FIG. 3.

Psoralen cross-linking of U1 snRNA to m⁷GpppG- and ApppG-capped Ad1 substrates. The substrates used had two alternative 5′ splice sites, each of which was either consensus (C), wild-type (W), or mutant (M), as indicated above each lane. (A) Pre-mRNA capped with m⁷GpppG (m⁷G) or ApppG (A) was incubated for 20 min in splicing mixes containing AMT-psoralen in the presence (lanes 1 to 8) or absence (lanes 9 to 16) of magnesium, ATP, and phosphocreatine. Cross-linked products were resolved on a 5% polyacrylamide gel. The positions of the snRNA cross-links are shown diagramatically. Bands a and b are minor forms of the two U1 cross-links.