snoRNA Maturation of eukaryotic rRNA requires the action of a large population of snoRNAs (small nucleolar RNAs) produced in the nucleus (see snoRNA production). Some of the snoRNAs function during the hydrolytic processing of the primary transcript to the individual rRNA species. However, the vast majority function by guiding modification of individual nucleotides in the rRNAs. rRNA contains two major types of nucleotide modifications; pseudouridines (psi) and methylation of the ribose, 2’-O-methylation. Mammalian rRNA has more than 100 2’-O-methyl groups and approximately 95 pseudouridines. These modifications are inserted at precise locations in the rRNA. The modification sites are identified by the snoRNAs through base pairing between the snoRNA and sequences around the specific modification site in the rRNA (see roll over in figure below).

The actual modification of the nucleotide, i.e. conversion of uridine to psedouridine and methylation of a ribose at the 2’-O position requires enzymes which recognise the snoRNA-rRNA complex. The snoRNAs can roughly be divided into two classes depending on their content of one of the two conserved structural elements known as the C/D and H/ACA boxes. All snoRNAs that direct 2’-O-methylation belong to the box C/D class while the snoRNAs that direct pseudouridylation are members of the H/ACA box family.