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Abstract

The ribosome contains three tRNA binding sites, the A, P and E sites. Although the E site is separated from the A via the intervening P site, there is a striking communication between these sites. This crosstalk plays an important role for the accuracy of the decoding process. Codon-anticodon interaction at the E site seems to be the signal to switch into the post-translocational (POST) state characterized by a low affinity of the A site. This low affinity state forces the ternary complexes aminoacyl-tRNA•EF-Tu•GTP to enter the A site via the decoding center preventing the selection of non-cognate aminoacyl-tRNAs and incorporation of the amino acid. This has the important consequence that only 1 in 400 misincorporations affects protein functions. Another aspect of the three tRNA binding sites is that at least two tRNAs are present on the ribosome. Since the tRNAs are firmly bound by the ribosome whereas the mRNA practically only via codon-anticodon interaction during the elongation phase, the movement of the tRNA during translocation pulls the mRNA through the ribosome. In fact, the six base pairs of two adjacent codon-anticodon interactions are instrumental for maintaining the reading frame. Without the codon-anticodon interaction of the E tRNA the reading frame would be lost at least after the incorporation of about 50 amino acids into the nascent chain.