The mRNA sequence AUG corresponds to the DNA sequence TAC. This is because during transcription, adenine (A) in DNA pairs with uracil (U) in mRNA, thymine (T) in DNA pairs with adenine (A) in mRNA, cytosine (C) pairs with guanine (G), and guanine (G) pairs with cytosine (C).
Thus, the original DNA triplet sequence that would correspond to the mRNA sequence AUG is:
TAC.