FIGURE 10-42 snoRNP-directed modification of pre-rRNA. (a) A class of snoRNAs called box C+D snoRNAs is involved in ribose 2′-O-methylation. Sequences in the snoRNA illustrated here hybridize to two different regions in the pre-rRNA, directing methylation at the indicated sites. (b) Box H+ACA snoRNAs fold into two stem-loops with internal single-stranded bulges in the stems. Pre-rRNA hybridizes to the single-stranded bulges, demarcating a site of pseudouridylation. (c) Conversion from uridine to pseudouridine involves rotation of the pyrimidine ring. See T. Kiss, 2001, EMBO J. 20:3617.