Ames test: what are the possible mutation positions?

you have been hired to design an Ames test for mutagens that cause polymerases to insert untemplated As. You choose to construct a histidine auxotroph by mutating the essential (hypothetical) HIS7 gene, such that it has an early stop codon that can revert to the wild type sequence upon a single A insertion. Use the 5'-end of the open reading frame of the wild type HIS7 gene shown below to indicate which position(s) in the DNA would serve this purpose by circling each site that would function. The enzyme sequence is amino
Met    Tyr      Ile     Tyr     Val      Tyr    Val----  
ATG    TAT    ATA   TAT   GTG    TAT   GTA