Box H and box ACA are nucleolar localization elements of U17 small nucleolar RNA

The nucleolar localization elements (NoLEs) of U17 small nucleolar RNA (sno RNA), which is essential for rRNA processing and belongs to the box H/ACA s noRNA family, were analyzed by fluorescence microscopy. Injection of mutant U17 transcripts into Xenopus laevis oocyte nuclei revealed that deletion o f stems 1, 2, and 4 of U17 snoRNA reduced but did not prevent nucleolar loc alization. The deletion of stem 3 had no adverse effect. Therefore, the hai rpins of the hairpin-hinge-hairpin-tail structure formed by these stems are not absolutely critical for nucleolar localization of U17, nor are sequenc es within stems 1, 3, and 4, which may tether U17 to the rRNA precursor by base pairing. In contrast, box H and box ACA are major NoLEs; their combine d substitution or deletion abolished nucleolar localization of U17 snoRNA. Mutation of just box H or just the box ACA region alone did not fully aboli sh the nucleolar localization of U17. This indicates that the NoLEs of the box H/ACA snoRNA family function differently from the bipartite NoLEs (cons erved boxes C and D) of box C/D snoRNAs, where mutation of either box alone prevents nucleolar localization.