Trans-splicing reaction catalyzed by the group I intron.

First step → Intron finds its target RNA sequence through complimentary base pairing with the internal and external guide sequences (IGS and EGS, respectively). The intron then sequesters the 3’-OH of a free-floating guanosine (GTP, GDP, GMP, or guanosine alone). The 3’ GNP OH attacks the phosphodiester backbone directly downstream of the reactive uracil on the 5’ exon. Second step → The 3’ exon is brought into proximity with the newly freed 3’-OH on the cleavage uracil, guided by the P10 helix. The 3’-OH attacks the phosphodiester backbone just upstream of the 3’ exon in another transesterification reaction, resulting in the 5’ exon and the 3’ exon being joined covalently. Third step → The end result is a new mRNA molecule, functional and ready for translation, formed of two separate RNA molecules. Adopted from Figure 1 in reference [49].