Figure I (Box 1).

Splicing involves two sequential transesterification reactions. (a) The reactive groups in each reaction are specified by splice site consensus sequences, shown for metazoa and budding yeast [88]. (b) In the first step of splicing, known as 5' splice site cleavage, the 2' hydroxyl of a conserved branch site adenosine attacks the 5' splice site phosphate forming a branched, lariat intermediate and free 5' exon. In the second step, the 3' hydroxyl of the cleaved 5' exon attacks the 3' splice site phosphate, ligating the exons and excising the lariat intron. N, any nucleotide; R, purine; Y, pyrimidine.