Figure 4.

A representation of RNAs and metal ions in the yeast spliceosome before the first catalytic step. Intermolecular helices between U6 and U2 (Ia and Ib), U2 and pre-mRNA at the branch site (U2/BS), U6 and pre-mRNA at the 5′ splice site (U6/5′SS), as well as the 3′ intramolecular stem-loop (3′ISL) in U6 are depicted^1-3,8. The branchpoint adenine is highlighted with a star. The U₈₀ of U6 is shaded. Three Mg²⁺ ions are shown as circles: one stabilizes the leaving oxygen at the 5′ scissile phosphate⁶ (lined); one activates the attacking oxygen⁵ (dotted); and one binds to the pro-S_P oxygen of U₈₀ (filled). The tertiary interaction between G₅₂ of U6 and A₂₅ of U2 (refs 26, 27), a putative interaction between 3′ISL of U6 and the U6/5′SS helix based on the λ-λ′ interaction in a group II intron²⁸, and the potential relationship between the metals are linked with a line.