In yeast, glycolysis plays a major role in energy production, and glucose is far and away its preferred carbon source. Genes involved in the metabolism of other carbon sources are usually repressed when glucose is available. When glucose is not available, however, yeast activate genes that metabolize other available energy sources, such as galactose. Galactose increases the transcription of several genes for enzymes that transport galactose into cells and ultimately convert it into glucose-6-phosphate (G6P), an intermediate in glycolysis. The first gene in the pathway induced by galactose, GAL1, encodes galactokinase, which phosphorylates galactose to galactose-1-phosphate. (Check out the GAL1 pathways link in SGD.) The GAL1 promoter has been incorporated upstream of the ORF site in both the pBG1805 and pYES2.1 plasmids and therefore controls transcription of plasmid-encoded MET/Met and lacZ genes in transformed cells.
The figure on the opposite page provides a simple overview of gene expression from theGAL1 promoter in the presence of glucose, raffinose and galactose. The promoter contains both negative and positive regulatory sites encoded within its DNA sequence. In the presence of glucose, repressor proteins bind to the negative regulatory sites and repress transcription. The Gal4p transcriptional activator binds to positive regulatory sites. Gal4p is a transcription factor that binds to DNA as a dimer. (The figure at the beginning of this chapter shows the crystal structure of the DNA binding and dimerization domains of Gal4p complexed with DNA.) In the presence of glucose, Gal4p is inactive, because it is bound to the repressor protein, Gal80p.
Glucose repression can be relieved by growing cells in a poor carbon source, such as raffinose. Raffinose is a trisaccharide composed of galactose, fructose and glucose. Raffinose is not able to induce high levels of GAL1 expression, which requires galactose. In the presence of galactose, expression of the GAL1 gene increases ~1000-fold above the level observed in the presence of glucose. This stimulation is primarily due to the activity of Gal4p, which is no longer bound to the inhibitory Gal80p protein. Gal4p acts as a master regulator of galactose metabolism. In addition to activating GAL1 transcription, Gal4p also binds to the promoters of the GAL7 and GAL10 genes, which are situated adjacent to the GAL1 gene on yeast chromosome 2. Like GAL1, the GAL7 and GAL10 genes encode proteins involved in galactose metabolism.
Regulation of the GAL1 promoter. In the presence of glucose, transcription is repressed because repressor proteins bind to regulatory sites
in the DNA and to the Gal4p transcriptional activator. Glucose repressionis relieved in the presence of raffinose, but Gal4p remains inactive.
Gal4p activates transcription in the presence of galactose due to the removal of the Gal80p protein.