Anatomical features of box C/D small (nucleolar) (sno) ribonucleoprotein particles (RNPs), U14, and U3 snoRNAs. The secondary structure of an archetypical C/D RNA is depicted by lines and conserved box C (RUGAUGA) and D (CUGA) sequences are boxed. The core proteins are represented by filled ovals and labeled with yeast protein names. The corresponding names in human and archaea are listed under the cartoon. Parentheses indicate alternative names. Each C/D box is associated with an antisense region (D or D′ guide) located upstream of box D (D guide) or D′ (D′ guide) that base-pairs with the target RNA. For methylation C/D s(no)RNAs, the target RNA is methylated on the 2′-ribose of the nucleotide base-paired with the 5th guide nucleotide upstream of box D or D′. Both U14 and U3 snoRNAs in yeast, though associated with the same proteins, do not completely comply with this rule and participate in ribosomal RNA processing.
