Nucleic Acids Research Advance Access published online on June 27, 2008
Nucleic Acids Research, doi:10.1093/nar/gkn384
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Mrd1p binds to pre-rRNA early during transcription independent of U3 snoRNA and is required for compaction of the pre-rRNA into small subunit processomes
1Department of Molecular Biology and Functional Genomics, Stockholm University, SE-106 91, Stockholm, Sweden and 2Department of Microbiology, University of Virginia Health System, Charlottesville, VA 22908, USA
*To whom correspondence should be addressed. Tel: +46 8 161720; Fax: +46 8 166488; Email: lars.wieslander{at}molbio.su.se
Received March 7, 2008. Revised May 7, 2008. Accepted May 30, 2008.
In Saccharomyces cerevisiae, synthesis of the small ribosomal subunit requires assembly of the 35S pre-rRNA into a 90S preribosomal complex. SnoRNAs, including U3 snoRNA, and many trans-acting proteins are required for the ordered assembly and function of the 90S preribosomal complex. Here, we show that the conserved protein Mrd1p binds to the pre-rRNA early during transcription and is required for compaction of the pre-18S rRNA into SSU processome particles. We have exploited the fact that an Mrd1p-GFP fusion protein is incorporated into the 90S preribosomal complex, where it acts as a partial loss-of-function mutation. When associated with the pre-rRNA, Mrd1p-GFP functionally interacts with the essential Pwp2, Mpp10 and U3 snoRNP subcomplexes that are functionally interconnected in the 90S preribosomal complex. The fusion protein can partially support 90S preribosome-mediated cleavages at the A0–A2 sites. At the same time, on a substantial fraction of transcripts, the composition and/or structure of the 90S preribosomal complex is perturbed by the fusion protein in such a way that cleavage of the 35S pre-rRNA is either blocked or shifted to aberrant sites. These results show that Mrd1p is required for establishing productive structures within the 90S preribosomal complex.