Nucleolar localization signals of Box H/ACA small nucleolar RNAs

The two major families of small nucleolar RNAs (snoRNAs), Box C/D and Box H /ACA, are generated in the nucleoplasm and transported to the nucleolus whe re they function in rRNA processing and modification. We have investigated the sequences involved in the intranuclear transport of Box H/ACA snoRNAs b y assaying the localization of injected fluorescent RNAs in Xenopus oocyte nuclear spreads. Our analysis of U17, U64 and U65 has revealed that disrupt ion of either of the conserved sequence elements, Box H or Box ACA, elimina tes nucleolar localization. In addition, the stem present at the base of th e 3' hairpin is required for efficient nucleolar localization of U65. Fragm ents or rearrangements of U65 that consist of Box H and Box ACA flanking ei ther the 5' or 3' hairpin are targeted to the nucleolus. The targeting is d ependent on the presence of the Box sequences, but not on their orientation . Our results indicate that in each of the two major families of snoRNAs, a motif composed of the signature conserved sequences and an adjacent struct ural element that tethers the sequence elements directs the nucleolar local ization of the RNAs, We demonstrate that telomerase RNA is also targeted to the nucleolus by a Box ACA-dependent mechanism.