Reactions (20 µl each) were carried out as described in the legend to Fig. 1, A and B, except that EDTA was present at a final concentration of 0.1 mm in order to remove contaminating divalent cations; and then CaCl2, MgCl2, MnCl2, or ZnCl2 was added to a final concentration of 5 mm prior to the pre-incubation step. The amount of ATP hydrolyzed in the presence of poly (dA) (A) or M13 RF (B) is presented. The average of two independent experiments is shown, and the error bars indicate the standard deviation.