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Ch. 18 - Control of Gene Expression in Bacteria
Freeman - Biological Science 8th Edition
Freeman8th EditionBiological ScienceISBN: 9780138276263Not the one you use?Change textbook
Chapter 18, Problem 6

Predict what would happen to regulation of the lac operon if the lacI gene were moved 50,000 nucleotides upstream of its normal location.

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Understand the role of the lacI gene: The lacI gene encodes the lac repressor protein, which binds to the operator region of the lac operon to inhibit transcription in the absence of lactose. This regulation is crucial for the operon's function.
Consider the impact of moving the lacI gene upstream: If the lacI gene is relocated 50,000 nucleotides upstream, it will still produce the lac repressor protein, as long as its promoter and coding sequence remain intact. However, the spatial distance between the lacI gene and the lac operon may affect the efficiency of regulation.
Analyze the mechanism of repressor action: The lac repressor protein operates by diffusing through the cytoplasm to bind to the operator region of the lac operon. Since the lacI gene's location does not directly affect the protein's ability to diffuse, the repressor can still reach the operator.
Evaluate potential effects on transcriptional regulation: While the lac repressor can still function, the relocation of the lacI gene might slightly delay the production of the repressor protein due to transcriptional and translational processes. This could lead to minor changes in the timing of operon regulation.
Conclude the likely outcome: The lac operon would still be regulated by the lac repressor, but the efficiency or timing of regulation might be altered due to the increased distance between the lacI gene and the operon. This change is unlikely to completely disrupt the operon's function.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Lac Operon Structure

The lac operon is a set of genes in E. coli that are involved in the metabolism of lactose. It consists of three structural genes (lacZ, lacY, and lacA) and regulatory elements, including the promoter and operator. The operon is controlled by the lacI gene, which encodes the lac repressor protein that inhibits transcription in the absence of lactose.
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The Lac Operon

Gene Regulation

Gene regulation refers to the mechanisms that control the expression of genes, determining when and how much of a gene product is made. In the case of the lac operon, the presence of lactose leads to the inactivation of the lac repressor, allowing transcription of the operon. Moving the lacI gene could disrupt this regulatory mechanism, affecting the operon's response to lactose.
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Introduction to Regulation of Gene Expression

Transcriptional Control

Transcriptional control is a key aspect of gene expression regulation, involving the binding of proteins to specific DNA sequences to either promote or inhibit transcription. In the lac operon, the lac repressor binds to the operator region to block RNA polymerase from transcribing the structural genes. If the lacI gene is relocated, the spatial relationship between the repressor and the operator may be altered, potentially leading to changes in transcription levels.
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Specific Transcription Factors
Related Practice
Textbook Question

Activators bind to regulatory sequences in ________ and to ________ polymerase.

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Textbook Question

A regulon is a set of genes controlled by

a. One type of regulator of transcription

b. Two or more different alternative sigma proteins

c. Many different types of promoters

d. Glucose

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Textbook Question

Evaluate these statements about regulation of the lac operon. Select True or False for each statement.

T/FThe lac operon is transcribed at the highest rate when extracellular glucose and lactose are abundant.

T/FThe repressor protein is bound to DNA of the operator when lactose is present.

T/FA mutation in the operator is likely to prevent transcription of the lac operon under any condition.

T/FA mutation that alters the catabolite activator protein is predicted to alter the regulation of many different operons.

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Textbook Question

Explain why it makes sense for the lexA regulatory gene of the SOS regulon to be expressed constitutively.

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Textbook Question

IPTG is a molecule with a structure much like lactose. IPTG can be transported into cells by galactoside permease and can bind to the lac repressor protein. However, unlike lactose, IPTG is not broken down by ββ-galactosidase.

Predict what would occur to lac operon regulation if IPTG were added to E. coli growth medium containing no glucose or lactose.

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Textbook Question

In a mutant that lacks adenylyl cyclase, the enzyme that synthesizes cAMP, predict which of the following conditions of extracellular lactose and glucose would cause regulation of the lac operon to differ from that of wild-type cells.

a. No lactose, no glucose

b. No lactose, abundant glucose

c. Abundant lactose, no glucose

d. Abundant lactose, abundant glucose

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