The ribosome plays a pivotal part in protein synthesis, translating the genetic code with remarkable accuracy. The structural basis of this accuracy has long been a puzzle. In one proofreading step, conserved residues around the A site, which binds to the incoming aminoacylated transfer RNA (tRNA), ensure that the codon-anticodon pairing is correct before a conformational change in the ribosome occurs. As a model of the decoding process, Natalia Demeshkina et al. have solved six X-ray crystal structures of 70S ribosomal complexes with cognate or near-cognate tRNA. They propose that the ribosomal structure forces the first two nucleotides in the A site to adopt canonical Watson-Crick geometry.
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