Different activation domains of Sp1 govern formation of multimers and mediate transcriptional synergism.

The process of transcriptional activation in eukaryotes by site-specific DNA-binding proteins is a key step in gene regulation. Here we have examined the properties of four distinct activator domains of the human transcription factor Sp1. In vivo transient cotransfection assays with Sp1 show that templates bearing multiple Sp1 sites activate transcription with a high degree of synergism. However, there is no evidence of cooperative binding of Sp1 to adjacent sites. Using deletion mutants of Sp1 we have determined that the glutamine-rich activation domains A and B and the previously uncharacterized carboxy-terminal domain D are all required for Sp1 to activate transcription synergistically. Gel-shift, DNase footprinting, and chemical cross-linking experiments reveal a strong correlation between the ability of Sp1 mutants to form homomultimeric complexes and their ability to activate transcription synergistically when bound to multiple sites. We have also examined the process of superactivation, in which a molecule of Sp1 tethered to DNA via its zinc fingers can be transcriptionally enhanced by interacting directly with fingerless Sp1 molecules. The domains involved in superactivation appear to be a subset of those necessary to achieve synergistic activation. These findings suggest that different domains of Sp1 carry out distinct functions and that the formation of multimeric complexes may direct synergism and superactivation.