Practice question

Galactose sensing and induction depend on Gal3 protein, paralog of Gal1 galactokinase lacking kinase activity but preserving ligand-binding pocket. In presence of galactose and ATP, Gal3 undergoes conformational change promoting high-affinity interaction with Gal80 repressor both cytoplasmic and nuclear fractions, sequestering Gal80 away from Gal4-bound promoters. Gal3-Gal80 sequestration frees Gal4 activation domain recruiting coactivators initiating transcription of GAL1, GAL10, GAL7 genes required for galactose catabolism via Leloir pathway. Gal1 itself contributes to maintenance after initial induction, binding Gal80 similarly, establishing feedback memory that keeps GAL system primed for subsequent galactose exposures even transiently after inducer removed.