FIG. 3.

RNA-RNA interactions in the S. cerevisiae spliceosomes (A) and in the GU-AG spliceosomes of higher eukaryotes (B). The interaction between the 5′ and 3′ splice sites and branch-site (BS) consensus sequences with U1 and U2 snRNAs are shown in scheme I of panels A and B. UsnRNA-UsnRNA and UsnRNA-pre-mRNA interactions at the catalytic center of the spliceosome are shown in scheme II of panels A and B. Helices Ia, Ib, II, and III between U2 and U6 snRNAs are represented, as well as the base-pairing interaction between U2 snRNA and the branch-site sequence. The interaction between U6 snRNA and the UGU trinucleotide, close to the 5′ splice site, is indicated by overlined residues joined by an arrow; the interaction between the terminal loop I of U5 snRNA and the exon extremities is also shown (for a review of all of these interactions, see reference 54). The Ψ residues are boxed, and the base and ribose methylations are indicated in boldface (this study; for a review, see reference 55). Nucleotide positions, starting from the 5′ extremity of each UsnRNA, are indicated.