A strain of E. coli carries a substitution mutation causing the terminator codon 5’-UAG-3’ to oc

Usually UAG causes the protein to stop being made. But in this case, since there is a tRNA that recognized this codon and will deliver the amino acid tyrosine, what would happen in a) is that the mRNA with the internal stop codon could actually make a full lenght protein, instead of being terminated early at the UAG. If the mutant tRNA binds the UAG, instead of a termination factor, then the protein will continue being made with a tyr inserted as the amino acid in this spot (I don't know what the original aa would have been from the info provided - I'm assuming it would be a change of an aa from what was there originally)

b The bacteria would face a bigger problem, b/c this mutant tRNA can also interact with the stop codon UAG when found as it is usually at the end of every mRNA. If this occurred, you would end up with longer than usual protiens (extra aa's being added to the end) and many of these will be non-functional.

Some polypeptides have would a Trp in the place of the original amino acid. (other polypeptides would remain shortened depending whether or not they were made with a mutant tRNA-trp molecule or not).

What effect would the presence of the mutated tRNA molecule have on the synthesis of other polypeptides?

Some other polypeptides that use UAG as their stop codon will now have Trp instead of stopping, and thus will also be longer. Assays carried out on the strain of bacteria described above show that in addition to the mutant tyrosine (above you have a trp mutation not a tyr mutation)-tRNA carrying the anticodon 3 AUC 5, there is a wild type tyrosine tRNA which carries the normal anticodon 3 AUG 5.


No, but redundant genes (2 or more genes that code for the same thing/type of thing-in this case type of tRNA) could be working here.


There are not very many mutant tyrosine-tRNAs in the cell.

Thank you Very much both for your Help