The P5 Activator of a Group IC Ribozyme Can Replace the P7.1/7.2 Activator of a Group IA Ribozyme

The P5 or P7 extensions in the group I intron ribozyme serve as "modular activator units" by stabilizing the conserved core of the ribozyme. The P5 extension of a group IC1 intron was introduced to a barely active group IA2 intron lacking its original P7 extension. The inserted P5 extension significantly activated the chimeric construct. Because the CYT-18 protein factor is also known to activate mutant group IA2 and IC1 introns lacking their P7 and P5 extensions, respectively, the RNA and protein activator units function in an analogous manner.