Deck 16: Control of Gene Expression in Prokaryotes

Question 1

What is the function of allolactose in regulation of the lac operon?

A) inducer
B) repressor
C) activator
D) promoter
E) regulatory protein

Answer 1

A

Question 2

Which of the following is NOT a common DNA-binding motif?

A) the helix-loop-helix motif
B) the beta sheet-alpha helix motif
C) the zinc-finger motif
D) the homeodomain motif
E) the leucine-zipper motif

Answer 2

B

Question 3

An example of a gene product encoded by a regulatory gene is:

A) beta-galactosidase enzyme.
B) allolactose.
C) repressor protein.
D) an operator.
E) a terminator.

Answer 3

C

Question 4

When binding of the inducer to the repressor causes a conformational change, which then prevents the repressor from binding to DNA, the repressor is called a(n) _____ protein.

A) coactivator
B) allosteric
C) structural
D) operating
E) responsive

Question 5

When a structural gene is under negative inducible control, what would be the result of a mutation that eliminates the repressor protein?

A) The structural gene will be constitutively expressed due to the lack of negative inducible control.
B) The transcription of the structural gene will not be affected, as a repressor is not required.
C) The mutation will lead to activation of an activator upon the lack of a repressor protein, which will allow the transcription to continue.
D) Since transcription will require a repressor protein, the transcription will be turned off.
E) More cAMP will be produced in a cell to compensate for the lack of a repressor protein.

Question 6

Which of the following statements about DNA-binding proteins is NOT true?

A) Specific amino acids within the motif form hydrogen bonds with DNA.
B) These proteins can affect the expression of a gene.
C) Most DNA-binding proteins bind dynamically.
D) Some of these proteins incorporate a metal ion such as zinc.
E) Once bound, most of DNA-binding proteins remain on DNA permanently.

Question 7

Proteins with DNA-binding motifs predominantly bind to the _____ of DNA.

A) major groove
B) minor groove
C) paired nitrogenous bases
D) phosphate groups
E) deoxyribose sugar

Question 8

In the absence of tryptophan, what happens to the genes within the trp operon?

A) The regulator without tryptophan-binding prevents the genes from being transcribed.
B) The regulator dissociates from the operator and structural genes get transcribed.
C) Lack of tryptophan increases the level of cAMP, which leads to activation of CAP protein and gene expression.
D) The active repressor binds to the operator and genes do not get transcribed.
E) The active activator binds to the operator and transcription of structural gene takes place.

Question 9

An operon is controlled by a repressor. When the repressor binds to a small molecule, it is released from binding to DNA near the operon. The operon is never expressed if a mutation prevents the repressor from binding to the small molecule. The type of control illustrated is:

A) negative inducible.
B) negative repressible.
C) positive inducible.
D) positive repressible.
E) attenuation.

Question 10

When a structural gene is under positive inducible control, what would be the result of a mutation that eliminates the activator protein?

A) The structural gene will be constitutively expressed due to the lack of inducible control.
B) The transcription of the structural gene will not be affected, as an activator is not required.
C) The mutation will lead to activation of a repressor upon the lack of an activator protein, which will block transcription.
D) Since transcription will require an activator protein, the transcription will be turned off.
E) More cAMP will be produced in a cell to compensate for the lack of an activator protein.

Question 11

Which of the following types of eukaryotic gene regulation takes place at the level of DNA?

A) alternation of chromatin structure
B) mRNA processing
C) RNA interference
D) mRNA stability
E) posttranslational modification

Question 12

E. coli lac operon control by CAP is:

A) negative inducible.
B) negative repressible.
C) positive inducible.
D) positive repressible.
E) regulated by riboswitches.

Question 13

Which of the following statements about gene regulation concerning operons is INCORRECT?

A) A negative repressible gene is controlled by a regulatory protein that inhibits transcription.
B) For a gene under negative repressible control, a small molecule is required to prevent the gene's repressor from binding to DNA.
C) For a gene under positive repressible control, the normal state is transcription of a gene, stimulated by a transcriptional activator.
D) A regulator gene has its own promoter and is transcribed into an independent mRNA.
E) Presence of operons where genes of related functions are clustered is common in bacteria but not in eukaryotes.

Question 14

Which of the following DNA-binding motifs are composed of three alpha helices?

A) zinc-finger motif
B) leucine-zipper motif
C) homeodomain motif
D) helix-turn-helix motif
E) helix-loop-helix motif

Question 15

Which of the following statements about regulation of gene expression is CORRECT?

A) An inducible gene is transcribed when a specific substance is absent.
B) A gene is any DNA sequence that is transcribed into an mRNA molecule only.
C) All genes are transcribed at all times as long as they have a functional promoter.
D) The regulation of gene expression is the same in both eukaryotes and prokaryotes.
E) The regulation of gene expression is critical for the control of life processes in all organisms.

Question 16

An operon is controlled by a repressor. When the repressor binds to a small molecule, it binds to DNA near the operon. The operon is constitutively expressed if a mutation prevents the repressor from binding to the small molecule. The type of control illustrated is:

A) negative inducible.
B) negative repressible.
C) positive inducible.
D) positive repressible.
E) catabolite repression.

Question 17

What is the function of cAMP in regulation of the lac operon?

A) It activates a repressor protein.
B) It activates an activator protein.
C) It inactivates a repressor protein.
D) It inactivates an activator protein.
E) It causes attenuation.

Question 18

Which of the following is generally constitutively transcribed?

A) regulatory gene
B) structural gene
C) operator element
D) promoter element
E) operon

Question 19

What is the difference between negative control operons that use induction and systems that use repression?

Question 20

The _____ is a type of regulator protein that binds to a region of DNA in the promoter of a gene called the operator and prevents transcription from taking place.

A) inducer
B) repressor
C) activator
D) inactivator
E) terminator

Question 21

A lac operon of genotype lacI^- lacP⁺ lacO⁺ lacZ⁺ lacY⁺ will produce $\beta$ -galactosidase and permease when:

A) lactose is present.
B) lactose is absent.
C) lactose is present or not.
D) glucose is present.
E) glucose and lactose are both present.

Question 22

Where would the lac repressor be bound in a (nonmutant) E. coli cell that is growing in low glucose and high lactose? (I = lac repressor gene; Z, Y, A = lac operon structural genes; P = lac promoter; O = lac operator)

A) P
B) O
C) P and O
D) I, P, O
E) The repressor would not be bound.

Question 23

A lac operon of genotype lacI⁺ lacP⁺ lacO⁺ lacZ⁺ lacY^- will produce $\beta$ -galactosidase but not permease when:

A) lactose is present.
B) lactose is absent.
C) in the presence or absence of lactose.
D) glucose is present.
E) glucose and lactose are both present.

Question 24

Describe one similarity and one difference in how the trp and lac repressor proteins function.

Question 25

E. coli lac operon control by lacI is:

A) negative inducible.
B) negative repressible.
C) positive inducible.
D) positive repressible.
E) attenuation.

Question 26

Suppose that you perform an experiment where you construct a plasmid that carries a copy of the lac operator region (lacO) but no other part of the lac operon. (The lac repressor can bind to single operator regions.) This plasmid is placed in an E. coli cell, which has a normal copy of the lac operon in its chromosome. When this strain is grown, the number of plasmids reaches about 50 copies per bacterial cell. What is the expected phenotype of such a strain in the absence of glucose?

A) The lac operon will be turned on even in the absence of lactose.
B) The lac operon will be turned off even in the presence of lactose.
C) The lac operon will be regulated normally.
D) The lac operon will be initially turned on in the presence of lactose but eventually it will be turned off even though lactose is still present.
E) The lac operon will be expressed only if galactose is added to the Moderate.

Question 27

A mutant E. coli strain is found that synthesizes $\beta$ -galactosidase in the absence of glucose whether or not lactose is present. What two mutations might lead to this outcome?

A) lacI^s mutation or lacO^c mutation
B) lacP^- mutation or lacI^- mutation
C) lacP^- mutation or lacO^c mutation
D) lacI^- mutation or lacO^c mutation
E) lacP^- mutation or lacI^s mutation

Question 28

Which parts of the DNA region shown in the diagram encode proteins? (I = lac repressor gene; Z, Y, A = lac operon structural genes; P = lac promoter; O = lac operator)

A) P
B) I, P, O
C) P, O, Z, Y, A
D) I, Z, Y, A
E) I, P, O, Z, Y, A

Question 29

It is possible for a repressor to negatively regulate the expression of an operon because:

A) the repressor induces the expression of the inducer by binding to the promoter that comes before the inducer gene.
B) one of the structural genes expressed in the operon negatively regulates the repressor.
C) the repressor-binding site overlaps the promoter site of the operon, allowing it to physically block the binding of RNA polymerase.
D) the repressor-binding site on the DNA overlaps with the translation start site, thereby preventing the transcription.
E) the repressor physically blocks where the activator should be binding on the operator region.

Question 30

A partial diploid E. coli cell of lacI⁺lacP⁺lacO^c lacZ^-lacY^-/ lacI^- lacP⁺ lacO⁺lacZ^-lacY⁺ genotype will synthesize:

A) both lacZ and lacY gene products in the absence of lactose.
B) neither lacZ nor lacY gene products in the presence of lactose.
C) lacZ but not lacY gene product in the presence of lactose.
D) lacY but not lacZ gene product in the absence of lactose.
E) lacY but not lacZ gene product in the presence of lactose.

Question 31

In the presence of both lactose and glucose, which of the following occurs with the E. coli lac operon?

A) The lacI gene does not produce repressor.
B) The cAMP-CAP complex is not available to bind near or at the lac promoter.
C) The lacZ and lacY genes are fully expressed.
D) Lactose is converted to glucose and galactose.
E) Lactose binds to the operator.

Question 32

A promoter that affects only genes that are on the same piece of DNA is referred to as a(n) _____-acting promoter.

A) cis
B) trans
C) enhancer
D) positive
E) negative

Question 33

Assume that a mutation occurs in the promoter for the lacI regulatory gene and this mutation results in a tenfold increase in the transcription of lacI. What would be the expected consequences of such a mutation?

A) The lac structural genes will be fully expressed even in the presence of glucose and absence of lactose.
B) The lac operon would be expected to be regulated normally.
C) The lac structural genes will not be fully induced even in the absence of glucose and presence of lactose.
D) The lac structural genes will be partially expressed in the presence of glucose and fully expressed if lactose is also present.
E) The lac structural genes will now be fully expressed in the absence of glucose and lactose but not expressed in the absence of glucose and presence of lactose.

Question 34

A lac operon of genotype lacI⁺ lacP⁺ lacO⁺ lacZ^- lacY⁺ will not produce $\beta$ -galactosidase but will produce permease when:

A) lactose is present.
B) lactose is absent.
C) in the presence or absence of lactose.
D) glucose is present.
E) glucose and lactose are present.

Question 35

Which of the following E. coli strains will synthesize permease in the presence of lactose and absence of glucose?

A) lacI^- lacP^- lacO⁺ lacZ⁺ lacY⁺
B) lacI^s lacP^- lacO⁺ lacZ^- lacY⁺/lacI⁺ lacP⁺ lacO⁺ lacZ⁺ lacY⁺
C) lacI^- lacP⁺ lacO⁺ lacZ⁺ lacY^-
D) lacI^- lacP⁺ lacO^c lacZ^-lacY^-/lacI⁺ lacP^- lacO⁺ lacZ^- lacY⁺
E) lacI⁺ lacP⁺ lacO^c lacZ⁺ lacY^-/lacI^- lacP⁺ lacO⁺lacZ^- lacY⁺

Question 36

A mutant E. coli strain, grown under conditions that normally induce the lac operon, produces particularly high amounts of ß-galactosidase. What is a possible genotype of the cells?

A) lacI⁺ lacP⁺ lacO⁺ lacZ^- lacY⁺ lacA⁺
B) lacI⁺ lacP⁺ lacO^c lacZ⁺ lacY⁺ lacA⁺
C) lacI^- lacP⁺ lacO⁺ lacZ^- lacY⁺ lacA⁺
D) lacI⁺ lacP^- lacO⁺ lacZ⁺ lacY⁺ lacA⁺
E) lacI^- lacP^- lacO⁺ lacZ^- lacY⁺ lacA^-

Question 37

What would happen to the lac operon in the absence of allolactose?

A) The structural genes within the lac operon will be constitutively transcribed.
B) The activator protein will be bound to the operator, which will turn on the structural gene behind it.
C) The repressor regulator protein binds to the operator and prevents the transcription of the structural gene.
D) The catabolite activator protein becomes inactivated and no transcription occurs.
E) The cAMP level rises in the absence of allolactose, which in turn inactivates the transcription.

Question 38

The following table shows several bacterial strain lac operon genotypes (some are partial diploids).
a. Fill in the blanks in the "lactose absent" and "lactose present" columns in this table. (+) means significant levels of active ß-galactosidase enzyme can be detected. (-) means no significant levels of active ß-galactosidase enzyme are present. The first line is filled in for reference.

$\begin{array}{|l|c|c|}\hline\text { Strain genotype }&\text { Lactose absent } & \text { Lactose present } \\\hline 1. l a c I^{+} l a c P^{+} l a c O^{+} l a c Z^{+} l a c Y^{+} &-&+\\\hline \text { 2. } \operatorname{lacI}^{+} \operatorname{lac}^{+} \operatorname{lac} O^{+} \operatorname{lacZ^{+}} \operatorname{lac} Y^{+}\\\hline 3. l a c I^{+} l a c P^{+} \operatorname{lac} O^{+} \operatorname{lacZ^{-}lacY^{+}} \\4. l a c \mathrm{l}^{*} l a c P^{+} \operatorname{lac} O^{+} \operatorname{lacZ}^{+} l a c \mathrm{Y}^{+} \\\text { 5. } \operatorname{lacI}^{-} \operatorname{lacP^{+}} \operatorname{lacO^{+}\operatorname {lac}Z^{+}lacY^{+}/lacI^{+}}\\\text { 6. } \operatorname{lac} I^{\top} \operatorname{lacP^{+}} \operatorname{lac} O^{+} \operatorname{lacZ^{+}} \operatorname{lac} Y^{+} / \operatorname{lac} I^{+}\\\hline\end{array}$
b. Using your answers in the previous table, explain the dominance relationships among the three lacI alleles (lacI⁺, lacI⁻, lacI^s).

Question 39

If there are mutations that inactivate lacP and lacI, which of the following statements is true? (I = lac repressor gene; Z, Y, A = lac operon structural genes; P = lac promoter; O = lac operator)

A) These are mutations that are, respectively, cis- and trans-acting on lac operon expression.
B) These are mutations that are, respectively, trans- and cis-acting on lac operon expression.
C) These will affect the expression of I only.
D) These will affect the expression of only Z, Y, and A.
E) These mutations will have no effect.

Question 40

A mutant E. coli strain, grown under conditions that normally induce the lac operon, does not produce ß-galactosidase. What is a possible genotype of the cells?

A) lacI⁺ lacP⁺ lacO⁺ lacZ⁺ lacY^- lacA⁺
B) lacI⁺ lacP⁺ lacO^c lacZ⁺ lacY⁺ lacA⁺
C) lacl⁺ lacP⁺ lacO⁺ lacZ⁺ lacY⁺ lacA⁺
D) lacI⁺ lacP^- lacO⁺ lacZ⁺ lacY⁺ lacA⁺
E) lacI^- lacP⁺ lacO^- lacZ⁺ lacY⁺ lacA^-

Question 41

A deletion occurs in the trp operon DNA of E. coli and results in the loss of the attenuation region in the 5'UTR of the RNA. The DNA sequences of the structural genes and the operator/promoter region are not affected by deletion. What effect will this deletion be expected to have on the regulation of this mutant trp operon compared to a wild-type operon?

A) In the presence of tryptophan, transcription of the structural genes will be enhanced compared with a wild-type operon.
B) In the absence of tryptophan, transcription of the structural genes will be reduced compared with a wild-type operon.
C) In the presence of tryptophan, the repressor will bind to the operator/promoter region in the mutant operon more strongly than in a wild-type operon.
D) In the absence of tryptophan, RNA polymerase will not bind to the operator/promoter region in the mutant operon.
E) In the presence of tryptophan, transcription will be initiated at the second structural gene in the mutant operon.

Question 42

The bacterium Bacillus subtilis can grow on minimal media with a variety of sugars as a carbon source. One such sugar is mannose, metabolized by the products of the man operon. Expression of the operon is controlled by a regulatory protein encoded in a separate gene, manR. Depending on conditions, the regulatory protein may bind at one of two sites in the operon, as follows:
(i) When mannose is absent from the cell, the regulatory protein is in a conformation called R1. R1 can bind specifically at an operator site manO. Binding of R1 at manO reduces transcription of the operon fourfold from a basal level of 20 units.
(ii) When mannose is present in the cell, it binds to the regulatory protein, causing it to undergo an allosteric transition from conformation R1 to a new conformation, called R2. R2 cannot bind at manO. However, R2 can bind specifically at a different site called the initiator, manI. Binding of R2 at manI increases transcription of the operon twofold from the basal level.
Mutations m1-m3, which affect expression of this operon, were identified. Each mutation affects only a single component of the operon. Levels of operon activity were measured in haploids. They were also measured in partial diploids with an F' carrying the wild-type alleles of all genes and regulatory elements described above. $\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\text { Operon activity }$
$\quad\quad\quad\quad\quad\quad\quad\quad\quad$ Haploids $\quad\quad\quad\quad\quad\quad\quad$ Partial Diploids
$\begin{array}{lcccc}&(-) \text { mannose }&(+) \text { mannose } &(-) \text { mannose }&(+) \text { mannose }\\\text { wild type } & 5 & 40 & 10 & 80 \\\mathrm{~m} 1 & 20 & 20 & 10 & 80 \\\mathrm{~m} 2 & 5 & 20 & 10 & 60 \\\mathrm{~m} 3 & 20 & 40 & 25 & 80\end{array}$ For each mutation, describe which component is affected. In addition, explain the observed activity in the haploid and partial diploid in each case.

Question 43

A mutation occurs in the trp operon DNA of E. coli and results in the change to the two UGG tryptophan codons in the 5' UTR of the RNA to UAG stop codons. What effect will this mutation be expected to have on the regulation of this mutant trp operon compared to a wild-type operon?

A) In the presence of tryptophan, transcription of the structural genes will be reduced compared with a wild-type operon.
B) In the absence of tryptophan, transcription of the structural genes will be reduced compared with a wild-type operon.
C) In the presence of tryptophan, the repressor will bind to the operator/promoter region with the mutant operon more strongly than with a wild-type operon.
D) In the absence of tryptophan, RNA polymerase will not bind to the operator/promoter region with the mutant operon.
E) In the presence of tryptophan, transcription of the structural genes will be enhanced compared with a wild-type operon.

Question 44

Fill in the blanks in the following table for the transcription status of the lac operon ((+) for transcription and (-) for no transcription) in the absence or presence of lactose. The first line is filled in for reference.

Question 45

Fill in the blanks in this table with "yes" or "no" for each condition of lac operon regulation. The strain is wild type with no partial diploidy. The first line is filled in for reference.

Question 46

You have isolated two mutations linked to the lac operon, which you designate Lac1^- and Lac2^- that cause constitutive expression of the operon. You construct strains carrying a lac operon with a mutant lacY gene on an F'. You test both ß-galactosidase activity and Lac permease activity in the strains you constructed using the artificial inducer IPTG. $$\begin{array}{l}~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~\text { B-galactosidase activity Lac permease activity }\\\begin{array} { ccccc } & \text { - IPTG}&{+ IPTG } & \text { - IPTG } & \text { + IPTG } \\\mathrm { Lac1 } ^ { - } \operatorname { lac } Z^- \operatorname { lac } Y ^ { + } / \mathrm { F } ^ { \prime } \operatorname { lac } Z ^ { + } \operatorname { lac } Y ^ { - } & - & + & + & + \\\mathrm { Lac2 } ^ { - } \operatorname { lac } Z^- \operatorname { lac } Y ^ { + } / \mathrm { F } ^ { \prime } \operatorname { lac } Z ^ { + } \operatorname { lac } Y ^ { - } & + & + & + & +\end{array}\end{array}$$ Are Lac1^- and Lac2^- dominant or recessive? Do they act in cis or in trans? Indicate how you can determine both of these properties for each mutation. What type of lac mutations best fit the properties of Lac1^- and of Lac2^-?

Question 47

The trp operon is known to operate by both negative repressible regulation of operator and attenuation. Which of the following statements does NOT support the reason as to why dual control exists to regulate the operon?

A) The repression alone is never complete, and some transcription can be initiated.
B) Combined mechanism provides a much finer tuning of tryptophan synthesis regulation.
C) Attenuation and repression allow the cell to more sensitively respond to the tryptophan level.
D) It is most likely due to the fact that the attenuation is the evolutionary relic, which by accident has remained.
E) Repression responds to the cellular levels of tryptophan, while attenuation responds to the number of tRNA charged with tryptophan.

Question 48

Transcriptional control that acts by regulating the continuation of transcription is called:

A) riboswitching.
B) antitermination.
C) negative control.
D) operator mutation.
E) attenuation.

Question 49

The dotted line in the following graph shows levels of glucose in a culture of wild-type E. coli grown in Moderate that initially contains both glucose and lactose. The solid line shows levels of transcription of the lac operon. Describe what is happening to the culture and the lac operon, referring to the lac repressor, allolactose, cAMP, and CAP (catabolite activator protein).

Question 50

Fill in the blanks in the following table with "yes" or "no" for each condition of trp operon regulation. The strain is wild type, with no partial diploidy. $\begin{array}{|lr|c|c|c|}\hlineModerate &t r p ~repressor &\text { trp repressor } & \text { Antiterminator } & \text { Transcription } \\&bound~ to~ trp?&\text { bound to } & \text { hairpin formed in } & \text { attenuated? }\\&&\text { operator? } & \text { mRNA? }\\high ~tryptophan&& \\low ~tryptophan&& &\\\hline\end{array}$

Question 51

An E. coli strain of chromosomal genotype lacI^- lacP⁺ lacO⁺ lacZ⁺ lacY⁺ constitutively expresses the genes of the lac operon. The strain is converted to wild-type lac operon regulation by the addition of an extra piece of DNA. What gene or genes are contained on this extra DNA that explain this conversion to wild type? Include an explanation of cis- or trans-acting factors and how they work.

Question 52

The formation of 1+2 and 3+4 secondary structures of 5' UTR region mRNA from the trp operon is triggered when:

A) the tryptophan level inside the bacterial cell is extremely low.
B) the tryptophan level inside the bacterial cell is high.
C) the repressor protein fails to bind to the operator.
D) there is a spontaneous mutation introduced into the 5' UTR.
E) the structural gene transcription within the trp operon gets initiated.

Question 53

In the experiments described in the text, Jacob and Monad deciphered that the lacI⁺ gene product can function in trans and regulate the lac operon either on the plasmid or on the chromosome. What genotype of the partial diploid bacterial strain was key to their experiments and led them to such a conclusion? Explain why this strain helped them to reach that conclusion.

Question 54

If a mutation prevents the formation of the antiterminator 2+3 loop in the trp operon, what would be the effect?

A) transcription only when tryptophan is absent
B) transcription only when tryptophan is present
C) constitutive attenuation of transcription
D) constitutive transcription
E) no effect, as 2+3 loop has no function

Question 55

Fill in the blanks in the "level of transcription" column of this table with (+) for high levels of transcription, (+/-) for Moderate levels of transcription, and (-) for minimal levels of transcription of the lac operon. Consider regulation by both the lac repressor and CAP (catabolite activator protein). The strain is wild type with no partial diploidy. The first line is filled in for reference.

Question 56

The trp operon in E. coli contains a 5' UTR sequence that is translated into a small polypeptide of 14 amino acids, which includes two tryptophans. If the two trp codons in the 5' UTR of the RNA are changed to serine codons and the resulting cells are starved for tryptophan but not for any other amino acid, what will be the effect of the mutant codons on the operation of the trp operon?

A) The trp repressor will be synthesized in greater amounts.
B) Expression of the trp structural genes will be less than with a normal trp operon.
C) Expression of the trp structural genes will be greater than with a normal trp operon.
D) The leader polypeptide will not be synthesized.
E) The expression of the operon will not be affected by these mutations.

Question 57

Which of the following secondary structures causes attenuation of structural genes of the trp operon?

A) 1+2 loop
B) 1+3 loop
C) 2+4 loop
D) 2+3 loop
E) 3+4 loop

Question 58

Imagine the following scenario: You take the regulatory region of the trp operon (including the promoter, operator, and 5' UTR) and attach it upstream of the structural genes of the lac operon. You then introduce this artificial construct into a mutant strain in which its own lac operon is completely nonfunctional. Indicate the level of ß-galactosidase activity in each of the following cases and explain why you expect that level of activity:
a. No tryptophan, no lactose
b. High tryptophan, high lactose

Question 59

Explain why glucose-dependent catabolite repression in E.coli is important and how it is possible to achieve this repression without influencing glucose metabolism.

Question 60

Aside from the case of lacO^c, where the operator is unable to bind to the repressor protein, briefly describe another mutant that could have caused constitutive expression of lac operon.

Question 61

The 5' UTR of the trp operon RNA contains several UGG tryptophan codons. What would be the effect on transcription-attenuation gene regulation if the trp codons were converted to UGC cysteine codons?

Question 62

Which of the following facts about riboswitches is INCORRECT?

A) Binding of certain molecules to the riboswitches results in the formation of specific secondary structures of mRNA.
B) Certain molecules that bind to riboswitches may act as repressors or inducers of transcription.
C) Riboswitches are not only found in bacterial cells but also in archaeal, fungal, or plant cells.
D) Riboswitches are typically found in the 3'UTR of the mRNA structure.
E) The secondary structure that forms riboswitches typically contains a base stem and several branching hairpins.

Question 63

RNA-mediated repression is carried out by:

A) nonsense RNA.
B) sense RNA.
C) antisense RNA.
D) riboswitches.
E) ribozymes.

Question 64

There are enzymes called aminoacyl-tRNA synthetases that attach tRNAs to the appropriate amino acid. Assume that the aminoacyl-tRNA synthetase that attaches tryptophan to its tRNA in an E. coli mutant strain is only partially active. It is active enough for the strain that carries it to be viable but is much less active than that of wild-type cells. What would be the expected effect of this mutation on attenuation in the trp operon?

A) Pairing of regions 3 and 4 in the 5' UTR of the RNA will be enhanced so attenuation will increase in the presence of tryptophan.
B) Pairing of regions 2 and 3 in the 5' UTR of the RNA will be decreased so attenuation will increase in the presence of tryptophan.
C) Pairing of regions 3 and 4 in the 5' UTR of the RNA will be decreased so attenuation will decrease in the presence of tryptophan.
D) Attenuation will not change but the trp repressor will be more active and the transcription of the trp operon structural genes will be decreased in the presence of tryptophan.
E) Attenuation will be decreased in the absence of tryptophan but will be enhanced in the presence of tryptophan.

Question 65

RNA molecules that are complementary to particular sequences on mRNA are called:

A) complementary RNA.
B) sense RNA.
C) antisense RNA.
D) riboswitches.
E) ribozymes.

Question 66

A partial diploid E. coli cell of lacI⁺ lacP^- lacO^c lacI⁺ lacY⁺ / lacI^s lacP⁺lacO⁺ lacZ⁺ lacY^- will synthesize:

A) both lacZ and lacY functional gene products in the absence of lactose.
B) neither lacZ nor lacY functional gene products in the presence of lactose.
C) both lacZ and lacY functional gene products in the presence of lactose.
D) lacY but not a functional lacZ gene product in the absence of lactose.
E) lacZ but not a functional lacY gene product in the absence of lactose.

Question 67

The lysC gene in Bacillus subtilis encodes the first enzyme in the metabolic pathway that generates lysine from its precursor molecule. In the presence of high concentrations of lysine, lysC expression is reduced. Researchers have generated mutant bacteria that produce the LysC enzyme constitutively, even in the presence of lysine. A mutant identified in this manner had two nucleotide changes in the 5' UTR of the lysC gene itself. These nucleotide changes are thought to affect the function of a riboswitch regulating transcription of lysC. Describe two possible ways in which the mutations might affect the function of this riboswitch.

Question 68

Draw a diagram of the transcription and translation of the trp operon under high tryptophan conditions. In addition to the trp operon, show RNA polymerase; the 5' UTR RNA with regions 1, 2, 3, 4, and the string of uracils; and a ribosome.