On the activities of Escherichia coli exonuclease III

Exonuclease III (Exo III) of Escherichia coli is a DNA-modifying enzyme very frequently used in molecular biology. The experiments described here were carried out with the aims of reliably controlling exonuclease activity and of learning more about the enzyme's specificities. The dependence of Exo III activity on factors such as temperature (including heat inactivation), the concentrations of Exo III and NaCl, and the concentration and shape of 3' termini was investigated. Double stranded DNA was found to be a competitive inhibitor of the enzyme activity. Some four nucleotide 3' protrusions were shown to be sensitive to Exo III digestion. The synchronism of deletion was also examined. Implications for the proposed mechanism of activity are discussed.