Skip to main content
Pearson+ LogoPearson+ Logo
Ch. 18 - Control of Gene Expression in Bacteria
Freeman - Biological Science 8th Edition
Freeman8th EditionBiological ScienceISBN: 9780138276263Not the one you use?Change textbook
All textbooksFreeman 8th EditionCh. 18 - Control of Gene Expression in BacteriaProblem 7
Chapter 18, Problem 7

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

Verified step by step guidance
1
Understand the context: The SOS regulon is a group of genes in bacteria that are activated in response to DNA damage. The lexA gene encodes the LexA protein, which acts as a repressor for the SOS genes under normal conditions.
Recognize the role of LexA: LexA binds to the operator regions of SOS genes, preventing their transcription. This repression ensures that the SOS response is not activated unnecessarily, which could be wasteful or harmful to the cell.
Consider the need for constant regulation: Since DNA damage can occur at any time, the cell must always have a mechanism in place to repress the SOS genes when there is no damage. This requires a steady supply of LexA protein, which is achieved through constitutive expression of the lexA gene.
Understand the balance: Constitutive expression of lexA ensures that the cell maintains a baseline level of LexA protein. When DNA damage occurs, the RecA protein becomes activated and facilitates the self-cleavage of LexA, lifting the repression and allowing the SOS genes to be expressed.
Conclude the reasoning: Constitutive expression of lexA is essential because it ensures the cell is always prepared to regulate the SOS response efficiently, maintaining a balance between repression and activation depending on the presence or absence of DNA damage.

Verified video answer for a similar problem:

This video solution was recommended by our tutors as helpful for the problem above.
Video duration:
3m
Was this helpful?

Key Concepts

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

SOS Response

The SOS response is a cellular mechanism in bacteria, particularly in Escherichia coli, that is activated in response to DNA damage. This response involves the expression of a set of genes that help repair damaged DNA and ensure cell survival. The SOS regulon includes various genes, including those regulated by the lexA protein, which plays a crucial role in controlling this response.
Recommended video:
Guided course
03:06
Inflammatory Response

lexA Gene

The lexA gene encodes a repressor protein that inhibits the expression of SOS response genes under normal conditions. When DNA damage occurs, the lexA protein is cleaved, leading to the derepression of these genes. This mechanism ensures that the cell can quickly respond to and repair DNA damage, which is vital for maintaining genomic integrity.
Recommended video:
Guided course
05:02
Genes & Alleles

Constitutive Expression

Constitutive expression refers to the continuous production of a gene product, regardless of environmental conditions. In the case of the lexA gene, its constitutive expression ensures that the repressor is always available to regulate the SOS response. This allows for a rapid response to DNA damage, as the system is primed to activate repair mechanisms immediately when needed.
Recommended video:
Guided course
06:40
Introduction to Regulation of Gene Expression
Related Practice
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

844
views
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.

528
views
Textbook Question

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

763
views
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.

806
views
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

512
views
Textbook Question

X-gal is a colorless, lactose-like molecule that can be split into two fragments by ββ-galactosidase. One of these product molecules creates a blue color. The photograph here shows E. coli colonies growing in a medium that contains X-gal. Find three colonies whose cells have functioning copies of ββ-galactosidase.

Find three colonies whose cells might have mutations in the lacZ or the lacY genes.

Suppose you analyze the protein-coding sequence of the lacZ and lacY genes of cells from the three mutant colonies and find that these sequences are wild type (normal).

What other region of the lac operon might be altered to account for the mutant phenotype of these colonies?

884
views