What regulates the action of GAL4?

The action of GAL4 is regulated primarily by the presence of specific sugars, particularly galactose. GAL4 is a transcription factor in yeast that activates the expression of genes involved in galactose metabolism. When galactose is present in the environment, it binds to a regulatory protein called Gal80, which normally inhibits GAL4's activity. In the presence of galactose, Gal80 undergoes a conformational change, leading to its release from GAL4 and allowing GAL4 to activate the transcription of target genes. This mechanism allows the yeast to utilize galactose as a carbon source only when it is available, demonstrating how the presence of specific nutrients directly regulates gene expression in response to environmental conditions.

The other options, while potentially relevant to various biological processes, do not specifically influence the regulation of GAL4.