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Free keywords:
Ribosome; tRNA; E-site; Shine-Dalgarno
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.
