NUCLEOLAR LOCALIZATION ELEMENTS IN U8 SNORNA DIFFER FROM SEQUENCES REQUIRED FOR RIBOSOMAL-RNA PROCESSING

U8 small nucleolar RNA (snoRNA) is essential for metazoan ribosomal RN A (rRNA) processing in nucleoli. The sequences and structural features in Xenopus U8 snoRNA that are required for its nucleolar localization were analyzed. Fluorescein-labeled U8 snoRNA was injected into Xenopu s oocyte nuclei, and fluorescence microscopy of nucleolar preparations revealed that wild-type Xenopus U8 snoRNA localized to nucleoli, rega rdless of the presence or nature of the 5' cap on the injected U8 snoR NA. Nucleolar localization was observed when loops or stems in the 5' portion of U8 that are critical for U8 snoRNA function in rRNA process ing were mutated. Therefore, sites of interaction in U8 snoRNA that po tentially tether it to pre-rRNA are not essential for nucleolar locali zation of U8. Boxes C and D are known to be nucleolar localization ele ments (NoLEs) for U8 snoRNA and other snoRNAs of the Box C/D family. H owever, the spatial relationship of Box C to Box D was not crucial for U8 nucleolar localization, as demonstrated here by deletion of sequen ces in the two stems that separate them. These U8 mutants can localize to nucleoli and function in rRNA processing as well. The single-stran ded Cup region in us, adjacent to evolutionarily conserved Box C, func tions as a NoLE in addition to Boxes C and D. Cup is unique to U8 snoR NA and may help bind putative protein(s) needed for nucleolar localiza tion. Alternatively, Cup may help to retain U8 snoRNA within the nucle olus.