Retinoblastoma binding factor 1 site in the core promoter region of the human RB gene is activated by hGABP/E4TF1

We previously reported two oncogenic point mutations present in the RB (retinoblastoma) gene promoter region, found at consensus Spl and ATF sites, respectively, and in two separate hereditary RB families. However, Spl protein was shown not to bind to the Spl site; this indicated that the Spl consensus site mutation was blocking the action of an alternative transcription factor, which we called RBF-1 (retinoblastoma binding factor 1). Subsequent purification of RBF-1 revealed it to be hGABP/E4TF1, a transactivator from the adenovirus early-region 4 promoter. In this study, we directly examined the effects of hGABP/E4TFI on transactivation of the RB gene promoter through the RBF-1 site. As expected, hGABP/E4TF1 enhanced the core RB promoter activity, whereas it did not stimulate a mutant RBF-1 site. We therefore conclude that the most essential transcription factor in the human RB gene is likely to be hGABP/E4TF1.