Identification and functional analysis of 20 box H/ACA small nucleolar RNAs (snoRNAs) from Schizosaccharomyces pombe

Considering all small nucleolar RNAs (snoRNAs) enriched in the nucleolus, we generated a specialized cDNA library of small nuclear RNAs from Schizosaccharomyces pombe and isolated, for the first time, 20 novel box H/ACA snoRNAs. Thirteen of these were characterized as novel guides that were predicted to direct 19 pseudouridylations in 18 S and 25 S rRNAs. The remaining seven snoRNAs were considered as orphan guides that lack sequence complementarity to either rRNAs or snRNAs. We have experimentally demonstrated the function of the 10 novel snoRNAs by gene deletion in the fission yeast. The snoRNAs were shown to be dispensable for the viability of S. pombe, although an impact of snR94 depletion on yeast growth, especially at 23 degrees C, was revealed. A total of 30 pseudouridylation sites were precisely mapped in the S. pombe rRNAs, showing a distinctive pseudouridylation pattern in the budding yeast. Interestingly, the absence of pseudouridylation on U2347 in S. pombe 25 S rRNA pointed out a critical role for Psi 2345 in conferring a growth advantage for yeast. In contrast to the intron-encoded box C/D snoRNAs in yeast, all box H/ACA snoRNAs appeared to be transcribed independently from intergenic regions between two protein-coding genes, except for snR35, which was nested in an open reading frame encoding for a hypothetical protein, although expressed from the opposite strand. Remarkably, snR90 was cotranscribed with an intron-encoded box C/D snoRNA, and this is the first demonstration of a non-coding RNA gene that encodes two different types of snoRNAs by its exon and intron. A detailed comparison of the S. pombe snoRNAs, with their functional homologues in diverse organisms, suggests a mechanism by which the snoRNAs have evolved in coordination with rRNAs to preserve the post-transcriptional modification sites among distant eukaryotes.