The DNA polymerase delta catalytic subunit gene (POLD1) was studied as a transcriptional target of p53. Northern blotting showed that a significantly decreased steady-state level of POLD1 mRNA was associated with increased wild-type p53 expression in cells treated with methyl methanesulfonate. When ectopic wild-type p53 expression was induced to a physiologically relevant level in "tet-off" cultured cells in which p53 expression was tightly regulated by tetracycline, it was found that POLD1 steady-state mRNA was repressed by about 65%. Transient cotransfection experiments using a POLD1 promoter luciferase reporter construct showed that: (i) POLD1 promoter activity was inhibited by transfected wild-type p53 plasmid to a maximum of about 86%; (ii) p53 mediated a large part of the transcriptional repression through a sequence-specific interaction with a site identified as the P4 site of the POLD1 promoter; (iii) tumor-derived p53 mutations in the p53 DNA-binding domain completely abolished the p53 transrepression activity. Moreover, transfection assays demonstrated that p53 was able to repress Sp1-stimulated POLD1 promoter activity and that this repression was largely due to the loss of the sequence-specific interaction between Sp1 protein and the P4 Sp1-binding site, which overlaps the P4 p53-binding site. Finally, gel shift assays suggested that p53 competes with Sp1 protein for binding to the P4 sequence of the POLD1 promoter.