Small Cajal body-specific RNA 17

scaRNAs are a specific class of small nuclear RNAs which localise to the Cajal bodies and guide the modification of RNA polymerase II transcribed spliceosomal RNAs U1, U2, U4, U5 and U12 [1]. The complete human U12-22/U4-8 scaRNA is composed of two tandem C/D box domains (termed U12-22 and U4-8). The 5' and 3' C/D domains are predicted to guide the 2'O-ribose methylation of residue U22 in U12 (RF00007) and residue C8 in U4 (RF00015) snRNAs respectively [1]. This family only includes only the 5' C/D box domain (U12-22) as the 3' C/D box is represented by RF00283. The 3' C/D domain (U4-8) was also cloned previously by Darzacq [2] and called U91. Both the doublet (U12-22/U4-8) and singlet (U4-8) forms of this snRNA have been purified from Hela cells [1]. The doublet form U12-22/U4-8 has been shown to localise to the nucleoplasm and is proposed to reside in the Cajal bodies whereas the U4-8 single domain appears to accumulate in the nucleolus [1]. In humans the genomic location of U12-88/U4-8 is intergenic and the purified transcript has been shown to possess a methylated guanosine cap suggesting it is independently transcribed by RNA pol II [1].