Regulation of GAL genes by the proteins Gal3p, Gal4p, and Gal80p. (a) Gal4p binds UASGAL, but Gal80p binds Gal4p and prevents its activation of Pol II and the general transcription factors. (b) Galactose is a small-molecule effector for Gal3p, causing it to bind Gal80p and alter Gal80p conformation, which frees Gal4p to activate transcription.