18S rRNA processing requires base pairings of snR30 H/ACA snoRNA to eukaryote-specific 18S sequences

The H/ACA RNAs represent an abundant, evolutionarily conserved and functionally diverse class of non-coding RNAs. Many H/ACA RNAs direct pseudouridylation of rRNAs and snRNAs, while members of the rapidly growing group of ‘orphan' H/ACA RNAs participate in pre-rRNA processing, telomere synthesis and probably, in other nuclear processes. The yeast snR30 ‘orphan' H/ACA snoRNA has long been known to function in the nucleolytic processing of 18S rRNA, but its molecular role remained unknown. Here, we provide biochemical and genetic evidence demonstrating that during pre-rRNA processing, two evolutionarily conserved sequence elements in the 3′-hairpin of snR30 base-pair with short pre-rRNA sequences located in the eukaryote-specific internal region of 18S rRNA. The newly discovered snR30-18S base-pairing interactions are essential for 18S rRNA production and they constitute a complex snoRNA target RNA transient structure that is novel to H/ACA RNAs. We also demonstrate that besides the 18S recognition motifs, the distal part of the 3′-hairpin of snR30 contains an additional snoRNA element that is essential for 18S rRNA processing and that functions most likely as a snoRNP protein-binding site

Etude fonctionnelle du snoARN à boîte H/ACA snR30, essentielle à la viabilité chez la levure saccharomyces cerevisiae

TOULOUSE3-BU Sciences (315552104) / SudocSudocFranceF

U17/snR30 Is a Ubiquitous snoRNA with Two Conserved Sequence Motifs Essential for 18S rRNA Production

Saccharomyces cerevisiae snR30 is an essential box H/ACA small nucleolar RNA (snoRNA) required for the processing of 18S rRNA. Here, we show that the previously characterized human, reptilian, amphibian, and fish U17 snoRNAs represent the vertebrate homologues of yeast snR30. We also demonstrate that U17/snR30 is present in the fission yeast Schizosaccharomyces pombe and the unicellular ciliated protozoan Tetrahymena thermophila. Evolutionary comparison revealed that the 3′-terminal hairpins of U17/snR30 snoRNAs contain two highly conserved sequence motifs, the m1 (AUAUUCCUA) and m2 (AAACCAU) elements. Mutation analysis of yeast snR30 demonstrated that the m1 and m2 elements are essential for early cleavages of the 35S pre-rRNA and, consequently, for the production of mature 18S rRNA. The m1 and m2 motifs occupy the opposite strands of an internal loop structure, and they are located invariantly 7 nucleotides upstream from the ACA box of U17/snR30 snoRNAs. U17/snR30 is the first identified box H/ACA snoRNA that possesses an evolutionarily conserved role in the nucleolytic processing of eukaryotic pre-rRNA