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Molecular and Cellular Biology, September 1999, p. 6012-6019, Vol. 19, No. 9
1European Molecular Biology Laboratory, Gene
Expression Programme, 69012 Heidelberg,
Germany, and 2 Institute of Cell and Molecular
Biology, University of Edinburgh, Edinburgh EH9 3JR, United
Kingdom
Received 15 March 1999/Returned for modification 23 April
1999/Accepted 28 May 1999
The loop of a stem structure close to the 5' end of the 18S rRNA is
complementary to the box A region of the U3 small nucleolar RNA
(snoRNA). Substitution of the 18S loop nucleotides inhibited pre-rRNA
cleavage at site A1, the 5' end of the 18S rRNA, and at
site A2, located 1.9 kb away in internal transcribed spacer 1. This inhibition was largely suppressed by a compensatory mutation in
U3, demonstrating functional base pairing. The U3-pre-rRNA base
pairing is incompatible with the structure that forms in the mature 18S
rRNA and may prevent premature folding of the pre-rRNA. In the
Escherichia coli pre-rRNA the homologous region of the 16S
rRNA is also sequestered, in that case by base pairing to the 5'
external transcribed spacer (5' ETS). Cleavage at site A0
in the yeast 5' ETS strictly requires base pairing between U3 and a
sequence within the 5' ETS. In contrast, the U3-18S interaction is not
required for A0 cleavage. U3 therefore carries out at least two functionally distinct base pair interactions with the pre-rRNA. The
nucleotide at the site of A1 cleavage was shown to be
specified by two distinct signals; one of these is the stem-loop
structure within the 18S rRNA. However, in contrast to the efficiency
of cleavage, the position of A1 cleavage is not dependent
on the U3-loop interaction. We conclude that the 18S stem-loop
structure is recognized at least twice during pre-rRNA processing.
0270-7306/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
Base Pairing between U3 Small Nucleolar RNA and the
5' End of 18S rRNA Is Required for Pre-rRNA Processing
*
Corresponding author. Mailing address: Institute of Cell and
Molecular Biology, University of Edinburgh, King's Buildings, Edinburgh EH9 3JR, United Kingdom. Phone: 44 131 650 7092. Fax: 44 131 650 7040. E-mail: d.tollervey{at}ed.ac.uk.
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