Cryo-EM of a nucleotide-polymerizing ribozyme enables its predictive improvement

Ribozymes capable of self-replication from nucleotides would have been central to the hypothesized RNA World. The leading laboratory models for such molecules were converted from a class I ligase by in vitro evolution but then developed without 3D structures. Here, scaffolded cryo-EM of the substrate-free tC19Z RNA polymerase ribozyme at 3.1 Å resolution shows how this conversion was achieved. An accessory domain evolved from random sequence grips the ancestral ligase through a loop-loop contact, a seam of magnesium ions, and a six-base stack, and rebuilds the ligase’s substrate binding site from different residues of its own. A previously unrecognized pairing, present before substrate binds, sequesters the 5′ end that must otherwise pair with the template. Compensatory mutations to the ribozyme and template, designed to break this ectopic pairing, increase the extension rate. These results suggest that accelerating RNA structure determination may speed progress toward nucleotide-based self-replication.