Synergistic activation of p53-dependent transcription by two cooperating damage recognition pathways

High level activation of p53-dependent transcription occurs following cellu lar exposure to genotoxic damaging agents such as UV-C, while ionizing radi ation damage does not induce a similarly potent induction of p53-dependent gene expression. Reasoning that one of the major differences between UV-C a nd ionizing radiation damage is that the latter does not inhibit general tr anscription, we attempted to reconstitute p53-dependent gene expression in ionizing irradiated cells by co-treatment with selected transcription inhib itors that alone do not activate p53. p53-dependent transcription can be dr amatically enhanced by the treatment of ionizing irradiated cells with low doses of DRB, which on its own does not induce p53 activity, The mechanism of ionizing radiation-dependent activation of p53-dependent transcription u sing DRB is more likely due to inhibition of gene transcription rather than prolonged DNA damage, as the non-genotoxic and general transcription inhib itor Roscovitine also synergistically activates p53 function in ionizing ir radiated cells, These results identify two distinct signal transduction pat hways that cooperate to fully activate p53-dependent gene expression: one r esponding to lesions induced by ionizing radiation and the second being a k inase pathway that regulates general RNA Polymerase it activity.