One consideration of exon bridging is that when exons are very small (less than 50 nucleotides) steric hindrance inhibits assembly of complexes at each end of the exon. There are a number of very small exons in both plants and animals. In animals these require specific elements in the flanking introns (Intron Splicing Enhancers or ISEs) which bind factors that activate splicing at one or other splice site.

Plant invertase genes contain a nine nucleotide mini-exon. For splicing, two elements: a branchpoint and adjacent U-rich region in the upstream intron, are required. The distance of the branchpoint from the mini-exon (50-80 nt) is important. Our model is that factors binding to the branchpoint/U-rich region interact, via bridging, with a complex at the 5' splice site, to activate splicing of the downstream intron. This is followed by splicing of the upstream intron to give the final mRNA.

Simpson et al. (2000) RNA 6, 422-433.