The 30S A site—the decoding site. The central participants are the universally conserved G530, C1054, A1492, and A1493 of 16S rRNA, which contact the codon–anticodon duplex. A1492 and A1493 in helix h44 form A-minor type interactions with the minor groove of the helix formed between the codon of the mRNA (green) and the anticodon of the incoming aminoacyl-tRNA (magenta); G530 in h18 and C1054 in h34 pack against the wobble nucleotide at the bottom of the anticodon (Ogle et al. 2001; Jenner et al. 2010; Watson et al. 2020). These 4 nt form a simple three-dimensional RNA cage that enforces Watson–Crick pairing of the anticodon with the A-site codon (Ogle et al. 2001; Demeshkina et al. 2012). The adjacent invariant nucleotide G1494 forms a tertiary Watson–Crick base pair with the conserved C1407 (C = 1.987). This interaction likely helps to fix the positions of the crucial A1492 and A1493. The noncanonical A532–G1206 base pair (see Fig. 10) forms a connection between h18 and h34, which positions C1054 and G530 in proximity to each other at the bottom of the anticodon as shown here, connecting the A-site elements of the head and body domains of the 30S subunit (see Figs. 10, 12). Universal 16S rRNA nucleotides are shown in yellow.
