Question
Under what circumstances is an upstream AUG in good context not a barrier to initiation at a downstream AUG? Present evidence.
Step 1
AUG is the start codon that signals the beginning of protein synthesis. In eukaryotic cells, the ribosome typically scans the mRNA from the 5' end to find the first AUG in a favorable context for initiation. Show more…
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RECALL Why do scientists now believe that AUG is not always the start codon?
The results shown in Fig. 8.5 may have struck you as incongruous because many synthetic RNAs that lacked AUG start codons (such as poly-U) were nonetheless translated into polypeptides in vitro. The reason this experiment was possible is that Marshall Nierenberg found that a high concentration of $\mathrm{Mg}^{2+}$ ions in the test tube, much higher than that found in cells, allows ribosomes to initiate translation at any position on an RNA molecule. Predict the outcomes of in vitro translation with each of the following synthetic mRNAs at both high and low $\mathrm{Mg}^{2+}$ concentrations: a. poly-UG (UGUGUG ...) b. poly-CAUG (CAUGCAUGCAUG ...) c. poly-GUAU (GUAUGUAUGUAU ...)
REFLECT AND APPLY Why is it advantageous to have a mechanism to override the effect of stop codons in protein synthesis?
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