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Crystal structures of bacterial and yeast ribosomes

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The crystal structure of the yeast 80S ribosome determined at 4.15 Å resolution reveals the higher complexity of eukaryotic ribosomes, which are 40% larger than their bacterial counterparts. Our crystals capture the ribosome in the ratcheted state which is essential for translocation of mRNA and tRNA and where the small ribosomal subunit has rotated with respect to the large subunit. We describe the conformational changes in both ribosomal subunits that are involved in ratcheting, and their implications to mRNA and tRNA translocation. Structural rearrangements of the ribosome in the tRNA binding step have been studied on bacterial ribosome model. Discrimination of tRNA on the ribosome occurs in two consecutive steps: initial selection and proofreading. We propose a proofreading mechanism based on comparison of crystal structures of the 70S ribosome with an empty A site or the A site occupied by cognate or non-cognate tRNA. We have shown involvement of tales of ribosomal proteins in stabilization of correct tRNA on the ribosome. We suggest that proofreading begins with stabilization of tRNA anticodon loop with involvement of magnesium ions, following by stabilization of elbow region and accommodation of the acceptor end in the peptidyl transferase center.

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