Secondary structure of a trans-splicing group I intron ribozyme.

The ribozyme (black) is base-paired to the target site of the mutated CAT mRNA (red). The 5′-splice site and 3′-splice site are indicated with arrowheads. During the trans-splicing reaction (arrows from left to right) the 3′-exon of the ribozyme (blue) replaces the 3′-portion of the mutated CAT mRNA, leading to the repaired CAT mRNA (red/blue). The positions of the P8 and P6b stem-loops in the ribozyme are indicated, together with the four positions 236, 238, 239, and 241.