Movement of the guide sequence during RNA catalysis by a group I ribozyme.

Ribozymes derived from the self-splicing pre-ribosomal RNA of Tetrahymena act as sequence-specific endonucleases. The reaction involves binding an RNA or DNA substrate by base pairing to the internal guide sequence (IGS) to form helix P1. Site-specific photo-crosslinking localized the 5' end of the IGS in helix P1 to the vicinity of conserved bases between helices P4 and P5, supporting a major feature of the Michel-Westhof three-dimensional structure model. The crosslinked ribozyme retained catalytic activity. When not base-paired, the IGS was still specifically crosslinked, but the major site was 37 A distant from the reactive site in the experimentally supported three-dimensional model. The data indicate that a substantial induced-fit conformational change accompanies P1 formation, and they provide a physical basis for understanding the transport of oligonucleotides to the catalytic core of the ribozyme. The ability of RNA to orchestrate large-scale conformational changes may help explain why the ribosome and the spliceosome are RNA-based machines.