Base-pairing interactions in the in vitro-assembled complex of U2–U6 and the branch oligonucleotide (Br). The shaded boxes mark the invariant regions in U6, and the three previously established base-paired helices between U2 and U6 and the intramolecular stemloop of U6 are indicated. Dashed lines enclose psoralen-crosslinked residues. The chemical groups involved in RNA X formation in U6 and Br are shown. Arrow indicates the reaction that results in formation of RNA X. Bold arrow points to the site of a pseudouridine modification in U2. The circled residues marked with thunderbolts can be joined together in a UV crosslink described previously. Numbers indicate nucleotide positions from the 5′ end.
