Figure 2: The G·U mismatch forms Watson–Crick-like pairs at the first and second codon–anticodon positions in the P-site of the 70S ribosome.

(a) Region of codon–anticodon interactions in the P-site (framed) in the context of the full 70S ribosome; tRNA bound in the A-, P- and E-sites are represented by red, blue and green, respectively. (b,c) The G·U mismatch mimics Watson–Crick pair at the first (b; left) and second (c; left) positions of the codon–anticodon duplex (b,c; right). Putative hydrogen bonds formed by an anticipated tautomeric form of G or U have 2.8–3.2 Å length (see text). (d) Geometry of canonical Watson–Crick pair and non-canonical wobble pair formed by keto isomers (left panel). Watson–Crick-like pairs formed by rare enol tautomers of uracil or guanosine (right panel; marked by asterisk). In b and d, the following figures schemes of the codon–anticodon duplexes are indicated and arrows mark the described mismatches. All graphical representations were rendered by PyMol. For all figures, the density maps are contoured at 1.6−1.8 σ level.