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. Author manuscript; available in PMC: 2020 May 1.
Published in final edited form as: FEBS Lett. 2019 Apr 24;593(10):1009–1019. doi: 10.1002/1873-3468.13383

Fig. 2. The tandem active site model for the ribosomal helicase.

Fig. 2.

A. Cartoon showing the tandem arrangement of helicase active sites on the ribosome. The proximal and distal active site correspond to positions +11 and +14, respectively. The proximal segment of mRNA (positions +12–14; blue) is shown in its S3-bound state, and the distal segment (positions +15–17; green) is shown in its open state. B and C. Encounter between a downstream mRNA duplex and the distal active site (in B) or the proximal active site (in C). During translation, encounter by the distal active site occurs first. Encounter by the proximal active site occurs in the next elongation cycle, after the distal active site fails to unwind the duplex.