Activity of Prp18Δ79. (A) Prp18 or Prp18Δ79 was added to Prp18-depleted yeast extract, and splicing of actin pre-mRNA was assayed. Lane 1 shows splicing in extract depleted with preimmune serum. Lane 2 shows splicing in extract depleted by anti-Prp18 with no added Prp18. For the splicing reactions displayed in lanes 3–8, the amount of Prp18 or Prp18Δ79 added is shown at the top of each lane in femtomol per microliter of yeast extract. The positions of lariat intermediate, lariat intron, pre-mRNA, mRNA, and exon1 are shown at the left of both A and B. (B) Prp18, Prp18Δ79, or hPrp18 was added to hPrp18-depleted HeLa-cell nuclear extract, and splicing of β-globin pre-mRNA was assayed. Lane 1 shows splicing in hPrp18-depleted extract alone. For lanes 2–12, the amount of Prp18, Prp18Δ79, or hPrp18 added is shown at the top in femtomol per microliter of nuclear extract.