FIG. 6.

RNA composition of complex X. Spliceosomes were assembled on unlabeled actin pre-mRNA. Splicing reactions were performed using wild-type (WT) or snu17Δ extracts at 30°C, either mock treated, treated to destroy U6, or treated with 8 mM EDTA as indicated above each lane. Aliquots were withdrawn after 0, 5, and 20 min, blocked on ice, and treated with a buffer containing heparin. Nondenaturing gels were run, blotted, and sequentially hybridized with probes for the actin precursor (Actin) (a) and subsequently for the U2 (b), U1 (c), U6 (d), U5 (e), and U4 (f) snRNAs. The complexes are indicated on the left. Complex X is an unusual slow-migrating complex seen only in snu17Δ extracts.