FIG. 5.

Deletion of SNU17 leads to accumulation of an unusual spliceosomal complex. (A) Spliceosomes were assembled on a ³²P-labeled actin pre-mRNA by incubation in mock-treated wild-type (WT) and snu17Δ extracts at 30°C. Aliquots were withdrawn after 0, 10, and 20 min and treated with a buffer containing heparin. Nondenaturing gels were run for 5 h (7). The identities of the complexes are indicated on the left. The unusual, slow-migrating complex seen only in snu17Δ extracts is designated complex X. (B) Yeast spliceosome assembly, as in reference 7. RNase H-mediated digestion of U6 snRNA (ΔU6) blocks spliceosome assembly at the B complex stage, whereas addition of 5 to 8 mM EDTA blocks spliceosome assembly at the A1 complex stage.