Figure 2.

Structural mechanism of decoding as visualized by cryoelectron microscopy (cryo-EM). (Top) Intermediates of cognate decoding by elongation factor (EF)-Tu•GDP–Phe-tRNA^Phe. (Left) Schematic of the cognate codon–anticodon interaction between the UUC mRNA codon and the AAG anticodon of Phe-tRNA^Phe. Other panels show decoding intermediates from the T state prior to codon reading, A*/T, where the codon has been recognized but EF-Tu did not move onto the sarcin-ricin loop (SRL) of the SSU and A/T state with the correct codon–anticodon interaction and EF-Tu docked on the SRL. Insets on top show the orientation of the G-domain of EF-Tu relative to the SRL. GCP, nonhydrolyzable GTP analog GDPCP. Insets at the bottom show the codon–anticodon complex and the key residues of 16S ribosomal RNA (rRNA) interacting with it. (Middle) Same as above for a near-cognate pair with a single G–U mismatch in the second position of the codon–anticodon complex. (Bottom) Intermediates of cognate decoding by SelB•GDPNP/Sec-tRNA^Sec. GNP, nonhydrolyzable GTP analog GDPNP; IB, initial binding prior to codon reading; CR, codon reading complex in which the anticodon of the tRNA comes into the proximity of the codon, but prior to base pairing; GA, GTPase-activated complex analogous to the A/T state. (Figure was prepared using structure coordinates from Fischer et al. 2016 and Loveland et al. 2017, PDB 5UYK, 5UYL, 5UYM, 5UYN, 5UYP, 5UYQ, 5LZB, 5LZC and 5LZD.)