Video 1. mRNA +1 frameshifting of ASL^Pro in the peptidyl (P) site is dependent on the slippery mRNA codon.

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An overview of the 70S ribosome (50S is light cyan and the 30S in gray) showing how the anticodon stem-loop (ASL) of tRNA^Pro (blue) interacts with either a cognate CCG codon or a slippery CCC-U codon (black). The m¹G37 modified nucleotide is shown in orange. A zoomed-in view of the anticodon nucleotides (G36, G35, and C34) interacting with the cognate C₊₁, C₊₂ and G₊₃ mRNA respectively, in the 0 frame. All 2F_o-F_c electron density maps shown are contoured at 1.0 σ. A morph from the 0 frame structure to the structure of ASL^Pro interacting with a slippery CCC-U codon is shown to demonstrate the movement of the mRNA into the +1 frame. The fourth nucleotide of the mRNA codon is shown in green. In either the 0 or +1 frame, a cis Watson–Crick interaction at the third base-pair forms (C₊₃•C34 or U₊₄•C34). mRNA numbering starts at +1 according to the first position of the mRNA in the P site.