[Show abstract][Hide abstract] ABSTRACT: The regulation of the Saccharomyces cerevisiae GAL genes in response to galactose as a source of carbon has served as a paradigm for eukaryotic transcriptional control over
the last 50 years. Three proteins—a transcriptional activator (Gal4p), an inhibitor (Gal80p), and a ligand sensor (Gal3p)—control
the switch between inert and active gene expression. The molecular mechanism by which the recognition of galactose within
the cell is converted into a transcriptional response has been the subject of considerable debate. In this study, using a
novel and powerful method of localizing active transcription factors within the nuclei of cells, we show that a short-lived
complex between Gal4p, Gal80p, and Gal3p occurs soon after the addition of galactose to cells to activate GAL gene expression. Gal3p is subsequently replaced in this complex by Gal1p, and a Gal4p-Gal80p-Gal1p complex is responsible
for the continued expression of the GAL genes. The transient role of the ligand sensor indicates that current models for the induction and continued expression of
the yeast GAL genes need to be reevaluated.