Bud23 methylates G1575 of 18S rRNA and is required for efficient nuclear export of pre-40S subunits

Section of Molecular Genetics and Microbiology, Institute for Cellular and Molecular Biology, Center for Systems and Synthetic Biology, Department of Chemistry and Biochemistry, the University of Texas at Austin, Austin, TX, 78712; Bioinformatics Laboratory, Institute of Molecular Biology and Biotechnology, Adam Mickiewicz University, Umultowska 89, PL-61-614 Poznan, Poland; Laboratory of Bioinformatics and Protein Engineering, International Institute of Molecular and Cell Biology, Trojdena 4, PL-02-109 Warsaw, Poland

^* To whom correspondence should be addressed. Email: arlen{at}mail.utexas.edu.

	Abstract

BUD23 was identified from a bioinformatics analysis of yeast genes involved in ribosome biogenesis. Deletion of BUD23 leads to severely impaired growth, reduced levels of the small (40S) ribosomal subunit and a block in processing 20S to 18S rRNA, a late step in 40S maturation. Bud23 belongs to the SAM-dependent Rossmann-fold methyltransferase (RFM) superfamily and is related to small molecule methyltransferases. Nevertheless, we considered that Bud23 methylates rRNA. Methylation of G1575 is the only mapped modification for which the methylase has not been assigned. Here, we show that this modification is lost in bud23 mutants. The nuclear accumulation of the small subunit reporters Rps2-GFP, Rps3-GFP as well as the rRNA processing intermediate 5'-ITS1, indicate that bud23 mutants are defective for small subunit export. Mutations in Bud23 that inactivated its methyltransferase activity complemented a bud23 mutant. In addition, mutant ribosomes in which G1575 was changed to adenosine supported growth comparable to wild-type ribosomes. Thus, Bud23 protein, but not its methyltransferase activity, is important for biogenesis and export of the 40S subunit in yeast.