Abstract
Protein synthesis in all organisms proceeds by stepwise translocation of the ribosome along messenger RNAs (mRNAs), during which the helicase activity of the ribosome unwinds encountered structures in the mRNA. This activity is known to occur near the mRNA tunnel entrance, which is lined by ribosomal proteins uS3, uS4, and uS5. However, the mechanism(s) of mRNA unwinding by the ribosome and the possible role of these proteins in the helicase activity are not well understood. Here, we present a crystal structure of the Escherichia coli ribosome in which single-stranded mRNA is observed beyond the tunnel entrance, interacting in an extended conformation with a positively charged patch on ribosomal protein uS3 immediately outside the entrance. This apparent binding specificity for single-stranded mRNA ahead of the tunnel entrance suggests that product stabilization may play a role in the unwinding of structured mRNA by the ribosomal helicase.