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Mol Cell Biol, April 1998, p. 2360-2370, Vol. 18, No. 4
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?

Denis L. J. Lafontaine,1,* Thomas Preiss,2 and David Tollervey1

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.

* 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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