Functional redundancy of Spb1p and a snR52-dependent mechanism for the 2′-O-ribose methylation of a conserved rRNA position in yeast

In yeast, guide snoRNAs have been assigned to 51 of the 55 rRNA ribose methylation sites. LSU-Um(2918) is one of the four remaining positions. This residue is highly conserved and located in the peptidyl transferase center of the ribosome. The equivalent position on the E. coli 23S rRNA is methylated by FtsJ/RrmJ which has three yeast homologs: Spb1, involved in biogenesis of LSU; Trm7, a tRNA methyltransferase; and Mrm2, a mitochondrial 21S rRNA methyltransferase. We demonstrate that a point mutation in the AdoMet binding site of Spb1p affects cell growth but does not abolish methylation Of U-2918. When this mutation is combined with disruption of snR52 (a snoRNA C/D), cell growth is severely impaired and U-2918 is no longer methylated. In vitro, Spb1p is able to methylate U-2918 on 60S subunits. Our results reveal the importance of this methylation for which two mechanisms coexist: a site-specific methyltransferase (Spb1p) and a snoRNA-dependent mechanism.