RESUMEN
We have demonstrated previously that the seven-nucleotide (nt) motif TTTTGTA (the heptamer) that is present within the proximal 3' untranslated sequences of numerous immediate-early genes is essential for platelet-derived growth factor (PDGF)-stimulated induction of the MCP-1 immediate-early gene. On this basis, the heptamer was suggested to be a conserved regulatory element involved in immediate-early gene expression, although its mechanism of action was unknown. Herein, we demonstrate that the heptamer functions to remove an inhibition of PDGF induction of MCP-1 maintained by two independently acting inhibitory elements present in the MCP-1 5' flanking sequences (designated I* elements). PDGF treatment relieves the I*-mediated inhibition of MCP-1 expression only if the heptamer is also present. One inhibitory element is contained within a 59-nt portion of MCP-1 5' flanking sequences and functions in an orientation-independent and heptamer-regulated manner. Significantly, proteins binding to two DNA sequences contribute to the formation of a single multiprotein complex on the 59-nt I* element. The I*-binding complex contains Sp3, an Sp1-like protein, and a novel DNA-binding protein. Moreover, the complex does not form on two 59-nt sequences containing mutations that reverse the inhibition of PDGF induction maintained by the wild-type I* element. We propose to call the multiprotein I*-binding complex a repressosome and suggest that it acts to repress PDGF-stimulated transcription of MCP-1 in the absence of the heptamer TTTTGTA.