page_title: Small nucleolar RNA
section_title: C/D box
hierarchical_section_title: Small nucleolar RNA / snoRNA guide families / C/D box
caption_attribution_description: English: Image has been uploaded to the Commons as part of the Molecular and Cellular Biology WikiProject in collaboration with the public-domain database Rfam. Here is the link to the discussion.
context_page_description: Small nucleolar RNAs 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.
context_section_description: C/D box snoRNAs contain two short conserved sequence motifs, C (RUGAUGA) and D (CUGA), located near the 5' and 3' ends of the snoRNA, respectively. Short regions (~ 5 nucleotides) located upstream of the C box and downstream of the D box are usually base complementary and form a stem-box structure, which brings the C and D box motifs into close proximity. This stem-box structure has been shown to be essential for correct snoRNA synthesis and nucleolar localization. Many C/D box snoRNA also contain an additional less-well-conserved copy of the C and D motifs (referred to as C' and D') located in the central portion of the snoRNA molecule.  A conserved region of 10-21 nucleotides upstream of the D box is complementary to the methylation site of the target RNA and enables the snoRNA to form an RNA duplex with the RNA.  The nucleotide to be modified in the target RNA is usually located at the 5th position upstream from the D box (or D' box).   C/D box snoRNAs associate with four evolutionary conserved and essential proteins—fibrillarin (Nop1p), NOP56, NOP58, and Snu13 (15.5-kD protein in eukaryotes; its archaeal homolog is L7Ae)—which make up the core C/D box snoRNP.
  There exists an eukaryotic C/D box snoRNA (snoRNA U3) that has not been shown to guide 2'-O-methylation.
  Instead, it functions in rRNA processing by directing pre-rRNA cleavage.
caption: Example of a C/D box snoRNA secondary structure taken from the Rfam database. This example is SNORD73 (RF00071).
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