Tumor suppressor protein p53 is a potent transcriptional activator and regulates cell growth negatively. To characterize the transcriptional activation domain (TAD) of p53, various point mutants were constructed in the context of Ga14 DNA binding domain and tested for their transactivation ability. Our results demonstrated that the positionally conserved hydrophobic residues shared with herpes simplex virus VP16 and other transactivators are essential for transactivation. Also, the negatively charged residues and proline residues are necessary for full activity, but not essential for the activity of p53 TAD. Deletion analyses showed that p53 TAD can be divided into two subdomains, amino acids 1-40 and 43-73. An in vitro glutathione S- transferase pull-down assay establishes a linear correlation between p53 TAD- mediated transactivation in vivo and the binding activity of p53 TAD to TATA- binding protein (TBP) in vitro. Mutations that diminish the transactivation ability of Ga14-p53 TAD also impair the binding activity to TBP severely. Our results suggest that at least TBP is a direct target for p53 TAD and that the binding strength of TAD to TBP (TFIID) is an important parameter controlling activity of p53 TAD. In addition, circular dichroism spectroscopy has shown that p53 TAD peptide lacks any regular secondary structure in solution and that there is no significant difference between the spectra of the wild type TAD and that of the transactivation-deficient mutant type.