DNA-dependent protein kinase acts upstream of p53 in response to DNA damage

The tumour suppressor p53 becomes activated as a transcription factor in response to DNA damage^1-3, but the mechanism for this activation is unclear. A good candidate for an upstream activator of p53 is the DNA- dependent protein kinase (DNA-PK) that depends on the presence of DNA breaks for its activity^4-6. Here we investigate the link between DNA damage and the activation of DNA-PK and of p53. To determine whether DNA-PK is an upstream mediator of the p53 DNA-damage response, we analysed a severe combined-immunodeficiency (SCID) mouse cell line, SCGR11 (refs 7, 8), and the human glioma cell line M059J (ref. 9). Both cell lines lack any detectable DNA-PK activity. We find that p53 is incapable of binding to DNA in the absence of DNA-PK, that DNA-PK is necessary but not sufficient for activation of p53 sequence-specific DNA binding and that this activation occurs in response to DNA damage. Our results establish DNA-PK as a link between DNA damage and p53 activation, and reveal the existence of a mammalian DNA- damage-response pathway.