Identification of a new ribose methylation in the 18S rRNA of S. cerevisiae.

Methylation of ribose sugars at the 2'-OH group is one of the major chemical modifications in rRNA, and is catalyzed by snoRNA directed C/D box snoRNPs. Previous biochemical and computational analyses of the C/D box snoRNAs have identified and mapped a large number of 2'-OH ribose methylations in rRNAs. In the present study, we systematically analyzed ribose methylations of 18S rRNA in Saccharomyces cerevisiae, using mung bean nuclease protection assay and RP-HPLC. Unexpectedly, we identified a hitherto unknown ribose methylation at position G562 in the helix 18 of 5' central domain of yeast 18S rRNA. Furthermore, we identified snR40 as being responsible to guide snoRNP complex to catalyze G562 ribose methylation, which makes it only second snoRNA known so far to target three ribose methylation sites: Gm562, Gm1271 in 18S rRNA, and Um898 in 25S rRNA. Our sequence and mutational analysis of snR40 revealed that snR40 uses the same D' box and methylation guide sequence for both Gm562 and Gm1271 methylation. With the identification of Gm562 and its corresponding snoRNA, complete set of ribose methylations of 18S rRNA and their corresponding snoRNAs have finally been established opening great prospects to understand the physiological function of these modifications.