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FIG. 8.   Leaky scanning and translational frameshifting during retroviral mRNA translation. (A) Leaky scanning during HIV mRNA translation accounts for synthesis of the Vpu and Env proteins. The HIV gene structure (upper) consists of several overlapping cistrons encoded within a heterogeneous array of mRNAs (438). Synthesis of the Vpu and Env proteins (lower diagram) of HIV proceeds via a leaky-scanning mechanism in which the scanning ribosome bypasses the weak vpu AUG codon (shown) to initiate translation from the env AUG codon (bent arrow). Translation initiation occurs at the vpu AUG codon at a low frequency and may account for the stoichiometric ratios of Vpu and Env protein accumulation during HIV infection (413). (B) Translational frameshifting at the gag-pro junction during MMTV mRNA translation. An RNA pseudoknot near the 3' end of the gag gene causes the elongating ribosome to pause at the gag-pro slippery sequence element. As a result, the mRNA slips backward by 1 nt and the ribosome-bound tRNAs mediate new anticodon base pairing in the 1 reading frame (12). Frameshifting is favored by weak codon-tRNA anticodon base pairing in the original reading frame and strong base pairing in the new reading frame. Synthesis of the entire Gag-Pro-Pol polyprotein is facilitated by a second frameshift at the downstream pro-pol slippery site. Model adapted from reference 12.

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