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 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 (). 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.