SEQUENCE AND STRUCTURAL ELEMENTS CRITICAL FOR U8 SNRNP FUNCTION IN XENOPUS OOCYTES ARE EVOLUTIONARILY CONSERVED

We have generated mutants in Xenopus U8 RNA, a nucleolar snRNA require d for the maturation of 5.8S and 28S rRNAS, to identify sequences and structural domains essential for RNA stability, particle assembly, and function of the U8 RNP. Activity of the mutants was assayed by microi njection of in vitro-synthesized U8 RNAs into the cytoplasm of Xenopus oocytes. Most of the mutant RNAs were stable, bound fibrillarin, a pr otein common to several of the nucleolar-specific snRNPs, and became h ypermethylated. Although hypermethylation of the 5' cap of U8 RNA and fibrillarin binding can occur in either the cytoplasmic or nuclear com partment of Xenopus oocytes, neither is required for nuclear import. W e find that the trimethylguanosine cap, although present on the endoge nous U8 RNA, is not essential for stability, particle assembly, or fun ctioning of U8 in the coordinate processing of pre-rRNA at sites 3' of 28S and 5' of 5.8S RNA. Several conserved single- and double-stranded sequences within the 5' domain of U8 RNA are essential for function.