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Molecular and Cellular Biology, November 2009, p. 5763-5774, Vol. 29, No. 21
0270-7306/09/$08.00+0     doi:10.1128/MCB.00428-09
Copyright © 2009, American Society for Microbiology. All Rights Reserved.

Mrd1p Is Required for Release of Base-Paired U3 snoRNA within the Preribosomal Complex{triangledown} ,{dagger}

Pär Lundkvist,1 Sara Jupiter,1 Åsa Segerstolpe,1 Yvonne N. Osheim,2 Ann L. Beyer,2 and Lars Wieslander1*

Department of Molecular Biology and Functional Genomics, Stockholm University, SE-106 91 Stockholm, Sweden,1 Department of Microbiology, University of Virginia Health System, Charlottesville, Virginia 229082

Received 2 April 2009/ Returned for modification 12 May 2009/ Accepted 28 July 2009

In eukaryotes, ribosomes are made from precursor rRNA (pre-rRNA) and ribosomal proteins in a maturation process that requires a large number of snoRNPs and processing factors. A fundamental problem is how the coordinated and productive folding of the pre-rRNA and assembly of successive pre-rRNA-protein complexes is achieved cotranscriptionally. The conserved protein Mrd1p, which contains five RNA binding domains (RBDs), is essential for processing events leading to small ribosomal subunit synthesis. We show that full function of Mrd1p requires all five RBDs and that the RBDs are functionally distinct and needed during different steps in processing. Mrd1p mutations trap U3 snoRNA in pre-rRNP complexes both in base-paired and non-base-paired interactions. A single essential RBD, RBD5, is involved in both types of interactions, but its conserved RNP1 motif is not needed for releasing the base-paired interactions. RBD5 is also required for the late pre-rRNP compaction preceding A2 cleavage. Our results suggest that Mrd1p modulates successive conformational rearrangements within the pre-rRNP that influence snoRNA-pre-rRNA contacts and couple U3 snoRNA-pre-rRNA remodeling and late steps in pre-rRNP compaction that are essential for cleavage at A0 to A2. Mrd1p therefore coordinates key events in biosynthesis of small ribosome subunits.

* Corresponding author. Mailing address: Department of Molecular Biology and Functional Genomics, Stockholm University, SE-106 91 Stockholm, Sweden. Phone: 46-8-161720. Fax: 46-8-166488. E-mail: Lars.Wieslander{at}molbio.su.se

{triangledown} Published ahead of print on 24 August 2009.

{dagger} Supplemental material for this article may be found at http://mcb.asm.org/.

Molecular and Cellular Biology, November 2009, p. 5763-5774, Vol. 29, No. 21
0270-7306/09/$08.00+0     doi:10.1128/MCB.00428-09
Copyright © 2009, American Society for Microbiology. All Rights Reserved.





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