Crystal Structure of the 70S Ribosome Bound with the Q253P Mutant Form of Release Factor RF2

Bacterial translation termination is mediated by release factors RF1 and RF2, which recognize stop codons and catalyze hydrolysis of the peptidyl-tRNA ester bond. The catalytic mechanism has been debated. We proposed that the backbone amide NH group, rather than the side chain, of the glutamine of the universally conserved GGQ motif participates in catalysis by H-bonding to the tetrahedral transition-state intermediate and by product stabilization. This was supported by complete loss of RF1 catalytic activity when glutamine is replaced by proline, the only residue that lacks a backbone NH group. Here, we present the 3.4 angstrom crystal structure of the ribosome complex containing the RF2 Q253P mutant and find that its fold, including the GGP sequence, is virtually identical to that of wild-type RF2. This rules out proline-induced misfolding and further supports the proposal that catalytic activity requires interaction of the Gln-253 backbone amide with the 3' end of peptidyl-tRNA.