FIG. 3.

Inserting a copy of MUP intron 5 into TPI intron 6 moves the boundary between nonsense codons that do and do not reduce TPI mRNA abundance to the new penultimate exon. L-cell transfections and the purification and analysis of total-cell RNA were as described in the legend to Fig. 1. XS MUP intron 5 denotes the insertion of MUP intron 5 into the EcoRI site of TPI intron 7, where plus (⊕) and minus (⊝) signs specify proper and improper orientations of the insertion, respectively. Values shown are an average of the values obtained from two independently performed experiments, which did not differ by more than 7%.