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Mol Cell Biol, April 1998, p. 2360-2370, Vol. 18, No. 4
Institute of Cell and Molecular Biology, The
University of Edinburgh, EH9 3JR Edinburgh,
Scotland,1 and
European Molecular
Biology Laboratory, Gene Expression, Heidelberg,
Germany2
Received 24 October 1997/Returned for modification 5 December
1997/Accepted 21 January 1998
One of the few rRNA modifications conserved between bacteria and
eukaryotes is the base dimethylation present at the 3' end of the small
subunit rRNA. In the yeast Saccharomyces cerevisiae, this
modification is carried out by Dim1p. We previously reported that
genetic depletion of Dim1p not only blocked this modification but also
strongly inhibited the pre-rRNA processing steps that lead to the
synthesis of 18S rRNA. This prevented the formation of mature but
unmodified 18S rRNA. The processing steps inhibited were nucleolar, and
consistent with this, Dim1p was shown to localize mostly to this
cellular compartment. dim1-2 was isolated from a library of
conditionally lethal alleles of DIM1. In dim1-2
strains, pre-rRNA processing was not affected at the permissive
temperature for growth, but dimethylation was blocked, leading to
strong accumulation of nondimethylated 18S rRNA. This demonstrates that
the enzymatic function of Dim1p in dimethylation can be separated from
its involvement in pre-rRNA processing. The growth rate of
dim1-2 strains was not affected, showing the dimethylation
to be dispensable in vivo. Extracts of dim1-2 strains,
however, were incompetent for translation in vitro. This suggests that
dimethylation is required under the suboptimal in vitro conditions but
only fine-tunes ribosomal function in vivo. Unexpectedly, when
transcription of pre-rRNA was driven by a polymerase II PGK
promoter, its processing became insensitive to temperature-sensitive
mutations in DIM1 or to depletion of Dim1p. This
observation, which demonstrates that Dim1p is not directly required for
pre-rRNA processing reactions, is consistent with the inhibition of
pre-rRNA processing by an active repression system in the absence of
Dim1p.
0270-7306/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
Yeast 18S rRNA Dimethylase Dim1p: a Quality Control
Mechanism in Ribosome Synthesis?
*
Corresponding author. Mailing address: Institute of
Cell and Molecular Biology, University of Edinburgh, Swann Building,
King's Buildings, EH9 3JR Edinburgh, Scotland. Phone: 44 131 650 7093. Fax: 44 131 650 7040 or 8650. E-mail:
denis.lafontaine{at}ed.ac.uk.
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