GAL Regulation - Online Tutor, Practice Problems & Exam Prep

The gal gene regulation system in yeast is an example of eukaryotic gene regulation, functioning similarly to prokaryotic operons. This inducible system activates transcription of genes responsible for processing galactose only in its presence. The UAS region upstream of the gal genes is crucial for this regulation. In the absence of galactose, the gal80 protein inhibits transcription by binding to gal4. When galactose is present, it interacts with gal3, allowing gal4 to promote transcription, thus optimizing energy use in the cell.

GAL Regulation

So your books like to use the example of the GAL gene regulation as an example of gene regulation in lower eukaryotic cells. Things like yeast, which is where the GAL system exists, are focused on. The GAL gene system reminds me of the operons in prokaryotic cells, but there are differences because operons are very much a prokaryotic way of regulating. However, the GAL gene system produces genes that transport and break down a sugar called galactose, and we're focusing on yeast cells here. This system is given a fancy name called "inducible." The reason it's called inducible is that the GAL genes are regulated by the presence or absence of galactose sugar. This sounds very similar to the lac operon or the trp operon, which we talked about in the prokaryotic gene regulation chapter. But this is eukaryotic, so it is a little different. Here's the summary: if there's no galactose, no transcription occurs. If there is galactose, transcription control happens. This type of regulation is called "positive control" because it's activated if it's present.

Let's now get into the details. The transcription is controlled by a specific region upstream, before the gene start site of the GAL genes, called the UAS region. A protein encoded later called GAL4 will bind to this section at four different sites. The second protein you need to know about is GAL80, which negatively regulates the GAL4. So, GAL4 gene, which makes the GAL4 protein. If it's all uppercase, that's the gene. If it has lowercase in it, this is the protein. So, if there's an absence of galactose, GAL80 binds to GAL4 and prevents transcription. If galactose is present, it interacts with another protein called GAL3, which binds to the UAS GAL4 section and promotes transcription.

Let's review the mechanism. Here we have DNA, here's your UAS sequence, and here are your GAL genes. This setup is somewhat akin to a bacterial operon. Transcription doesn't occur if glucose isn't available because the GAL4 protein binds to the GAL80 protein, blocking transcription at the UAS region. If galactose and GAL3 become available, GAL80 leaves, allowing GAL4 to activate, promoting transcription of these genes. This mechanism makes sense because the GAL genes are responsible for processing galactose, and you only need to process galactose if it is present. When galactose is available, you need these genes and when it's not, there's no need to waste energy producing these genes because they're not necessary without the sugar. This is an example of gene regulation in eukaryotic, but specifically lower eukaryotic cells like yeast. With that, let's now move on.

Which of the following is the purpose of the GAL gene system?

• A. To synthesize galactose
• B. To break down galactose
• C. To synthesize glucose
• D. To break down glucose

The GAL gene system is activated in which of the following conditions?

• A. Absence of galactose
• B. Presence of galactose
• C. Absence of glucose
• D. Presence of glucose

When the GAL gene system is activated, galactose binds to the UAS regulatory region.

• A. True
• B. False

Here’s what students ask on this topic:

What is the GAL gene regulation system in yeast?

The GAL gene regulation system in yeast is an example of eukaryotic gene regulation. It functions similarly to prokaryotic operons but is specific to lower eukaryotic cells like yeast. This inducible system activates the transcription of genes responsible for processing galactose only in its presence. The upstream activating sequence (UAS) region, located before the gal genes, is crucial for this regulation. In the absence of galactose, the GAL80 protein inhibits transcription by binding to GAL4. When galactose is present, it interacts with GAL3, allowing GAL4 to promote transcription. This system optimizes energy use by ensuring that the genes for processing galactose are only expressed when galactose is available.

How does the presence of galactose affect the GAL gene regulation system?

In the presence of galactose, the GAL gene regulation system in yeast activates the transcription of genes responsible for processing galactose. Galactose interacts with the GAL3 protein, which then binds to the GAL80 protein. This interaction causes GAL80 to dissociate from GAL4, allowing GAL4 to bind to the UAS region upstream of the gal genes. This binding promotes the transcription of the gal genes, enabling the yeast cell to process and utilize galactose. This inducible system ensures that the genes are only expressed when galactose is available, optimizing the cell's energy use.

What role does the UAS region play in the GAL gene regulation system?

The UAS (Upstream Activating Sequence) region plays a crucial role in the GAL gene regulation system in yeast. Located upstream of the gal genes, the UAS region is the binding site for the GAL4 protein. When galactose is absent, the GAL80 protein binds to GAL4, preventing it from interacting with the UAS region and thus inhibiting transcription. When galactose is present, it interacts with GAL3, which binds to GAL80, causing it to dissociate from GAL4. This allows GAL4 to bind to the UAS region, promoting the transcription of the gal genes. The UAS region is essential for the inducible activation of these genes.

How does the GAL80 protein regulate the GAL gene system?

The GAL80 protein plays a key regulatory role in the GAL gene system in yeast. In the absence of galactose, GAL80 binds to the GAL4 protein, preventing it from interacting with the UAS region upstream of the gal genes. This inhibition blocks the transcription of the gal genes, conserving the cell's energy. When galactose is present, it interacts with the GAL3 protein, which then binds to GAL80. This interaction causes GAL80 to dissociate from GAL4, allowing GAL4 to bind to the UAS region and promote the transcription of the gal genes. Thus, GAL80 acts as a negative regulator in the absence of galactose.

What is the function of the GAL4 protein in the GAL gene regulation system?

The GAL4 protein is a transcriptional activator in the GAL gene regulation system in yeast. It binds to the UAS (Upstream Activating Sequence) region located upstream of the gal genes. In the absence of galactose, the GAL80 protein binds to GAL4, preventing it from interacting with the UAS region and inhibiting transcription. When galactose is present, it interacts with the GAL3 protein, which binds to GAL80, causing it to dissociate from GAL4. This allows GAL4 to bind to the UAS region, promoting the transcription of the gal genes. GAL4 is essential for the inducible activation of these genes in the presence of galactose.