ARE VERTEBRATE EXONS SCANNED DURING SPLICE-SITE SELECTION

PAIRWISE recognition of splice sites as a result of a scanning mechanism is an attractive model to explain the coordination of vertebrate splicing1. Such a mechanism would predict a polarity-of-site recognition in the scanned unit, but no evidence for a polarity gradient across introns has been found2-4. We have suggested that the exon rather than the intron is the unit of recognition in vertebrates5 and that polyadenylation and splicing factors interact during recognition of 3'-terminal exons6-8. Interaction is reflected in maximal rates of in vitro polyadenylation. If scanning across the exon is operating during this interaction, then insertion of a 5' splice site should depress polyadenylation. Here we report recognition in vitro and in vivo of a 5' splice site situated within a 3'-terminal exon, and a concomitant depression of polyadenylation and ultraviolet crosslinking of a polyadenylation factor. Decreased crosslinking was only found when the 3' and 5' splice sites were within 300 nucleotides of each other. These results are consistent with an exon scanning mechanism for splice-site selection.