Galactose metabolism in yeast is controlled by the gal switch. This is a regulatory mechanism allowing the determination of whether galactose is present, controlling the expression of the Leloir pathway to convert galactose to glucose.
Galactose binds to Gal3, which acts as a sensor to detect galactose is present. On binding to Gal3, Gal80 (the repressor) is removed from the DNA, allowing the expression of the enzymes necessary. This allows the expression of Gal1, Gal2 permease, Gal7 and Gal10. Gal4 is a specific transcription factor (sTF), allowing the control of gene expression of the regulon, especially of the repressor (Gal80).
This process, similar to the lac operon in E. coli, allows the selective expression of these metabolic enzymes only in the presence of the substrate sugar (galactose in the case of the gal switch, and lactose in the case of the lac operon).