a, Conformation of the GGQ-loop (teal) of eRF1AAQ (purple) within the peptidyl transferase centre. The AAQ tripeptide, positioned next to the CCA end of the P-site tRNA (green), closely resembles the conformation adopted by GGQ of a bacterial release factor (RF1, grey) bound to the ribosome. b, Positions of NIKS, YxCxxxF, and GTS (teal) motifs in the N domain of eRF1AAQ (purple) relative to the mRNA (slate). The positions of the stop codon (+1 to +3) and the following base (+4) are indicated.