p53 represses Sp1 DNA binding and HIV-LTR directed transcription

The HIV-LTR region contains binding sites for, and is regulated by, a number of transcription factors including Sp1 and NF-kB. The wild-type p53 tumor suppressor protein represses transcription from the HIV-LTR promoter while oncogenic mutant forms of p53 stimulate expression from the HIV-LTR. We have shown previously that wild-type p53 is a site specific DNA binding protein that binds to a region of the SV40 virus which contains GC-box DNA binding sites for the ubiquitously expressed transcription factor Sp1. In this study using DNase I footprinting, we have shown that purified p53 is able to protect the Sp1 binding sites and the adjacent NF-kB site of the HIV-LTR. Furthermore we have demonstrated that when p53 and Sp1 are mixed together both proteins change each other's interaction with DNA. Interestingly, we noted that oncogenic mutant p53 is also able to change the interaction of Sp1 with DNA. We confirmed p53 dependent repression of HIV-LTR driven transcription by comparing the expression from an HIV-LTR reporter construct in the presence and absence of p53. EMSA of an oligonucleotide sequence derived from the HIV-LTR sequence demonstrated a slight decrease in Sp1 DNA binding activity with nuclear extract derived from the cell line expressing a high level of wild-type p53. These data suggest that the influence of p53 on the transcription of promoters with Sp1 binding sites may be partially due to a change in the DNA binding ability of Sp1.