The human U6/U2 snRNA complex and the snRNA-mediated splicing reaction. (A) The human U6/U2 complex. The location of the base-pairing cassette used for binding Exon2 is shown. Position of helices I, II, and III and the U6 ISL are marked on the complex. Numbers refer to the human numbering system. Arrows point to the previously determined phosphorothioate interference sites. Asterisks mark locations that are also identified as metal binding sites in this study. The boxed nucleotides to the right refer to the sequence of a hyperstable loop used to join U6 and U2 together as a single transcript. (B) The snRNA-catalyzed splicing reaction. The sequence of Exon1 and Exon2 substrates and the products of each step of the reaction are shown. Numbers refer to position from the 5′ end. “Exonic” sequences are highlighted in gray. The base-pairing cassette in Exon2 used for binding to U6 is shown.
