(A) Upper panel, schematic representation of the NTR complex, composed of Prp43, Ntr1 and Ntr2; lower panel, Prp43_Ntr1GP in which the G-patch of Ntr1 is fused to the C-terminal domain of Prp43. (B) 10–30% glycerol gradient sedimentation of purified ILS incubated in solution with ATP plus NTR, (C) no recombinant protein, (D) Prp43 (E) Prp43 fused to Ntr1GP (Prp43_Ntr1GP), or (F) Prp43_Ntr1GP and Ntr2. U2, U5 and U6 snRNAs were visualized by Northern blotting followed by autoradiography. RNA identities are indicated on the left. Quantifications were performed with ImageQuant software (Molecular Dynamics, Pittsburg, PA). Numbers represent the percentage of intron-lariat RNA released in the top fractions (sum of fractions 1–11) or associated with the ILS (unreleased, sum of fractions 12–23) relative to the intron-lariat RNA distributed in all 23 fractions, the sum of which was set to 100%.