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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.
