NUCLEOLAR LOCALIZATION ELEMENTS OF XENOPUS-LAEVIS U3 SMALL NUCLEOLAR RNA

The Nucleolar Localization Elements (NoLEs) of Xenopus laevis U3 small nucleolar RNA (snoRNA) have been defined. Fluorescein-labeled wild-ty pe U3 snoRNA injected into Xenopus oocyte nuclei localized specificall y to nucleoli as shown by fluorescence microscopy. Injection of mutate d U3 snoRNA revealed that the 5' region containing Boxes A and A', kno wn to be important for rRNA processing, is not essential for nucleolar localization. Nucleolar localization of U3 snoRNA was independent of the presence and nature of the 5' cap and the terminal stem. In contra st, Boxes C and D, common to the Box C/D snoRNA family, are critical e lements for U3 localization. Mutation of the hinge region, Box B, or B ox C' led to reduced U3 nucleolar localization. Results of competition experiments suggested that Boxes C and D act in a cooperative manner. It is proposed that Box B facilitates U3 snoRNA nucleolar localizatio n by the primary NoLEs (Boxes C and D), with the hinge region of U3 su bsequently base pairing to the external transcribed spacer of pre-rRNA , thus positioning U3 snoRNA for its roles in rRNA processing.