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Ch. 17 - Transcriptional Regulation in Eukaryotes
Klug - Concepts of Genetics 12th Edition
Klug12th EditionConcepts of GeneticsISBN: 9780135564776Non è quello che usi tu?Cambia libro di testo
Capitolo 17, Problema 25a

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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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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Lac Operon Overview

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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Prokaryotic Transcription
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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 cis-regulatory elements of the lac operon and GAL gene system.

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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 how these two systems are negatively regulated such that they are downregulated in the presence of glucose.

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DNA methylation is commonly associated with a reduction of transcription. The following data come from a study of the impact of the location and extent of DNA methylation on gene activity in eukaryotic cells. A bacterial gene, luciferase, was inserted into plasmids next to eukaryotic promoter fragments. CpG sequences, either within the promoter and coding sequence (transcription unit) or outside of the transcription unit, were methylated to various degrees, in vitro. The chimeric plasmids were then introduced into cultured cells, and luciferase activity was assayed. These data compare the degree of expression of luciferase with differences in the location of DNA methylation [Irvine et al. (2002). Mol. and Cell. Biol. 22:6689–6696]. What general conclusions can be drawn from these data? 

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The interphase nucleus is a highly structured organelle with chromosome territories, interchromatin compartments, and transcription factories. In cultured human cells, researchers have identified approximately 8000 transcription factories per cell, each containing an average of eight tightly associated RNAP II molecules actively transcribing RNA. If each RNAP II molecule is transcribing a different gene, how might such a transcription factory appear? Provide a simple diagram that shows eight different genes being transcribed in a transcription factory and include the promoters, structural genes, and nascent transcripts in your presentation.

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