A tandem active site model for the ribosomal helicase

During protein synthesis, the messenger RNA ( mRNA ) helicase activity of the ribosome ensures that codons are made single stranded before decoding. Here, based on recent structural and functional findings, a quantitative model is presented for a tandem arrangement of two helicase active sites on the ribosome. A distal site encounters mRNA structures first, one elongation cycle earlier than a proximal site. Although unwinding of encountered mRNA structures past the proximal site is required for translocation, two routes exist for translocation past the distal site: sliding, which requires unwinding, and stick‐slip, which does not. The model accounts in detail for a number of findings related to the ribosomal helicase and provides a testable framework to further study mRNA unwinding.