The path of messenger RNA through the ribosome

Using X-ray crystallography, we have directly observed the path of mRNA in the 70S ribosome in Fourier difference maps at 7 Angstrom resolution. About 30 nucleotides of the mRNA are wrapped in a groove that encircles the neck of the 30S subunit. The Shine-Dalgarno helix is bound in a large cleft bet ween the head and the back of the platform. At the interface, only about ei ght nucleotides (-1 to +7), centered on the junction between the A and P co dons, are exposed, and bond almost exclusively to 16S rRNA. The mRNA enters the ribosome around position + 13 to + 15, the location of downstream pseu doknots that stimulate -1 translational frame shifting.