FIG. 1.

Activity of purified DNA-PK in the presence of a 59-bp DNA fragment and in the absence or presence of Ku. Reaction mixtures of 20 μl contained 1.2 × 10⁻⁹ M purified DNA-PK without (bottom curve) or with (top curve) 2.4 × 10⁻⁸ M purified Ku protein and increasing concentrations of the 59-bp DNA fragment. The probes containing Ku were preincubated with DNA for 10 min at room temperature before addition of the enzyme. The phosphorylation reaction was carried out at 37°C for 10 min as described in Materials and Methods. The curves from this experiment are representative of multiple experiments, and kinase determinations at each DNA concentration are within ±10% of the values obtained in this experiment.