When does UASg function as an enhancer?

UASg, or the Upstream Activating Sequence of the GAL genes in yeast, functions as an enhancer specifically in the presence of galactose. This sequence is essential for the transcriptional activation of the GAL genes when galactose is available as a carbon source.

In the presence of galactose, UASg binds to transcription factors, primarily Gal4, which then interacts with the transcriptional machinery to enhance the expression of genes required for metabolizing galactose. This regulatory mechanism allows yeast to switch its metabolic pathway in response to the availability of different sugars, showcasing a finely tuned response to environmental changes.

When galactose is absent, UASg does not function as an enhancer because the necessary activators are not present, leading to repression of the GAL genes. Similarly, in the presence of glucose, which is a preferred carbon source over galactose, the expression of GAL genes is generally repressed, demonstrating that the presence of glucose prevents UASg from being active in enhancing gene expression. Therefore, the enhancer activity of UASg is conditional, aligning specifically with the presence of galactose.