FIGURE 1.

Secondary structures of free U2 snRNA and sequence alignment for the 5′ end of U2 snRNA. (A) The 5′ end of U2 snRNA contains the branch point recognition sequence (blue) and three stem–loops, including stem I (red). (B) Watson–Crick pairing is universally conserved and wobble base pairs at the center of the helix (light-gray background) are either tandem U-U pairs in yeast or G-U and C-A pairs in higher eukaryotes. The U-rich tetraloop (dark-gray background) is highly conserved as well.