The crystal structure of the ribosome in complex with mRNA has revealed that the mRNA is in a singlestranded conformation in the narrow downstream tunnel (13, 14) . The ribosome therefore has to unwind mRNA secondary structure through its mRNA helicase activity (15, 16) . A mechanistic basis for mRNA helicase activity has been proposed involving ribosomal proteins S3, S4, S5 at the mRNA entrance and rotational movement of the head of the 30 S subunit (13, 15) . The 9 Å and the torsional restraint models (17) propose that -1 PRF is dependent on the mechanical tension induced when a pseudoknot resist unfolding by a moving translating ribosome. Possible effects of such tension were directly observed in the cryo-electron microscopic (Cryo-EM) images of eukaryotic ribosomes stalled in the process of -1 frameshifting in complex with eEF2, tRNA and a frameshifting mRNA pseudoknot (18) . The opposing actions of translocation catalyzed by eEF2 and resistance to unfolding by the mRNA strand generate strain that deforms the P-site tRNA which may weaken the codonanticodon interaction and promote the shift by one nucleotide into the 5 0 direction.