Baserga S J, Gilmore-Hebert M, Yang X W
Howard Hughes Medical Institute, Yale University School of Medicine, New Haven, Connecticut 06510.
Genes Dev. 1992 Jun;6(6):1120-30. doi: 10.1101/gad.6.6.1120.
Export to the cytoplasm of U3 RNA transcribed from a rat U3 gene injected into the nucleus of Xenopus oocytes indicates that the biogenesis of U3 RNA, like that of the previously studied Sm-precipitable nucleoplasmic snRNAs (U1, U2, U4, and U5), includes a cytoplasmic phase. The regulation of import of the U3 snRNA into the nucleus has been analyzed by injection of synthetic human U3 transcripts into the cytoplasm of Xenopus oocytes. Binding of the major autoantigenic protein of the U3 snRNP, fibrillarin, and cap trimethylation can occur in the cytoplasm, but neither are required for import. The 3'-terminal 13 nucleotides are required for optimal import and cap trimethylation and participate in a phylogenetically conserved U3 structural element, a short 3'-terminal stem. An artificial construct containing the 3'-terminal 13 nucleotides, including the 3'-terminal stem, but only 56 nucleotides of the 217 nucleotides in U3, appears to be sufficient for import. The presence of the 3'-terminal stem in all snRNAs known to be imported suggests that it might be a universal element required for nuclear import.
将注射到非洲爪蟾卵母细胞核中的大鼠U3基因转录的U3 RNA输出到细胞质中，这表明U3 RNA的生物合成，与之前研究的Sm可沉淀核质小核RNA（U1、U2、U4和U5）一样，包括一个细胞质阶段。通过将合成的人U3转录本注射到非洲爪蟾卵母细胞的细胞质中，分析了U3小核RNA进入细胞核的调控。U3小核核糖核蛋白的主要自身抗原蛋白原纤维蛋白的结合以及帽三甲基化可在细胞质中发生，但两者都不是进入细胞核所必需的。3'末端的13个核苷酸是最佳导入和帽三甲基化所必需的，并参与一个系统发育保守的U3结构元件，即一个短的3'末端茎。一个人工构建体，包含3'末端的13个核苷酸，包括3'末端茎，但在U3的217个核苷酸中只有56个核苷酸，似乎足以进行导入。已知所有可导入的小核RNA中都存在3'末端茎，这表明它可能是核导入所需的一个普遍元件。
