A SINGLE GAL4 DIMER CAN MAXIMALLY ACTIVATE TRANSCRIPTION UNDER PHYSIOLOGICAL CONDITIONS

Most eukaryotic promoters contain multiple binding sites for one or mo re transcriptional activators that interact in a synergistic manner. A common view is that synergism is a manifestation of the need for many contacts between activators and the general transcription machinery t hat a single activator presumably cannot fulfill. In this model, vario us combinations of protein-protein interactions control the level of g ene expression. However, we show here that under physiological conditi ons, a single binding site and presumably binding of a single dimer of the prototypical yeast activator GAL4 can activate transcription to t he maximum possible level ira vivo. Synergistic effects in this natura l system are shown to be consistent with cooperative DNA binding. Thes e results point to DNA occupancy as the major element in fine tuning g ene expression in the galactose regulon.