PROCESSING OF THE INTRON-ENCODED U18 SMALL NUCLEOLAR RNA IN THE YEASTSACCHAROMYCES-CEREVISIAE RELIES ON BOTH EXONUCLEOLYTIC AND ENDONUCLEOLYTIC ACTIVITIES

Many small nucleolar RNAs (snoRNAs) are encoded within introns of prot ein-encoding genes and are released by processing of their host pre-mR NA. We have investigated the mechanism of processing of the yeast U18 snoRNA, which is found in the intron of the gene coding for translatio nal elongation factor EF-1 beta. We have focused our analysis on the r elationship between splicing of the EF-1 beta pre-mRNA and production of the mature snoRNA. Mutations inhibiting splicing of the EF-1 beta p re-mRNA have been shown to produce normal U18 snoRNA levels together w ith the accumulation of intermediates deriving from the pre-mRNA, thus indicating that the precursor is an efficient processing substrate. I nhibition of 5' --> 3' exonucleases obtained by insertion of G cassett es or by the use of a rat1-1 xrn1 Delta mutant strain does not impair U18 release. In the Exo(-) strain, 3' cutoff products, diagnostic of a n endonuclease-mediated processing pathway, were detected. Our data in dicate that biosynthesis of the yeast U18 snoRNA relies on two differe nt pathways, depending on both exonucleolytic and endonucleolytic acti vities: a major processing pathway based on conversion of the debranch ed intron and a minor one acting by endonucleolytic cleavage of the pr e-mRNA.