FIGURE 2.

RNA duplexes and naming schemes used in these studies. The duplex ψBP, with strands representing the wild-type U2 snRNA (top oligomer) and intron (bottom oligomer) sequences from S. cerevisiae, includes two additional complementary nucleotides added to the 5′ side of the U2 strand and 3′ side of the intron strand to increase thermal stability. Other duplexes include modifications in the base pair A7-U22 that participates in the base triple identified in the solution structure of ψBP; in each case, the mutation from the native yeast sequence is the rectangle. Adenosine residues substituted by the fluorescent analog 2-aminopurine (2ap) are circled. The rationale for testing each of the mutated pairs is explained in the text.