Competition with TATA box-binding protein for binding to the TATA box implicated in human cytomegalovirus IE2-mediated transcriptional repression of cellular promoters.

The human cytomegalovirus IE2 is a sequence-specific DNA-binding protein. A consensus IE2-binding site (IBS) contains two copies of the dinucleotide CG separated by 10 not well-conserved but AT-rich nucleotides. In this report, we demonstrated that the TATA box of the insulin-like growth factor binding protein 4 (IGFBP4) promoter is embedded in an IBS. In a transient transfection study, IE2-mediated repression of a reporter driven either by a synthetic promoter containing the IGFBP4 TATA/IBS element or by the native IGFBP4 promoter was dependent on the intactness of the IBS. Competition with TBP for binding to the IGFBP4 TATA/IBS element may underlie the mechanism for the IE2-mediated repression, because IE2 and TATA box-binding protein (TBP) binding to the IGFBP4 TATA/IBS element are mutually elusive. Moreover, the TATA boxes of several other genes, including ADA and CPH-70, are likewise confined in IBS-like sequences. The IE2 interacts with those TATA/IBS elements in vitro and inhibits transcription driven by them in vivo, supporting the idea that competitive inhibition of TBP binding to the TATA box represents a novel mechanism exploited by IE2 to repress cellular gene expression.