Lecture 12 / Chapter 16 - Prokaryotic Gene Regulatory Logic

Question 1

Which term would be applied to a regulatory condition that occurs when protein greatly reduces transcription when associated with a particular section of DNA?

• activation
• negative control
• positive control
• Inhibition
• stimulation

Answer 1

Negative control.

Question 2

True or False?

Lactose is an inducer of the lac operon.

Answer 2

True.

Lactose indirectly induces or stimulates the transcription of genes involved in its metabolism.

Question 3

What would be the effect of a mutation in the lacI gene that prevented the repressor from binding to lactose?

• The lac Z, Y, and A genes would be repressed by lactose.
• The lac Z, Y, and A genes would be induced by lactose.
• The lac Z, Y, and A genes would be expressed constitutively.
• The lac Z, Y, and A genes would not be expressed.

Answer 3

The lac Z, Y, and A genes would not be expressed.

(If lactose could not bind to the repressor, the repressor would stay bound to the operator and repress the transcription of the lac Z, Y, and A genes.)

Question 4

What is the role of glucose in catabolite repression?

• It decreases the levels of cAMP in the cell, repressing transcription from the lac operon.
• It represses transcription from the lac operon, causing a decrease in cAMP levels in the cell.
• It stimulates transcription from the lac operon, causing an increase in the cAMP levels in the cell.
• It increases the levels of cAMP in the cell, stimulating transcription from the lac operon.

Answer 4

It decreases the levels of cAMP in the cell, repressing transcription from the lac operon.

(Glucose decreases the levels of cAMP in the cell, preventing formation of the CAP–cAMP complexes necessary for the stimulation of transcription from the lac operon.)

Question 5

Which term refers to a contiguous genetic complex that is under coordinate control?

• prototroph
• operon
• lysogen
• allosteric
• attenuation

Answer 5

Operon.

Question 6

Which term refers to a contiguous genetic complex that is under coordinate control?

• prototroph
• operon
• lysogen
• allosteric
• attenuation

Answer 6

Operon.

Question 7

In the lactose operon, the product of structural gene lacZ is capable of ________.

• splitting the Beta-linkage of lactose
• forming lactose from two glucose molecules
• replacing hexokinase in the early steps of glycolysis
• forming ATP from pyruvate
• nonautonomous replication

Answer 7

Splitting the Beta-linkage of lactose.

Question 8

Constitutive mutations may occur in various components of the lac operon. Mutations in which two genes are constitutive?
lac__ and lac ___.

Answer 8

I and O

Question 9

Which of the following mutations could lead to constitutive expression of the genes of the lac operon?

• a mutation in the lac-Z gene
• a mutation in the operator sequence
• a super repressor mutation
• a mutation in the lac-Y gene

Answer 9

A mutation in the operator sequence.

Such a mutation could prevent binding of the repressor, allowing expression under all conditions.

Question 10

Which of the following best describes the biological role of the lac operon?

• It ensures that a cell produces enzymes involved in lactose metabolism in a constitutive manner.
• It prevents other sugars from being metabolized until all available lactose has been used.
• It ensures that a cell dedicates resources to the production of enzymes involved in lactose metabolism only when lactose is available in the environment.
• It ensures that bacterial cells produce lactose only when no other food sources are available.

Answer 10

It ensures that a cell dedicates resources to the production of enzymes involved in lactose metabolism only when lactose is available in the environment.

(The cell expends energy to produce the proteins necessary for lactose metabolism only when lactose is present.)

Question 11

True or False?

The placement of the operator sequence between the promotor and the structural genes is critical to the proper function of the lac operon.

Answer 11

True.

When the repressor binds to the operator, RNA polymerase cannot transcribe the structural genes.

Question 12

What is the role of glucose in catabolite repression?

• It decreases the levels of cAMP in the cell, repressing transcription from the lac operon.
• It represses transcription from the lac operon, causing a decrease in cAMP levels in the cell.
• It increases the levels of cAMP in the cell, stimulating transcription from the lac operon.
• It stimulates transcription from the lac operon, causing an increase in cAMP levels in the cell.

Answer 12

It decreases the levels of cAMP in the cell, repressing transcription from the lac operon.

(Glucose decreases the levels of cAMP in the cell, preventing formation of the CAP-cAMP complexes necessary for the stimulation of transcription from the lac operon.)

Question 13

When considering the binding of cAMP-CAP and RNA polymerase to the lac operon, both bind more efficiently than either singly. Which term is applied to this increased efficiency of binding?

Answer 13

cooperative binding

Question 14

A(n) ______ is a stretch of DNA consisting of an operator, a promoter, and genes for a related set of proteins, usually making up an entire metabolic pathway.

Answer 14

operon

Question 15

The ______ is/are arranged sequentially after the promoter.

Answer 15

genes of an operon

Question 16

A(n) ______ is a specific nucleotide sequence in DNA that binds RNA polymerase, positioning it to start transcribing RNA at the appropriate place.

Answer 16

promoter

Question 17

A(n) _______ codes for a protein, such as a repressor, that controls the transcription of another gene or group of genes.

Answer 17

regulatory gene

Question 18

Regulatory proteins bind to the ______ to control expression of the operon.

Answer 18

operator

Question 19

A(n) _______ is a protein that inhibits gene transcription. In prokaryotes, this protein binds to the DNA in or near the promoter.

Answer 19

repressor

Question 20

A(n) ______ is a specific small molecule that binds to a bacterial regulatory protein and changes its shape so that it cannot bind to an operator, thus switching an operon on.

Answer 20

inducer

Question 21

Under what conditions would an operon not be transcribed?

Answer 21

• lac operon: lactose absent

- trp operon: tryptophan present

Question 22

Under what conditions would an operon be transcribed, but not sped up through positive control?

Answer 22

• lac operon: lactose present, glucose present

- trp operon: tryptophan absent

Question 23

Under what conditions would an operon be transcribed quickly through positive control?

Answer 23

-lac operon: lactose present, glucose absent

Question 24

Which of the following statements best describes the trp operon?

• It is repressible and under positive control.
• It is inducible and under positive control.
• It is repressible and under negative control.
• It is inducible and under negative control.

Answer 24

It is repressible and under negative control.

(The trp operon controls the transcription of genes necessary for the synthesis of tryptophan. If tryptophan is present, these gene products are not needed and are therefore not synthesized. The trp operon is repressible, meaning that transcription occurs only when its substrate is absent. The gene will be transcribed unless it is shut down (negative control) by the presence of tryptophan.)

Question 25

This question relates to the regulation of enzymes involved in the synthesis of tryptophan in E. coli. If tryptophan is present in the medium and available to the bacterium, ________.

• the repressor is inactive and the tryptophan operon is “on”
• the repressor is inactive and the tryptophan operon is “off”
• positive control is occurring
• the five structural genes associated with tryptophan synthesis are being transcribed at “induced” levels
• the repressor is bound to the operator, thus blocking transcription

Answer 25

The repressor is bound to the operator, thus blocking transcription.

Question 26

Which of the following occurs as a result of an abundance of tryptophan in E. coli?

• The 5 trp genes (TrpA – TrpE) are not transcribed.
• The 5 trp genes (TrpA – TrpE) are transcribed, but not translated.
• Stalling of the ribosome at trp codons in the leader sequence
• The leader sequence is not transcribed.

Answer 26

The 5 trp genes (TrpA – TrpE) are not transcribed.

(When trp is abundant, the genes involved in tryptophan synthesis are negatively regulated at the level of transcription.)

Question 27

Which of the following features of the trp operon is likely least essential to the process of attenuation?

• The ability of sequences within the leader mRNA to pair with one another
• Trp codons near the beginning of the leader sequence
• The order of the structural genes, E, D, C, B, A
• Transcription and translation of the leader sequence occur simultaneously.

Answer 27

The order of the structural genes, E, D, C, B, A.

(Because each gene encodes an enzyme involved in the trp synthetic pathway, the order in which the genes occur is likely not important in terms of the attenuation process.)

Question 28

True or False?

Attenuator systems such as the one described for regulation of tryptophan synthesis would be just as likely to occur in eukaryotes as in prokaryotes.

Answer 28

False.

(Regulation by attenuation requires that translation of a given transcript can begin before transcription is completed. This is not possible in eukaryotes, as the two processes are spatially separated by the nuclear membrane.)

Question 29

Within the control region of the tryptophan operon is a section of DNA that is sensitive to levels of tryptophan in the system. What is the name of this region?

Answer 29

leader region

Question 30

True or False?

Attenuation involves the termination of mRNA synthesis.

Answer 30

True.

Question 31

Which of the following statements about the mechanism of attenuation is true?

• The levels of tryptophan in the environment affect only repression of the trp operon and do not affect attenuation.
• When tryptophan levels are low, transcription is initiated and continues through the leader region into the structural genes, resulting in the production of a polycistronic mRNA and, ultimately, the enzymes required for the biosynthesis of tryptophan.
• When tryptophan levels are low, transcription of the leader sequence proceeds, but mRNA synthesis is terminated approximately 140 nucleotides along the transcript.
• When tryptophan is present at high concentrations, the repressor binds to the operator and transcription is initiated and not terminated. As a result, the enzymes required for the biosynthesis of tryptophan are translated.

Answer 31

When tryptophan levels are low, transcription is initiated and continues through the leader region into the structural genes, resulting in the production of a polycistronic mRNA and, ultimately, the enzymes required for the biosynthesis of tryptophan.

(This describes the regulation of the trp operon by attenuation.)

Question 32

Keeping in mind the life cycle of bacteriophages discussed in Chapter 6, consider the following problem: During the reproductive cycle of a temperate bacteriophage, the viral DNA inserts into the bacterial chromosome where the resultant prophage behaves much like a Trojan horse. It can remain quiescent, or it can become lytic and initiate a burst of progeny viruses. Several operons maintain the prophage state by interacting with a repressor that keeps the lytic cycle in check. Insults (ultraviolet light, for example) to the bacterial cell lead to a partial breakdown of the repressor, which in turn causes the production of enzymes involved in the lytic cycle. As stated in this simple form, would you consider this system of regulation to be operating under positive or negative control?

• Positive control.
• Negative control.

Answer 32

Negative control.