Multiple and essential Sp1 binding sites in the promoter for transforming growth factor-beta type I receptor

Maximal gene expression driven by the promoter for the transforming growth factor beta type I receptor (TGF-betaRI) occurs with a 1. 0-kilobase pair fragment immediately upstream of exon 1. This region lacks a typical TATA box but contains CCAAT boxes, multiple Sp1, and PEBP2/CBFalpha binding sites among other possible cis-acting elements. Alterations within two CCAAT box sequences do not mitigate reporter gene expression driven by the basal promoter, and no nuclear factor binds to oligonucleotides encompassing these sites. In contrast, other deletions or site-specific mutations reveal an essential Sp1 site in the basal promoter and several dispersed upstream Sp1 sites that contribute to maximal reporter gene expression. The proportions of transcription factors Sp1 and Sp3, and their ratios of binding to consensus elements, are maintained in bone cells at different stages of differentiation. Finally, nuclear factor that binds to PEBP2/CBFalpha-related cis-acting elements in the basal promoter sequence also occurs in osteoblasts. Our studies reveal that constitutive expression of TGF-betaRI may be determined by constitutive nuclear factor binding to Sp1 sites, whereas other elements may account for the variations in TGF-betaRI levels that parallel changes in bone cell differentiation or activity.