The complete set of H/ACA snoRNAs that guide rRNA pseudouridylations in Saccharomyces cerevisiae

Abstract

Conversion of uridines into pseudouridines (Ψs) is the most frequent base modification in ribosomal RNAs (rRNAs). In eukaryotes, the pseudouridylation sites are specified by base-pairing with specific target sequences within H/ACA small nucleolar RNAs (snoRNAs). The yeast rRNAs harbor 44 Ψs, but, when this work began, 15 Ψs had completely unknown guide snoRNAs. This suggested that many snoRNAs remained to be discovered. To address this problem and further complete the snoRNA assignment to Ψ sites, we identified the complete set of RNAs associated with the H/ACA snoRNP specific proteins Gar1p and Nhp2p by coupling TAP-tag purifications with genomic DNA microarrays experiments. Surprisingly, while we identified all the previously known H/ACA snoRNAs, we selected only three new snoRNAs. This suggested that most of the missing Ψ guides were present in previously known snoRNAs but had been overlooked. We confirmed this hypothesis by systematically investigating the role of previously known, as well as of the newly identified snoRNAs, in specifying rRNA Ψ sites and found all but one missing guide RNAs. During the completion of this work, another study, based on bioinformatic predictions, also reported the identification of most missing guide RNAs. Altogether, all Ψ guides are now identified and we can tell that, in budding yeast, the 44 Ψs are guided by 28 snoRNAs. Finally, aside from snR30, an atypical small RNA of heterogeneous length and at least one mRNA, all Gar1p and Nhp2p associated RNAs characterized by our work turned out to be snoRNAs involved in rRNA Ψ specification.