Quantification of T4 DNA polymerase exonuclease activity. Sac II/ Nde I-digested plasmid DNA (3 fragments, lane C) was treated with T4 DNA polymerase for 10 minutes at 25°C, 20°C, 15°C and 10°C. The T4 DNA polymerase was then inactivated by incubation at 80°C for 5 min. The single-stranded ends generated by the 3′ to 5′ exonuclease activity T4 DNA polymerase were removed by using Mung Bean nuclease. The size of the resulting fragments was analyzed by agarose gel electrophoresis. As a control for the heat inactivation of T4 DNA polymerase, digested plasmid DNA was inactivated at 80°C for 5 minutes immediately after addition of T4 DNA polymerase (lane H).