bacterial exonuclease enzyme . It is composed of two nonidentical subunits; one large subunit and 4 small ones. [ 1 ] Exonuclease VII catalyses exonucleolytic cleavage in either 5' - 3' or 3'-5' direction to yield 5'-phosphomononucleotides. The large subunit also contains an N-terminal OB-fold domain that binds to nucleic acids