| U3 | |
| Width: | 220px |
| Symbol: | U3 |
| Altsymbols: | RNU3P2, |
| Rfam: | RF00012 |
| Rna Type: | snoRNA |
| Tax Domain: | Eukaryota |
| Entrezgene: | 26844 |
| Hgncid: | 10176 |
| Omim: | 180710 |
In molecular biology, U3 snoRNA is a non-coding RNA found predominantly in the nucleolus.U3 has C/D box motifs that technically make it a member of the box C/D class of snoRNAs; however, unlike other C/D box snoRNAs, it has not been shown to direct 2'-O-methylation of other RNAs.Rather, U3 is thought to guide site-specific cleavage of ribosomal RNA (rRNA) during pre-rRNA processing.[1]
The box C/D element is a subset of the six short sequence elements found in all U3 snoRNAs, namely boxes A, A', B, C, C', and D.[2] The U3 snoRNA secondary structure is characterized by a small 5' domain (with boxes A and A'), and a larger 3' domain (with boxes B, C, C', and D), the two domains being linked by a single-stranded hinge. Boxes B and C form the B/C motif, which appears to be exclusive to U3 snoRNAs, and boxes C' and D form the C'/D motif. The latter is functionally similar to the C/D motifs found in other snoRNAs. The 5' domain and the hinge region act as a pre-rRNA-binding domain. The 3' domain has conserved protein-binding sites. Both the box B/C and box C'/D motifs are sufficient for nuclear retention of U3 snoRNA. The box C'/D motif is also necessary for nucleolar localization, stability and hyper-methylation of U3 snoRNA.[3] Both box B/C and C'/D motifs are involved in specific protein interactions and are necessary for the rRNA processing functions of U3 snoRNA.
S. cerevisiae secondary structure determined by chemical mapping of U3A RNA in a purified snoRNP is available.[4] A human structure model has also been proposed.[5] Like yeast and human, protozoan protist Entamoeba histolytica : a primitive eukaryote adopted the same conserved secondary structure of U3 snoRNA.[6] Four consensus structures specific to metazoa, fungi, plants and basal eukaryotes have been proposed.[7]