Yeast snoRNA accumulation relies on a cleavege-dependent/polyadenylation-independent 3 '-processing apparatus

In Saccharomyces cerevisiae, snoRNAs are encoded by independent genes and w ithin introns. Despite this heterogenous organization, snoRNA biosynthesis relies on a common theme: entry sites for 5'-3' and 3'-5' exonucleases are created on precursor molecules allowing the release of mature snoRNAs, In i ndependently transcribed snoRNAs, such entry sites are often produced by th e Rnt1p endonuclease, In many cases, cleavage sites are absent in the 3' po rtion of the pre-snoRNAs, suggesting that processing starts from the 3' end of the primary transcript. Here we show that cleavage/polyadenylation site s driving efficient polyadenylation, such as CYC1, prevent production of ma ture and functional snoRNPs. With these sites, snoRNA accumulation is resto red only if polyadenylation activity is inhibited. Analysis of sequences do wnstream of snoRNA-coding units and the use of strains carrying mutations i n RNA polymerase II (polII) cleavage/polyadenylation activities allowed us to establish that formation of snoRNA mature 3' ends requires only the clea vage activity of the polII 3'-processing machinery. These data indicate tha t, in vivo, uncoupling of cleavage and polyadenylation is necessary for an essential cellular biosynthesis.