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June 2012 research highlights

Understanding decoding

SBKB [doi:10.1038/nsmb.2297]

Crystal structures of the 70S ribosome provided a more integrated view of decoding.

During protein synthesis, the ribosome accurately selects aminoacyl tRNAs in accordance with mRNA codons in the decoding center in the A site. The tRNA selection process proceeds in two consecutive steps: initial selection and proofreading. During proofreading, the ribosome has a second chance to reject the tRNA if it does not match the codon in the A site. Although the mechanism underlying the decoding process was previously studied on the basis of structures of the isolated 30S subunit, Yusupova and colleagues have now determined six X-ray structures of the bacterial 70S ribosome, in which they modeled cognate and near-cognate states of the decoding center at the proofreading step by using long mRNA and tRNAs whose anticodons carried a mismatch corresponding to the first, second or third position of the mRNA triplet in the A site. They show that the 30S subunit undergoes an identical domain closure upon binding of either cognate or near-cognate tRNA. Remarkably, single U•G and G•U mismatches in the first or second codon-anticodon position, which were expected to form standard wobble base pairs, instead adopt Watson-Crick–like base pairs, owing to positional restrictions imposed by the decoding center on these base pairs. By contrast, no such restraints exist for the third base pair. The authors propose that geometrical restriction of the first two base pairs of the codon-anticodon duplex by the very tight decoding pocket causes the near-cognate tRNA to be expelled from the ribosome. ( Nature doi:10.1038/nature10913, published online 21 March 2012)

Arianne Heinrichs

 

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