CONSERVED BOX-C AND BOX-D ARE ESSENTIAL NUCLEOLAR LOCALIZATION ELEMENTS OF U14 AND U8 SNORNA

Sequences necessary for nucleolar targeting were identified in Box C/D small nucleolar RNAs (snoRNAs) by fluorescence microscopy. Nucleolar preparations were examined after injecting fluorescein-labelled wildty pe and mutated U14 or U8 snoRNA into Xenopus oocyte nuclei. Regions in U14 snoRNA that are complementary to 18S rRNA and necessary for rRNA processing and methylation are nob required for nucleolar localization . Truncated U14 molecules containing Boxes C and D with or without the terminal stem localized efficiently. Nucleolar localization was aboli shed upon mutating just one or two nucleotides within Boxes C and D. M oreover, the spatial position of Boxes C or D in the molecule is essen tial. Mutations in Box C/D of U8 snoRNA also impaired nucleolar locali zation, suggesting the general importance of Boxes C and D as nucleola r localization sequences for Box C/D snoRNAs. U14 snoRNA is shown to b e required for 18S rRNA production in vertebrates.