DNA damage induces phosphorylation of the amino terminus of p53. This study demonstrates that DNA damage leads to specific post-translational modifications of p53, including phosphorylation of serine residues. Using various techniques, a unique p53 phosphopeptide was identified, which contains three phosphorylated serine residues. This phosphopeptide was found to be phosphorylated on serine-15 in response to DNA damage, including ionizing radiation (IR) and ultraviolet (UV) irradiation. Phosphorylation of p53 on serine-15 correlates with increased transcription of downstream p53 target genes. The study also shows that p53 phosphorylation on serine-15 occurs in both normal and ataxia-telangiectasia (AT) lymphoblasts, although the kinetics and levels differ between the two. Phosphorylation of p53 on serine-15 does not affect its ability to bind or dissociate from DNA. The results indicate that p53 is phosphorylated in response to DNA damage, and this de novo phosphorylation may be involved in the subsequent induction and activation of p53. While ATM affects the kinetics of p53 phosphorylation after IR, it is not absolutely required for phosphorylation of p53 on serine-15. The study also highlights the importance of p53 phosphorylation in the regulation of its transcriptional activity and in the cellular response to DNA damage. The findings suggest that phosphorylation of p53 on serine-15 is a critical step in the activation of p53 and its subsequent role in DNA repair and cell cycle control.DNA damage induces phosphorylation of the amino terminus of p53. This study demonstrates that DNA damage leads to specific post-translational modifications of p53, including phosphorylation of serine residues. Using various techniques, a unique p53 phosphopeptide was identified, which contains three phosphorylated serine residues. This phosphopeptide was found to be phosphorylated on serine-15 in response to DNA damage, including ionizing radiation (IR) and ultraviolet (UV) irradiation. Phosphorylation of p53 on serine-15 correlates with increased transcription of downstream p53 target genes. The study also shows that p53 phosphorylation on serine-15 occurs in both normal and ataxia-telangiectasia (AT) lymphoblasts, although the kinetics and levels differ between the two. Phosphorylation of p53 on serine-15 does not affect its ability to bind or dissociate from DNA. The results indicate that p53 is phosphorylated in response to DNA damage, and this de novo phosphorylation may be involved in the subsequent induction and activation of p53. While ATM affects the kinetics of p53 phosphorylation after IR, it is not absolutely required for phosphorylation of p53 on serine-15. The study also highlights the importance of p53 phosphorylation in the regulation of its transcriptional activity and in the cellular response to DNA damage. The findings suggest that phosphorylation of p53 on serine-15 is a critical step in the activation of p53 and its subsequent role in DNA repair and cell cycle control.