FIG. 5.

Sequences within domain 3B dictate 5′ splice site specificity. For this experiment, the two enhancers were considered to contain four domains. Compared to the domains identified in Fig. 4, the extra domain arises by subdivision of domain 3 into domain 3A, which contains the first 6 nucleotides of domain 3, with the sequence (GAR)₂ and domain 38, which contains the terminal enhancer-specific. Caldesmon (black boxes) and cTNT (white boxes) domains are indicated. One mutant enhancer was prepared by replacing domain 3B of the caldesmon enhancer with domain 3B from the cTNT enhancer (lane 3). Two point mutations were made by altering the two C’s in the cTNT domain 3B to U’s (UU) (lane 4) or deleting both C’s (XX) (lane 5). Splicing reactions were performed for 45 min. Product RNAs are identified as for Fig. 2. Quantification of the indicated product RNAs is shown beneath the gel. The amounts of products are shown in arbitrary PhosphorImager units as a percentage of total RNA precursor and products, normalized for uridine content.