Transactivation of the mdm2-gene by p53 is Regulated by DNA Topology
Sequence-specific transactivation is one of the best characterized molecular properties of the tumor suppressor p53. A large number of putative target genes for p53, some of which directing seemingly opposing functions, has been identified, emphasizing the need for a tight regulation of p53-mediated gene activation. We recently suggested that, in addition to known modulators, like post-translational modifications of the p53 protein, or its interaction with regulatory proteins, the DNA topology within the respective p53-responsive promoter might determine the interaction of p53 with its target gene. In support of this hypothesis we showed that different DNA conformations of the same p53 consensus elements either promoted or inhibited sequence-specific DNA binding (SSDB) of p53 to these elements.
The mdm2-gene was chosen to demonstrate the relevance of the promoter DNA topology for the regulation of p53-mediated transactivation. In vitro studies revealed that, under conditions of DNA supercoiling, the mdm2 (P2) promoter undergoes a transition from a B-duplex to a non-B-DNA conformation. This transition results in local distortion of the DNA duplex within and in close vicinity to the p53 binding sites, and leads to inhibition of SSDB and transactivation of the mdm2 (P2) promoter by p53. Relaxation of the mdm2 (P2) DNA by treatment with topoisomerase I abolished these topological distortions and relieved this inhibition. Mutational analysis indicated that the specific architecture of the p53 responce element within the mdm2 (P2) promoter is responsible for the conformational flexibility of the mdm2 (P2) promoter DNA. We propose that transactivation of different p53 target genes can be regulated by transient changes in DNA conformation. Such a mechanism could provide a means by which conflicting signals can be avoided, which otherwise could result from the simultaneous induction of p53 target genes with opposing functions.
Ella Kim and Wolfgang Deppert
Department of Tumor Virology,