Small nucleolar RNAs (snoRNAs) are named for their subcellular localization within the nucleolus. The nucleolus is a dark-staining structure within the nucleus that is the site of ribosomal RNA transcription and maturation [Hadjiolov, 1985,Woolford, 1991]. After transcription by RNA polymerase I, the primary rRNA transcript undergoes numerous cleavages resulting in the 18S, 5.8S and 28S rRNAs, as well as dozens of specific nucleotide modifications. In the end, these trimmed and decorated rRNA molecules fold into complex scaffold structures that associate with a huge number of ribosomal proteins (at least 78 in yeast! [Planta & Mager, 1998]) to become a mature ribosome. Small ribonucleoprotein particle complexes (snoRNPs) associate with ribosomal RNA in the course of maturation, and are essential for proper rRNA cleavage and modification [Tollervey et al., 1991,Mattaj, 1993]. SnoRNAs are the RNA component of snoRNPs, and represent a large, complex population of small RNAs [Riedel et al., 1986].
SnoRNAs have been subdivided into two main classes, named for their conserved sequence motifs: the C/D box snoRNAs, and the H/ACA box snoRNAs [Balakin et al., 1996]. A single snoRNA, the RNA component of the ribonuclease for mitochondrial RNA processing (MRP RNA), fails to fit into either classification. SnoRNAs are technically a subgroup of the small nuclear RNAs (snRNAs), but should not be confused with the snRNAs involved in messenger RNA splicing, the spliceosomal RNAs (e.g., U1, U2,U4, U5, U6 [Guthrie & Patterson, 1988]).