Accessibility of alpha 2-repressed promoters to the activator Gal4.

It has been proposed that eukaryotic repressors of transcription can act by organizing chromatin, thereby preventing the accessibility of nearby DNA to activator proteins required for transcription initiation. In this study, we test this idea for the yeast alpha 2 repressor using a simple, artificial promoter that contains a single binding site for the activator protein Gal4 and a single binding site for the repressor alpha 2. When both the repressor and the activator are expressed in the same cell, the artificial promoter is efficiently repressed. In vivo footprinting experiments demonstrate that Gal4 can occupy its binding site even when the promoter is repressed. This result indicates that alpha 2-directed repression must result from interference with some stage in transcription initiation other than activator binding to DNA.