Introduction

The GAL gene regulatory network in the yeast Saccharomyces cerevisiae is one of the most well-characterized systems for understanding eukaryotic gene regulation. This network enables yeast to metabolize galactose efficiently by tightly controlling the expression of genes involved in galactose utilization. The regulation involves several key proteins—GAL4, GAL80, and GAL3—each playing distinct roles in activating or repressing GAL gene expression. In this article, we will analyze four common statements about GAL gene expression and identify which one is false, providing a detailed explanation of the underlying biology.

Overview of GAL Gene Regulation

The GAL regulon in yeast consists of several genes required for galactose metabolism, including GAL1, GAL2, GAL7, and GAL10. The expression of these genes is controlled by the interplay of three main regulatory proteins:

• GAL4: A transcriptional activator that binds to the upstream activating sequence (UAS) of GAL genes and drives their expression.

• GAL80: A negative regulator that binds to GAL4 and inhibits its activity in the absence of galactose.

• GAL3: A signal transducer that, in the presence of galactose, binds to GAL80 and relieves its inhibition of GAL4, allowing GAL gene expression234.

This regulatory network ensures that yeast only expresses the genes needed for galactose metabolism when galactose is present, conserving cellular resources.

Analyzing the Statements

Let’s examine each of the four statements in detail:

1. GAL4 is a positive regulator of GAL genes

Explanation:
GAL4 is the primary transcriptional activator for the GAL genes. In the presence of galactose, GAL4 is freed from inhibition and binds to the UAS of GAL genes, recruiting the transcriptional machinery and activating gene expression. This makes GAL4 a positive regulator of GAL gene expression234.

Conclusion:
True.

2. The UAS region that regulates the GAL gene expression harbors short, phased AT repeats every 10 base pairs

Explanation:
The UAS (upstream activating sequence) for GAL genes is characterized by specific binding sites for the GAL4 protein. While the UAS is known to have a particular sequence organization that allows GAL4 to bind and activate transcription, the presence of short, phased AT repeats every 10 base pairs is not a standard or widely recognized feature of the GAL UAS. The UAS is more accurately described by its GAL4 binding sites, not by periodic AT repeats25.

Conclusion:
Likely false or at least not a standard description. However, this is less commonly discussed and may be a distractor.

3. GAL80 is a positive regulator of GAL4

Explanation:
GAL80 is a negative regulator of GAL4. In the absence of galactose, GAL80 binds to GAL4 and prevents it from activating GAL gene expression. Only when galactose is present does GAL3 bind to GAL80, releasing GAL4 to activate transcription. GAL80 does not positively regulate GAL4; it inhibits it234.

Conclusion:
False.

4. GAL80 is negatively regulated by GAL3

Explanation:
In the presence of galactose, GAL3 binds to GAL80 and sequesters it away from GAL4. This relieves the inhibition of GAL4 and allows it to activate GAL gene expression. In this sense, GAL3 negatively regulates GAL80 by preventing it from inhibiting GAL4236.

Conclusion:
True.