Using X-ray crystallography, we have directly observed the path of mRNA in the 70S ribosome in Fourier difference maps at 7 Å 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 between the head and the back of the platform. At the interface, only about eight nucleotides (-1 to +7), centered on the junction between the A and P codons, are exposed, and bond almost exclusively to 16S rRNA. The mRNA enters the ribosome around position +13 to +15, the location of downstream pseudoknots that stimulate -1 translational frame shifting.
Yusupova, G. Z., Yusupov, M. M., Cate, J. H. D., & Noller, H. F. (2001). The path of messenger RNA through the ribosome. Cell, 106(2), 233–241. https://doi.org/10.1016/S0092-8674(01)00435-4