Figure 3.

Model of utilization of an exogenous IGS. In this example, the h•Z RNA molecule is multifunctional, performing three discrete roles: acting as the 3′ end of the ribozyme, acting as a substrate for transesterification, and acting as an exogenous IGS. In the last case, the h•Z molecule must be held in place in the catalytic core of the ribozyme complex via tertiary hydrogen-bonding interactions. Some potential examples of these are indicated in the diagram, although no solid evidence exists for any of these interactions. In addition, the loose interaction between the fragment supplying the exogenous IGS and the remainder of the ribozyme apparently can lead to mis-pairing between the IGS and the IGS complement (see text); here GUG5 is depicted as acting as the exogenous IGS and binding to CGC instead of CAU, which would lead to a 4-nt deletion in the splicing product.