Ch. 18 - Control of Gene Expression in Bacteria

Freeman - Biological Science 7th Edition

Freeman7th EditionBiological ScienceISBN: 9783584863285Not the one you use?Change textbook

Freeman 7th 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 glucoseb. no lactose, abundant glucosec. abundant lactose, no glucosed. abundant lactose, abundant glucose

Verified step by step guidance

Understand the role of adenylyl cyclase: Adenylyl cyclase is responsible for converting ATP to cyclic AMP (cAMP). In the context of the lac operon, cAMP binds to the cAMP receptor protein (CRP), which then enhances the transcription of the operon.

Consider the effect of glucose on cAMP levels: Normally, high levels of glucose inhibit the production of cAMP. Therefore, in the presence of glucose, cAMP levels are low, reducing the activity of CRP and thus decreasing the transcription of the lac operon.

Analyze the impact of lactose: Lactose acts as an inducer of the lac operon by binding to the repressor and preventing it from binding to the operator site. This allows RNA polymerase to transcribe the genes necessary for lactose metabolism.

Predict the regulation in mutant cells: In a mutant lacking adenylyl cyclase, cAMP cannot be synthesized regardless of glucose levels. This means that CRP cannot be activated, potentially altering the regulation of the lac operon.

Evaluate each condition: For each given condition (a-d), consider how the absence of cAMP synthesis would affect the regulation of the lac operon compared to wild-type cells, focusing on the presence or absence of lactose and glucose.

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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 responsible for 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

cAMP (cyclic adenosine monophosphate) 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 mutants lacking adenylyl cyclase, cAMP cannot be produced, disrupting this regulatory mechanism.

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.

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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

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?

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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.

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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.

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