FIG. 2.

Inefficient activation of DNA-PK by hairpin-ended DNA. DNA-PK (70% pure) and its peptide substrate were incubated with hairpin-ended DNA (closed circles), hairpin-ended DNA cleaved with EcoRI (open triangles), or open-ended DNA (closed squares). The uncleaved hairpin-ended DNA was treated identically to its cleaved counterpart by incubation with heat-inactivated EcoRI. Kinase activity was measured as the fold increase in counts from phosphorylation of peptide in the presence of DNA over background counts in the absence of DNA. Similar results were obtained with 95% pure DNA-PK.