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Recombinant human p53 isolated either from E. coli or from insect cells is poorly active for binding to DNA but it can be dramatically stimulated by phosphorylation, antibody binding to the
carboxy-terminal negative regulatory domain, short peptides derived from this negative regulatory domain or short single strands of DNA. We report here that Xenopus p53 has a very similar
behavior. Using a new set of monoclonal antibodies directed either to the amino- or the carboxy-terminus of Xenopus p53, we demonstrate that the frog protein can be activated by specific
carboxy-terminus monoclonal antibodies in order to bind to human p53 DNA response element. In addition, we report that such activation of both humans and frogs protein can also be achieved
by small peptides derived from the carboxy-terminus of both p53. Although, the sequence of this region is not conserved in the various p53 species, the presence of conserved basic residues
indicates that such activation is charge-dependent. This is confirmed by the finding that small poly-lysine peptides can activate both human and Xenopus p53. In vivo expression of Xenopus
p53 indicates that this protein is able to transactivate a wide variety of human p53 response elements as long as the experiments are performed at 32°C since activity at 37°C, a temperature
well above the natural temperature of Xenopus, is lost. Finally, we demonstrate that human mdm2 is able to down regulate the transcriptional activity of Xenopus p53.
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