The tumor suppressor activity of p53 is regulated by interactions with the ubiquitin ligase HDM2 and the general transcriptional coactivators CBP and p300. Using NMR spectroscopy and isothermal titration calorimetry, we have dissected the binding interactions between the N-terminal transactivation domain (TAD) of p53, the TAZ1, TAZ2, KIX, and nuclear receptor coactivator binding domains of CBP, and the p53-binding domain of HDM2. The p53 TAD contains amphipathic binding motifs within the AD1 and AD2 regions that mediate interactions with CBP and HDM2. Binding of the p53 TAD to CBP domains is dominated by interactions with AD2, although the affinity is enhanced by additional interactions with AD1. In contrast, binding of p53 TAD to HDM2 is mediated primarily by AD1. The p53 TAD can bind simultaneously to HDM2 (through AD1) and to any one of the CBP domains (through AD2) to form a ternary complex. Phosphorylation of p53 at T18 impairs binding to HDM2 and enhances affinity for the CBP KIX domain. Multisite phosphorylation of the p53 TAD at S15, T18, and S20 leads to increased affinity for the TAZ1 and KIX domains of CBP. These observations suggest a mechanism whereby HDM2 and CBP/p300 function synergistically to regulate the p53 response. In unstressed cells, CBP/p300, HDM2 and p53 form a ternary complex that promotes polyubiquitination and degradation of p53. After cellular stress and DNA damage, p53 becomes phosphorylated at T18 and other residues in the AD1 region, releases HDM2 and binds preferentially to CBP/p300, leading to stabilization and activation of p53.