The tumor suppressor protein p53 acts as a transcriptional activator that can mediate cellular responses to DNA damage by inducing apoptosis and cell cycle arrest. p53 is a nuclear phosphoprotein, and phosphorylation has been proposed to be a means by which the activity of p53 is regulated. The cyclin-dependent kinase (CDK)-activating kinase (CAK) was originally identified as a cellular kinase required for the activation of a CDK-cyclin complex, and CAK is comprised of three subunits: CDK7, cyclin H, and p36MAT1. CAK is part of the transcription factor IIH multiprotein complex, which is required for RNA polymerase II transcription and nucleotide excision repair. Because of the similarities between p53 and CAK in their involvement in the cell cycle, transcription, and repair, we investigated whether p53 could act as a substrate for phosphorylation by CAK. While CDK7-cyclin H is sufficient for phosphorylation of CDK2, we show that p36MAT1 is required for efficient phosphorylation of p53 by CDK7-cyclin H, suggesting that p36MAT1 can act as a substrate specificity-determining factor for CDK7-cyclin H. We have mapped a major site of phosphorylation by CAK to Ser-33 of p53 and have demonstrated as well that p53 is phosphorylated at this site in vivo. Both wild-type and tumor-derived mutant p53 proteins are efficiently phosphorylated by CAK. Furthermore, we show that p36 and p53 can interact both in vitro and in vivo. These studies reveal a potential mechanism for coupling the regulation of p53 with DNA repair and the basal transcriptional machinery.