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Discrete states of the A site of the 30S subunit, as deduced from two different crystal structures. In all panels, the tRNA anticodon stem-loop (ASL) (gold), A-site mRNA codon (purple), P-site mRNA (green), protein S12 (brown), and important bases (red) involved in conformational changes are shown. A few elements of the 16S RNA such as helix 44, 530 loop, and helix 34 are also shown. (A) The native 30S subunit. A1492 and A1493 (red, right) are stacked in the interior of H44, and G530 is in the syn conformation. C1054 is shown in the upper left corner. (B) When the codon and cognate tRNA-ASL bind in the A site, A1492 and A1493 flip out to monitor the codon-anticodon interaction, and G530 has switched to the anti conformation to interact with A1492, the anticodon in the second position, and the codon in the third. Two Mg2+ ions (magenta spheres) are visible near the region vacated by A1492 and A1493 in the interior of helix 44, and one is located close to the ribose of the codon in the wobble position. (Note: This figure has been modified from the original.)
From: J. M. Ogle, D. E. Brodersen, W. M. Clemons Jr., M. J. Tarry, A. P. Carter, and V. Ramakrishnan (2001) Recognition of Cognate Transfer RNA by the 30S Ribosomal Subunit, Science 292, 897-902. Click here for paper (html version) or (pdf version).
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