Figure 5.

Summary of 5′ and 3′ substrate structural effects on bimolecular exon ligation. The simplest model accounting for all data regarding 3′ splice site AG selection in bimolecular exon ligation assays (Upper) is one in which access to the active site for exon ligation is limited such that only linear 3′ substrates with no 5′ end structures can gain entry. Effects of structures in the 5′ substrate suggest that this space normally is occupied by intronic RNA downstream of the branch site in cis constructs (Lower).