p53 and NF-kappa B RelA are activated by various genotoxic agents and mutually suppress each other's ability to activate transcription, most likely through competition for transcriptional coactivators such as CBP or p300. However, we found that the inhibition by RelA of p53 transcriptional activity is not completely restored by CBP/p300 overexpression and that a p53 mutant can not suppress RelA activity despite of its ability to bind CBP/p300. In the present study, we further present evidence that these two transcriptional factors directly interact both in vivo and in vitro. These results therefore indicate that the cross transcriptional interference between p53 and RelA is partly caused by the direct interaction between these two transcription factors which is mediated by their dimerization/tetramerization domains and results in inhibition of each other's transcriptional activity. Finally, cells derived from RelA knockout mice showed enhanced p53 transcriptional activity, suggesting that this cross transcriptional interference is physiologically important in cellular response to genotoxic stress.