What happens when galactose is present in the cell?

When galactose is present in the cell, GAL3 binds to GAL80, which is crucial for the regulation of the GAL gene system in yeast. In the absence of galactose, GAL80 inhibits the transcriptional activation of genes needed for the metabolism of galactose by binding to GAL4, the transcription factor that promotes the expression of those genes. However, when galactose is available, GAL3 interacts with it and undergoes a conformational change, allowing GAL3 to bind to GAL80. This interaction leads to the release of GAL4 from inhibition, allowing transcription to proceed. This mechanism exemplifies how the presence of a specific substrate (in this case, galactose) can signal the cell to express the necessary genes for its utilization, promoting the cell's adaptability to varying nutrient availability.