The yeast Gal switch is used to control the conversion of galactose to glucose through the Leloir pathway. Similar to the lac operon in E. coli, the enzymes and proteins required for this metabolic pathway are only expressed when galactose is present. Using Gal1, Gal2 permease, Gal 7, and Gal10, the conversion can be catalysed.
Gal4, a key regulator protein, is integral in permitting the expression of the permease and enzymes. Galactose binds to Gal3, acting as a sensor and detecting the presence of galactose. This allows the removal of Gal80 from the nucleus, removing the repressor and upregulating the transcription of the genes. The necessary enzymes required for galactose metabolism are then expressed, on activation of the Gal gene switch.
Gal4 is a specific transcription factor, having a specific regulatory effect on a regulon. This allows control of transcription with Gal80 acting as the repressor.