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Small nucleolar RNAs (snoRNAs) are a class of small RNA molecules that primarily guide chemical modifications of other RNAs, mainly ribosomal RNAs, transfer RNAs and small nuclear RNAs. There are two main classes of snoRNA, the C/D box snoRNAs, which are associated with methylation, and the H/ACA box snoRNAs, which are associated with pseudouridylation. SnoRNAs are commonly referred to as guide RNAs but should not be confused with the guide RNAs that direct RNA editing in trypanosomes. ==snoRNA guided modifications == After transcription, nascent rRNA molecules (termed pre-rRNA) undergo a series of processing steps to generate the mature rRNA molecule. Prior to cleavage by exo- and endonucleases, the pre-rRNA undergoes a complex pattern of nucleoside modifications. These include methylations and pseudouridylations, guided by snoRNAs. *Methylation is the attachment or substitution of a methyl group onto various substrates. The rRNA of humans contain approximately 115 methyl group modifications. The majority of these are 2'O-ribose-methylations (where the methyl group is attached to the ribose group). *Pseudouridylation is the conversion (isomerisation) of the nucleoside uridine to a different isomeric form pseudouridine (Ψ). Mature human rRNAs contain approximately 95 Ψ modifications.〔 Each snoRNA molecule acts as a guide for only one (or two) individual modifications in a target RNA. In order to carry out modification, each snoRNA associates with at least four protein molecules in an RNA/protein complex referred to as a small nucleolar ribonucleoprotein (snoRNP). The proteins associated with each RNA depend on the type of snoRNA molecule (see snoRNA guide families below). The snoRNA molecule contains an antisense element (a stretch of 10-20 nucleotides), which are base complementary to the sequence surrounding the base (nucleotide) targeted for modification in the pre-RNA molecule. This enables the snoRNP to recognise and bind to the target RNA. Once the snoRNP has bound to the target site, the associated proteins are in the correct physical location to catalyse the chemical modification of the target base. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Small nucleolar RNA」の詳細全文を読む スポンサード リンク
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