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Gal4 regulates expression of the yeast GAL genes in response to galactose. In noninducing conditions (A), Gal4 is bound to the upstream activating sequences for galactose (UASGAL) but is prevented from activating transcription by the inhibitor Gal80. Rapid induction by galactose requires Gal3 (B). Recent experiments demonstrate that Gal3, when bound to galactose, directly interacts with Gal80 in the presence of ATP. Gal3-galactose is thought to cause induction of the GAL genes by producing a conformational change in the Gal4-Gal80 complex that allows interaction of the Gal4 activating domains with the general transcription factors. The induced conformation may involve a shift of Gal80 from the C terminus to the central region of Gal4.