Dimeric ligands define a role for transcriptional activation domains in reinitiation.

Eukaryotic transcriptional activators mediate transcriptional induction through stabilization of the preinitiation complex, probably through direct interactions with basal transcription factors. In vitro studies on the role of an activator in the maintenance of on-going transcription (reinitiation) have been contradictory, suggesting that, after formation of a preinitiation complex, an activator may or may not be necessary for transcription to be maintained. We have developed a means of regulating transcription in living cells through the use of both homodimeric and heterodimerizing synthetic ligands that allow the ligand-dependent association and disassociation of a transcriptional activation domain with a promoter. Here we report that maintaining the transcription of endogenous genes in vivo, in both yeast and human cells, requires the continuous presence of the activation domain. The use of synthetic ligands as a transcriptional on-off switch represents a powerful means of controlling the transcription in vitro and in vivo for both experimental and therapeutic purposes.