snoRNAs

Small nucleolar RNAs (snoRNAs) are a class of small non-coding RNAs that primarily guide chemical modifications of other RNAs, especially ribosomal RNA (rRNA), within the nucleolus of eukaryotic cells. They also participate in the processing and maturation of rRNA and, in some cases, small nuclear RNAs (snRNAs).

There are two major classes of canonical snoRNAs: C/D box snoRNAs and H/ACA box snoRNAs. C/D box snoRNAs guide site-specific 2'-O-methylation of target RNAs, while H/ACA box snoRNAs guide the formation of pseudouridine residues. Each class forms a small ribonucleoprotein (snoRNP) by associating with specific protein partners that provide catalytic activity: fibrillarin acts as the methyltransferase in C/D snoRNPs, together with NOP56, NOP58 and SNU13; dyskerin partners with GAR1, NOP10, NHP2 and NOP5 in H/ACA snoRNPs. The snoRNA contains conserved sequence motifs that define its class and guide function: C/D snoRNAs carry C and D boxes, whereas H/ACA snoRNAs possess H box and ACA sequences at defined locations.

Biogenesis and localization often involve transcription as introns of host genes or as independent transcripts, followed by processing and assembly into snoRNPs. Most snoRNPs localize to the nucleolus, the site of rRNA transcription and processing; some snoRNAs or their fragments also localize to Cajal bodies or other nuclear compartments.

Beyond rRNA modification, snoRNAs and their derived fragments can influence other RNA processing events and gene regulation. Their expression is evolutionarily conserved across eukaryotes, and misregulation of snoRNAs has been linked to human diseases, including cancer.