Textbook Question
Explain how the following mutations would affect transcription of the yeast GAL1 gene in the presence of galactose.
A deletion within the GAL4 gene that removes the region encoding amino acids 1 to 100.
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13. Gene Regulation in Eukaryotes
GAL Regulation
2:37 minutesProblem 25a
Textbook Question
Regulation of the lac operon in E. coli and regulation of the GAL system in yeast are analogous in that they both serve to adapt cells to growth on different carbon sources. However, the transcriptional changes are accomplished very differently. Consider the conceptual similarities and differences as you address the following.
Compare and contrast the roles of the lac operon inducer in bacteria and Gal3p in eukaryotes in the regulation of their respective systems.
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Step 1: Begin by defining the lac operon inducer in bacteria, which is allolactose (a lactose derivative). Explain that it functions by binding to the lac repressor protein, causing a conformational change that prevents the repressor from binding to the operator region, thereby allowing transcription of lac genes.
Step 2: Define Gal3p in yeast as a signal transducer protein involved in the GAL system. Describe how Gal3p binds galactose and ATP, then interacts with the Gal80p repressor to relieve its inhibition on the Gal4p activator, enabling transcription of GAL genes.
Step 3: Highlight the conceptual similarity that both the lac operon inducer and Gal3p act as sensors of the sugar (lactose or galactose) and trigger transcriptional activation by relieving repression, but through different molecular mechanisms.
Step 4: Contrast the mechanisms by noting that the lac operon inducer directly binds the repressor protein to prevent DNA binding, a relatively simple prokaryotic system, whereas Gal3p acts indirectly by binding to a repressor protein (Gal80p) to free an activator (Gal4p), reflecting the more complex eukaryotic regulation involving multiple protein-protein interactions.
Step 5: Summarize that the lac operon inducer functions by inactivating a repressor to allow RNA polymerase access, while Gal3p functions by modulating protein interactions to activate transcription factors, illustrating fundamental differences in bacterial versus eukaryotic gene regulation despite their analogous roles.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Lac Operon Induction in E. coli
The lac operon in E. coli is regulated by an inducer molecule, allolactose, which binds to the lac repressor and prevents it from blocking transcription. This allows the bacteria to express genes needed for lactose metabolism only when lactose is present, enabling efficient use of available carbon sources.
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Role of Gal3p in Yeast GAL System
Gal3p is a regulatory protein in yeast that senses galactose and interacts with the repressor Gal80p, relieving its inhibition on the activator Gal4p. This interaction promotes transcription of GAL genes, allowing yeast to metabolize galactose, demonstrating a eukaryotic mechanism of transcriptional regulation.
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Differences Between Prokaryotic and Eukaryotic Transcriptional Regulation
Prokaryotic regulation, like the lac operon, often involves direct repressor-operator interactions controlling RNA polymerase access, while eukaryotic systems use complex protein-protein interactions and chromatin remodeling. The lac operon inducer acts by inactivating a repressor, whereas Gal3p modulates activator-repressor interactions to regulate gene expression.
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