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Molecular and Cellular Biology, December 1999, p. 8412-8421, Vol. 19, No. 12
Department of Biochemistry and Molecular
Biology1 and Department of
Genetics,2 University of Georgia, Athens,
Georgia 30602
Received 26 July 1999/Returned for modification 27 August
1999/Accepted 3 September 1999
The processing and methylation of precursor rRNA is mediated by the
box C/D small nucleolar RNAs (snoRNAs). These snoRNAs differ from most
cellular RNAs in that they are not exported to the cytoplasm. Instead,
these RNAs are actively retained in the nucleus where they assemble
with proteins into mature small nucleolar ribonucleoprotein particles
and are targeted to their intranuclear site of action, the nucleolus.
In this study, we have identified the cis-acting sequences
responsible for the nuclear retention of U3 box C/D snoRNA by analyzing
the nucleocytoplasmic distributions of an extensive panel of U3 RNA
variants after injection of the RNAs into Xenopus oocyte
nuclei. Our data indicate the importance of two conserved sequence
motifs in retaining U3 RNA in the nucleus. The first motif is comprised
of the conserved box C' and box D sequences that characterize the box
C/D family. The second motif contains conserved box sequences B and C. Either motif is sufficient for nuclear retention, but disruption of
both motifs leads to mislocalization of the RNAs to the cytoplasm.
Variant RNAs that are not retained also lack 5' cap hypermethylation
and fail to associate with fibrillarin. Furthermore, our results
indicate that nuclear retention of U3 RNA does not simply reflect its
nucleolar localization. A fragment of U3 containing the box B/C motif
is not localized to nucleoli but retained in coiled bodies. Thus, nuclear retention and nucleolar localization are distinct processes with differing sequence requirements.
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Copyright © 1999, American Society for Microbiology. All rights reserved.
Nuclear Retention Elements of U3 Small
Nucleolar RNA
*
Corresponding author. Mailing address: Department of
Biochemistry and Molecular Biology, University of Georgia, Life
Sciences Building, Athens, GA 30602. Phone: (706) 542-1896. Fax: (706) 542-1752. E-mail: mterns{at}bchiris.bmb.uga.edu.
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