A ribosome is a ribozyme polymerizing amino acids, exploiting positional- and substrate-mediated chemical catalysis. We showed that peptide-bond formation is facilitated by the ribosomal architectural frame, provided by a sizable symmetry-related region in and around the peptidyl transferase centre, suggesting that the ribosomal active site was evolved by gene fusion. Mobility in tunnel components is exploited for elongation arrest as well as for trafficking nascent proteins into the folding space bordered by the bacterial chaperone, namely the trigger factor.