One key question in protein biosynthesis is how the ribosome couples mRNA and
tRNA movements to prevent disruption of weak codon-anticodon interactions and
loss of the translational reading frame during translocation. Here we report the
complete path of mRNA on the 70S ribosome at the atomic level (3.1-A
resolution), and we show that one of the conformational rearrangements that
occurs upon transition from initiation to elongation is a narrowing of the
downstream mRNA tunnel. This rearrangement triggers formation of a network of
interactions between the mRNA downstream of the A-site codon and the elongating
ribosome. Our data elucidate the mechanism by which hypermodified nucleoside
2-methylthio-N6 isopentenyl adenosine at position 37 (ms(2)i(6)A37) in
tRNA(Phe)(GAA) stabilizes mRNA-tRNA interactions in all three tRNA binding
sites. Another network of contacts is formed between this tRNA modification and
ribosomal elements surrounding the mRNA E/P kink, resulting in the anchoring of
P-site tRNA. These data allow rationalization of how modification deficiencies
of ms(2)i(6)A37 in tRNAs may lead to shifts of the translational reading frame.
