Combinatorial control in global repression of yeast GAL genes. Expression levels of a GAL gene are shown under three different growth conditions, with (+) or without (−) glucose or galactose. (a) In the absence of glucose and galactose, Gal4p occupies UAS_GAL, but the GAL gene is repressed by Gal80p. (b) In the presence of galactose and absence of glucose, Gal4p activates transcription of the GAL gene because Gal80p repression is relieved by binding of Gal3p. (c) In the presence of both glucose and galactose, glucose is the preferred carbon source; there is no transcription of the GAL gene because the Mig1-Tup1 complex represses its expression below basal levels.