16. Regulation of Expression

Prokaryotic Gene Regulation via Operons

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

Which of the following molecules is a protein that stops the transcription of a gene?

Inducible Operons

 No. A repressor may attach to the operator in this structure. 

 No. Regulatory genes are usually located elsewhere on the chromosome. 

 No. Enhancers are found in eukaryotic genomes. 

 No. Prokaryotes do not use transcription factors. 

Both lac and trp have these.

In prokaryotic genomes, groups of functionally related genes along with their promoters and operators are found together in __________.

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.


The diagram shown here is a model of the gene regulatory circuit for light production by V. fischeri cells. The lux operon contains genes for luminescence (luxCDABE) and a gene, luxI, that encodes an enzyme that catalyzes the production of an inducer. This inducer easily moves back and forth across the plasma membrane and acts as a signaling molecule. The lux operon is never completely turned off. The luxR gene codes for the activator LuxR. The inducer can bind to LuxR, and when it does, the LuxR–inducer complex can bind to a regulatory site to activate transcription of the lux operon and inhibit transcription of luxR. Explain how this gene regulatory circuit accounts for bacteria emitting light only when they reach a high cell density.

An inducible operon is turned on by the presence of an inducer molecule.

An inactive repressor protein requires binding of a corepressor molecule to become active.

A repressible operon is turned on when the repressor protein is not bound to the corepressor molecule.

An active repressor protein is inactivated by the binding of an activator molecule.

Which of the following statements is FALSE?

In the lac operon, the repressor inhibits transcription when
a. the repressor is bound to the inducer.
b. the repressor is not bound to the inducer.
c. the repressor is bound to glucose.
d. the repressor is not bound to the operator.

What characteristic of the light-producing regulatory circuit is consistent with the idea that it may be a regulon? What characteristic of this circuit stretches the definition for a regulon?

 No. The end product is usually a repressor in anabolic pathways, not the inducer in a catabolic pathway. 

 No. A corepressor works with a repressor protein to switch a repressible operon off. The question pertains to an inducible operon. 

 No. The end result of the substrate binding to the repressor is to render the repressor inactive. 

Inducible operons are generally found in catabolic pathways.

In an inducible operon, the inducer is often the __________ in the pathway being regulated; the inducer binds to the __________, which then becomes __________.

Positive/Negative; Repressible/Inducible Gene Regulation

Inducible & Repressible operons

When this is present in the cell, it binds to the repressor and the repressor can no longer bind to the operator:

They allow the organism the opportunity to simultaneously regulate transcription of multiple genes.

They allow the organism the opportunity to regulate transcription of a single gene.

They allow many genes to be expressed at the same time, even those unrelated in function.

They significantly increase the rate of DNA replication, thereby make transcription more efficient.

Which of the following is true about operons?

LuxR is allosterically regulated by the inducer molecule secreted by V. fischeri. What does it mean that LuxR is allosterically regulated?

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

 No. Prokaryotic mRNA does not undergo post-transcriptional processing. 

RNA polymerase acts at what point in gene expression?

Both repressible and inducible operons control gene expression at the level of __________.

Repressible Operons

Positive Inducible/Repressible Operons

Review of Inducible vs. Repressible Operons

What would occur if the repressor of an inducible operon were mutated so it could not bind the operator?
a. irreversible binding of the repressor to the promoter
b. reduced transcription of the operon’s genes
c. buildup of a substrate for the pathway controlled by the operon
d. continuous transcription of the operon’s genes

Negative Inducible/Repressible Operons

Organizing gene expression, so that genes are expressed in a given order.

Allowing each gene to be expressed an equal number of times.

Allowing a prokaryote to adjust to changes in environmental conditions.

Allowing environmental changes to alter a prokaryote's genome.

Altering patterns of gene expression in prokaryotes would likely increase a prokaryote’s survival by _______.

Repressible and Inducible Operons

Quorum sensing (introduced in Ch. 11, Section 11.4) allows bacteria to detect the number of neighboring cells and to trigger a response only when this number reaches a critical level. Quorum sensing is used by V. fischeri in light production and by many pathogenic bacteria, including Vibrio cholerae, to turn on genes for toxin production only when a critical cell density is reached. Why might quorum sensing be beneficial to pathogenic bacteria?

Structure of an Operon

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