Yeast gene regulation — Which promoter is specifically repressed by the presence of glucose via catabolite repression (consider standard Saccharomyces cerevisiae laboratory conditions)?

Introduction / Context:
Glucose repression (also called catabolite repression) is a classic regulatory circuit in yeast. When glucose is abundant, Saccharomyces cerevisiae downregulates genes required to metabolize alternative carbon sources such as galactose. Identifying which promoters are glucose-repressed versus phosphate- or growth-driven is essential in designing expression systems and interpreting transcriptional data in molecular biology labs.

Given Data / Assumptions:

• Organism of interest: Saccharomyces cerevisiae.
• Glucose present in the medium triggers catabolite repression via Mig1/Snf1 pathways.
• GAL system (GAL1, GAL10, GAL7) is the canonical glucose-repressed, galactose-inducible system.
• PHO system (PHO5/PHOS) is primarily phosphate regulated, not glucose dependent.
• PGK1 and ADH1 are robust glycolytic/housekeeping promoters active in glucose media.

Concept / Approach:
In glucose, the regulator Mig1 localizes to the nucleus and represses GAL promoters, preventing unnecessary expression of galactose-utilization genes. PHO5 responds to inorganic phosphate scarcity; its induction requires low phosphate rather than glucose absence. PGK1 and ADH1 are typically active in glucose-rich conditions, supporting high glycolytic flux and fermentation.

Step-by-Step Solution:

Verification / Alternative check:
Yeast expression vectors often use GAL1 for tightly controlled, off-in-glucose/on-in-galactose induction, confirming glucose repression in practice.

Why Other Options Are Wrong:

• b: PHO5 depends mainly on phosphate starvation, not on glucose levels.
• c: Both is incorrect because PHO5 is not characteristically glucose-repressed.
• d: PGK1 is highly active in glucose; it is not repressed by it.
• e: ADH1 is a strong constitutive promoter in glucose media.

Common Pitfalls:
Assuming any “inducible” promoter is also glucose-repressed; regulatory signals are pathway-specific (carbon vs. phosphate regulation).

Final Answer:
Galactokinase promoter (GAL1)