FIG. 6.

Chu is a processive enzyme. Refolded Chu (0.151 nM, as determined by titration) was incubated with PstI-cut pUC19 DNA (0.151 nM ends) for 2 min at 37°C, and reactions were then initiated with 5 mM Mg²⁺. Sufficient heparin to trap all of the enzyme was added after 10 s (open circles). DNA was digested under standard reaction conditions (see Materials and Methods) for 0, 0.5, 1, 2, 5, or 10 min. Reactions were quenched with 20 mM EDTA. The 0-min sample was quenched with EDTA prior to the addition of Mg²⁺. Heparin was added after quenching in the controls (squares). The digestion of 1,000 bp by a single turnover of Chu indicates that the enzyme is processive.