Processing of the intron-encoded U18 small nucleolar RNA in the yeast Saccharomyces cerevisiae relies on both exo- and endonucleolytic activities.

Many small nucleolar RNAs (snoRNAs) are encoded within introns of protein-encoding genes and are released by processing of their host pre-mRNA. We have investigated the mechanism of processing of the yeast U18 snoRNA, which is found in the intron of the gene coding for translational elongation factor EF-1beta. We have focused our analysis on the relationship between splicing of the EF-1beta pre-mRNA and production of the mature snoRNA. Mutations inhibiting splicing of the EF-1beta pre-mRNA have been shown to produce normal U18 snoRNA levels together with the accumulation of intermediates deriving from the pre-mRNA, thus indicating that the precursor is an efficient processing substrate. Inhibition of 5'-->3' exonucleases obtained by insertion of G cassettes or by the use of a rat1-1 xrn1Delta mutant strain does not impair U18 release. In the Exo- strain, 3' cutoff products, diagnostic of an endonuclease-mediated processing pathway, were detected. Our data indicate that biosynthesis of the yeast U18 snoRNA relies on two different pathways, depending on both exonucleolytic and endonucleolytic activities: a major processing pathway based on conversion of the debranched intron and a minor one acting by endonucleolytic cleavage of the pre-mRNA.