Impact of base pair identity 5′ to the spliceosomal branch site adenosine on branch site conformation

Changes in chemical shifts of nonexchangeable protons of the ψ-dependent branch site duplexes (ψBP and ψBP_2ap). Positive values for differences [δ (ψBP_2ap) − δ (ψBP)] are associated with downfield shifts and the negative values with upfield shifts. Differences are shown for the anomeric protons H1′, aromatic protons H6/8 (for pyrimidines/purines), and H2/5 (for adenosine/pyrimidines). (A) Nucleotides of the U2 snRNA strand; (B) nucleotides of the intron strand. The slight chemical shift changes in ψBP_2ap are consistent with a similar helical structure to the branch site helix ψBP.