The SV40 large T oncoprotein disrupts DNA-binding of the cell cycle-regulated transcriptional repressor CDF

A hallmark of neoplastic transformation by DNA tumor viruses is the deregulation of cell cycle genes. At least in some genes, this deregulation appears to be due to the oncoprotein-mediated disruption of complexes between E2F and pocket proteins and the ensuing generation of transcriptionally active free E2F. In the present study, we have analysed the effect of the SV40 large T oncoprotein (SV-LT) on the function of a different cell cycle-regulated transcriptional repressor, CDF, which is the principal regulator of the cdc25C, cyclin A and cdc2 genes. As shown by genomic footprinting of sorted G1 and G2 cell populations, transformation by SV-LT completely abrogated protection of the CDF binding site (CDE-CHR) in the cdc25C promoter. In agreement with this observation, expression of the SV-LT in fibroblasts led to a dramatic up-regulation of the cdc25C promoter in cells synchronized in G0. These findings indicate that the oncoprotein-mediated dissociation of the CDF repressor protein from its cognate DNA-binding site is a major mechanism in virus-induced transcriptional deregulation.