Ch. 18 - Control of Gene Expression in Bacteria

Freeman - Biological Science 8th Edition

Freeman8th EditionBiological ScienceISBN: 9780138276263Not the one you use?Change textbook

Freeman 8th Edition

Ch. 18 - Control of Gene Expression in Bacteria

Problem 9

Chapter 18, Problem 9

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

Verified step by step guidance

Understand the role of adenylyl cyclase and cAMP in the regulation of the lac operon. Adenylyl cyclase is responsible for synthesizing cyclic AMP (cAMP) from ATP. cAMP binds to the catabolite activator protein (CAP), forming a cAMP-CAP complex that enhances transcription of the lac operon when glucose levels are low.

Analyze the mutant condition. In the mutant lacking adenylyl cyclase, cAMP cannot be synthesized. This means the cAMP-CAP complex cannot form, and the lac operon will not be activated regardless of glucose levels.

Evaluate condition (a): no lactose, no glucose. In wild-type cells, the absence of lactose would prevent lac operon transcription because the repressor remains bound to the operator. In the mutant, the lack of cAMP would not affect this outcome since the operon is already repressed.

Evaluate condition (b): no lactose, abundant glucose. In wild-type cells, the presence of glucose would inhibit cAMP production, and the absence of lactose would keep the operon repressed. In the mutant, the lack of cAMP would not change this outcome, as the operon remains repressed due to the absence of lactose.

Evaluate conditions (c) and (d): abundant lactose, no glucose and abundant lactose, abundant glucose. In wild-type cells, abundant lactose would remove the repressor, and the presence or absence of glucose would determine the level of transcription via cAMP-CAP. In the mutant, even with lactose present, the lack of cAMP would prevent activation of the operon, leading to a difference in regulation compared to wild-type cells in these conditions.

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Key Concepts

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

Lac Operon Regulation

The lac operon is a set of genes in E. coli that are involved in the metabolism of lactose. Its regulation is influenced by the presence of lactose and glucose. When lactose is present, it binds to the repressor protein, allowing transcription of the operon. Conversely, high glucose levels inhibit the operon through catabolite repression, which prevents the synthesis of cAMP, a crucial signaling molecule.

cAMP and Adenylyl Cyclase

Cyclic AMP (cAMP) is a secondary messenger that plays a vital role in cellular signaling. It is synthesized from ATP by the enzyme adenylyl cyclase. In the context of the lac operon, cAMP levels are inversely related to glucose concentration; low glucose leads to high cAMP, which activates the CAP (catabolite activator protein) to enhance transcription of the lac operon. In the mutant lacking adenylyl cyclase, cAMP cannot be produced, affecting operon regulation.

Catabolite Repression

Catabolite repression is a regulatory mechanism that ensures bacteria preferentially utilize the most efficient energy source. In the presence of glucose, the synthesis of cAMP is inhibited, leading to reduced activation of the lac operon, even if lactose is available. This mechanism allows cells to conserve energy by prioritizing glucose metabolism over lactose, which is less efficient. Understanding this concept is crucial for predicting how the lac operon will behave in different nutrient conditions.

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Repressible Operons

Related Practice

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.

Textbook Question

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

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.

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?

Textbook Question

The Hawaiian bobtail squid (Euprymna scolopes) is able to glow from luminescent Vibrio fischeri bacteria held in its light organs. As it swims at night near the ocean surface, it adjusts the amount of light visible to predators below to match the light from the stars and moon. Predators have difficulty seeing the illuminated squid against the night sky.

The bacteria glow in response to a molecule that regulates expression of genes involved in light-producing chemical reactions. The regulator controls production of the genes' mRNA. Therefore, the light-producing genes are under

a. Transcriptional control.

b. Translational control.

c. Post-translational control.

d. Negative control.

Textbook Question

The light-producing genes of V. fischeri are organized in an operon that is under positive control by an activator protein called LuxR.

Would you expect the genes of this operon to be transcribed when LuxR is bound or not bound to a DNA regulatory sequence? Explain.