FIG. 1.

Schematic representation of the three RNA motifs bound by 15.5K. The secondary structures of the U4/U6 snRNA (reference 19 and references therein), U14 box C/D snoRNA (30, 31), and U3 snoRNA-specific box B/C motif (6, 31) are shown. The highly conserved internal loop elements recognized by 15.5K are depicted in white type on a black background. The two flanking stem structures (stems I and II) that along with the loop nucleotides are part of the k-turn structure are indicated. The conserved sequence of stem II in the box B/C and box C/D is indicated by black type on a gray background. Base pairing interactions between the U14 snoRNA and the 18S rRNA essential for modification and processing are shown. The Sm-binding site in the U4 snRNA is underlined. In addition, the minimal U6 sequence, which together with the U4 snRNA, is required for the binding of the CYPH/hPRP4/hPRP3 proteins is indicated by gray type (20).