The p53 tumor suppressor gene product is a transcriptional activator that may be associated with its ability to suppress tumor cell growth. The acidic amino terminus of the p53 protein has been shown to contain this trans-activation activity as well as the domains for mdm-2 and adenovirus 5 E1B 55-kD protein binding. An extensive genetic analysis of this amino-terminal p53 domain has been undertaken using site-specific mutagenesis. The results demonstrate that the acidic residues in the amino terminus of p53 may contribute to, but are not critical for, this trans-activation activity. Rather, the hydrophobic amino acid residues Leu-22 and Trp-23 of human p53 are both required for trans-activation activity, binding to the adenovirus E1B 55-kD protein and the human mdm-2-p53 protein in vitro. In addition, hydrophobic residues Leu-14 and Phe-19 are crucial for the interactions between p53 and human mdm-2 (hdm-2). Hydrophobic residues Trp-23 and Pro-27 are also important for binding to the adenovirus 5 (ADS) E1B 55-kD protein in vitro. These mutations have no impact on the ability of the p53 protein to bind to a p53-specific DNA element. These results suggest that 2-4 critical hydrophobic residues in the amino-terminal domain of the p53 protein interact with the transcriptional machinery of the cell resulting in transcriptional activation. These very same hydrophobic residues contact the hdm-2 and Ad5 E1B 55-kD oncogene products.[Key Words: p53 protein; trans-activation; mdm-2 binding; E1B 55-kD binding]

Transgenic mice that contain the simian virus 40 (SV40) enhancer-promoter and large tumor (T) antigen gene develop papillomas of the choroid plexus. The tumors remain well differentiated on histological examination and express normal levels of tissue-specific mRNAs for transthyretin (TTR) and the 5-HT,c serotonin receptor, two differentiated cell markers. Both Northern (RNA) blot analysis and in situ cytohybridization have been used to monitor the steady-state levels of the mRNAs from the viral oncogene (T antigen) and from several cellular oncogenes. In situ hybridization demonstrated, in serial sections, increased levels of both T antigen mRNA and p53 mRNA localized in the tumor tissue but not in the normal brain tissue. The ratios of the steady-state levels of mRNA for p53/TTR and p53/L32, a ribosomal protein gene, were 2to 20-fold higher in the tumor tissue than in the normal choroid plexus tissue. Several other oncogenes did not show elevated levels of mRNA in these tumors. p53 protein levels were not detectable in normal brain tissue, but p53 levels were very high in tumor tissue in which all of the p53 was found in a complex with the SV40 large T antigen. These data continue to show a close relationship between SV40 T-antigen-mediated tumorigenesis and the role of p53 in these tumors.