A single base substitution in the anticodon loop of one of the tRNAs for glutamic acid (tRNAglu) can hypothetically generate a mutant tRNA that is able to suppress translational termination by causing insertion of a glutamic acid residue at certain termination codons. Such mutant tRNAs are in fact encountered in nature and are referred to as suppressor tRNAs. Assume that there are two tRNA genes for glutamic acid (so that this hypothetical mutation would not be lethal) and that one of these tRNAs acquires a single base mutation in its anticodon loop. Assume also that this tRNA has a U residue at the wobble position (and can therefore base pair with both Glu codons) and that the mutation does NOT occur at this wobble position.

A) What is the nucleotide sequence of the wildtype anticodon of tRNAglu (5'?3' polarity) and what is the mutant sequence that would have the properties described above?

Wt sequence:

Mutant sequence:

B) At which stop codons would the mutant tRNAglu be able to suppress translational termination? (assume that G-U base pairing is possible at the wobble position)

C) Mutations of the sort described above are rarely encountered in nature, due in part to the existence of a key step in the chain of events that helps ensure translational fidelity. What is this step and how might it prevent suppressor tRNAs from being able to incorporate amino acids at stop codons?