FIGURE 9.
The 530 loop—a key component of the A site. (A,B) A pseudoknot structure formed by pairing of residues 507–509 and 524–526 in helix h18 (A) (Woese and Gutell 1989) creates a stacked coaxial 5-bp all-G–C helix (B) that stabilizes an unusual distortion of the RNA backbone of the hairpin loop of helix h18. (C) Note that the universal G530 and G532 are the sole bases that project from the pseudoknot, where their positions are fixed by numerous tertiary interactions involving conserved nucleotides, including the type XI A520–G529 pair and the reverse Hoogsteen U513–A535 pair. Pairing of A532 with G1206 connects the A-site elements h18 and h34 (Figs. 10, 12).
