences are separated by spacer sequences named 5′ETS (5′ external transcribed spacer), ITS1 (the internal transcribed spacer 1), ITS2 (internal transcribed spacer 2), and 3′ETS (3′ external transcribed spacer). The guide snoRNPs are involved in modification reactions; C/D snoRNPs direct 2′‐O‐ribose methylation (Me), and H/ACA snoRNPs direct pseudouridylation (Ψ). The processing snoRNPs (represented by spheres) are involved in cleavage reactions. Cleavages are likely orchestrated by a large complex containing many snoRNPs, which is referred to as the ‘processome’ (by analogy with the spliceosome). The transcribed spacers are removed by a series of endo‐ and exonucleolytic reactions, and mature rRNAs are liberated and packaged into ribosomal subunits.

Figure 3.

Expression of snoRNAs. Various types of snoRNA production are illustrated: synthesis from independent genes, pre‐mRNA introns, gene clusters and intronic gene clusters. The promoter regions (P) are shown as white boxes, exons (E) as grey boxes, snoRNA coding sequences as coloured boxes and snoRNAs as coloured arrows.


Further Reading

Bachellerie J‐P, Cavaillé J and Huttenhofer A (2002) The expanding snoRNA world. Biochimie 84: 775–790.

Brown JW, Echeverria M and Qu LH (2003) Plant snoRNAs: functional evolution and new modes of gene expression. Trends in Plant Science 8: 42–49.

Decatur WA and Fournier MJ (2003) RNA‐guided nucleotide modification of ribosomal and other RNAs. Journal of Biological Chemistry 278: 695–698.

Filipowicz W and Pogacic V (2002) Biogenesis of small nucleolar ribonucleoproteins. Current Opinion in Cell Biology 14: 319–327.

Gerbi SA, Borovjagin AV and Lange TS (2003) The nucleolus: a site of ribonucleoprotein maturation. Current Opinion in Cell Biology 15: 318–325.

Granneman S and Baserga SJ (2004) Ribosome biogenesis: of knobs and RNA processing. Experimental Cell Research 296: 43–50.

Kiss T (2004) Biogenesis of small nuclear RNPs. Journal of Cell Science 117: 5949–5951.

Rogelj B and Giese KP (2004) Expression and function of brain specific small RNAs. Reviews in the Neurosciences 15: 185–198.

Tran E, Brown J and Maxwell ES (2004) Evolutionary origins of the RNA‐guided nucleotide‐modification complexes: from the primitive translation apparatus? Trends in Biochemical Sciences 29: 343–350.

Uliel S, Liang XH, Unger R and Michaeli S (2004) Small nucleolar RNAs that guide modification in trypanosomatids: repertoire, targets, genome organisation, and unique functions. International Journal for Parasitology 34: 445–454.

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Dragon, François, Lemay, Vincent, and Trahan, Christian(Apr 2006) snoRNAs: Biogenesis, Structure and Function. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1038/npg.els.0003813]