Regulation of GAL gene transcription

(a) When galactose is absent, Gal80 protein binds the activation domain of Gal4 to inactivate that protein and block GAL gene transcription. (b) When galactose is present, Gal3 protein binds Gal80 protein to prevent it from binding Gal4 protein. The activation domain of Gal4 protein is then available to initiate GAL gene transcription(a) When galactose is absent, Gal80 protein binds the activation domain of Gal4 to inactivate that protein and block GAL gene transcription. (b) When galactose is present, Gal3 protein binds Gal80 protein to prevent it from binding Gal4 protein. The activation domain of Gal4 protein is then available to initiate GAL gene transcription

(a) Galactose absent Gal80 Activation domain . _ Ga|4 is bound by Gal80 and is , i H unable to activate transcription. Ga|4 homodimer GAL genes No transcription Gal80 is bound by Gal3, releasing Gal4 to activate transcription. DNA-binding domain homodimer GAL genes Ifi—l Transcription