Functional analysis of yeast snoRNA and snRNA 3' end formation mediated by uncoupling of cleavage and polyadenylation

Many nuclear and nucleolar small RNAs are accumulated as nonpolyadenylated species and require 3′-end processing for maturation. Here, we show that several genes coding for box C/D and H/ACA snoRNAs and for the U5 and U2 snRNAs contain sequences in their 3′ portions which direct cleavage of primary transcripts without being polyadenylated. Genetic analysis of yeasts with mutations in different components of the pre-mRNA cleavage and polyadenylation machinery suggests that this mechanism of 3"-end formation requires cleavage factor IA (CF IA) but not cleavage and polyadenylation factor activity. However, in vitro results indicate that other factors participate in the reaction besides CF IA. Sequence analysis of snoRNA genes indicated that they contain conserved motifs in their 3" noncoding regions, and mutational studies demonstrated their essential role in 3"-end formation. We propose a model in which CF IA functions in cleavage and polyadenylation of pre-mRNAs and, in combination with a different set of factors, in 3"-end formation of nonpolyadenylated polymerase II transcripts.