Chapter 18: Bio 151

X-gal is a colorless molecule cleaved by Beta-galactosidase into two products, one of which is blue. Cells with a mutation in the lac operator that makes this sequence non-functional were plated onto a medium that contains X-gal. What color or colors would you expect the colonies from these cells to be?

All blue Colonies of cells with a non-functional lac operator would be all blue on X-gal plates. These cells would constitutively express lacZ because the lac repressor could not bind the lac operator and block transcription. The Beta-galactosidase protein encoded by lacZ would cleave the X-gal and the blue product would turn these cells blue. White colonies would indicate the lack of expression of the lacZ gene.

Which of the following choices (A-D) best represent(s) the mechanisms for control of gene expression that can occur in bacteria? A. reduced production of mRNA B. increased life span of mRNA molecules C. decreased efficiency of mRNA translation D. protein inactivation

All of them (A, B, C, and D) can occur in bacteria.

second module X-gal is a colorless molecule cleaved by Beta-galactosidase into two products, one of which is blue. Cells with mutations in both the lac operator and lac operon promoter that makes these sequences non-functional were plated onto a medium that contains X-gal. What color or colors would you expect the colonies from these cells to be?

All white Colonies from cells with both a non-functional promoter sequence and a non-functional operator sequence plated on X-gal plates would be all white. Cells with a non-functional operator sequence alone would constitutively express lacZ because the lac repressor could not bind the lac operator and block transcription. However these cells have a non-functional promoter, which would not be able to bind RNA Polymerase to initiate transcription of the lac operon. Therefore, no transcription of the lacZ gene and therefore no production of the Beta-galactosidase protein. Without the Beta-galactosidase the X-gal would remain uncleaved and the colonies would be their normal white color.

X-gal is a colorless molecule cleaved by Beta-galactosidase into two products, one of which is blue. Cells with a mutation in the promoter of the lac operon that makes this sequence non-functional were plated onto a medium that contains X-gal. What color or colors would you expect the colonies from these cells to be?

All white Colonies of cells with a non-functional lac operon promoter would be all white on X-gal plates. A lac operon promoter that is non-functional would not be able to bind RNA Polymerase to initiate transcription of the lac operon. Therefore, there would be no transcription of the lacZ gene and, therefore, no production of the Beta-galactosidase protein. Without the Beta-galactosidase, the X-gal would remain uncleaved and the colonies would be their normal white color. Blue colonies would indicate the expression of the lacZ gene and the presence of the Beta-galoctosidase enzyme.

X-gal is a colorless molecule cleaved by Beta-galactosidase into two products, one of which is blue. Cells with a mutation in the lacZ gene that makes the encoded enzyme non-functional were plated onto a medium that contains X-gal. What color or colors would you expect the colonies from these cells to be?

All white Colonies of cells with a non-functional lacZ gene would be all white on X-gal plates. Because the lacZ gene if transcribed would be translated into a non-functional Beta-galactosidase protein, the X-gal would not be cleaved and the cells would remain their normal color of white.

X-gal is a colorless molecule cleaved by Beta-galactosidase into two products one of which is blue. Cells with mutations in both the lac operator that make this sequence non-functional and the lacZ gene that make the encoded enzyme non-functional were plated onto a medium that contains X-gal. What color or colors would you expect the colonies from these cells to be?

All white Colonies of cells with mutations in both the lac operator that make this sequence non-functional and the lacZ gene that make the encoded enzyme non-functional plated onto a medium that contains X-gal would be all white. Cells with a non-functional operator sequence alone would constitutively express lacZ because the lac repressor could not bind the lac operator and block transcription. This lacZ gene, however, encodes a non-functional Beta-galactosidase protein and the X-gal would not be cleaved and the cells would remain their normal color of white.

A hypothetical bacterium isolated from a Martian sea uses an abundant, silicon-based sugar called silicose as its main energy source. Which of the following would be the most efficient type of control for the production of silicase, the enzyme used to metabolize silicose?

Constitutive transcription of silicase gene

A mutation resulting in a major conformational change in lacI protein that made it unable to bind to the operator would result in what cellular response?

Continuous production of β-galactosidase

What would occur if the repressor for an operon under negative control were mutated so that it could not bind the operator?

Continuous transcription of the operon's genes

What would you expect the levels of transcription of the SOS response genes to be in a cell containing a frameshift mutation in the gene for the common repressor protein?

High levels of transcription, whether exposed to UV light or not The levels of transcription of the SOS response genes in a cell containing a frameshift mutation in the gene for the common repressor protein are expected to be high whether the cells were exposed to UV or not. Frameshift mutations add or delete bases in the coding sequence of the gene that shifts the reading frame of the mRNA. The common repressor protein would likely therefore be inactive. This inactive repressor would result in the constitutive expression of these genes because transcription of these genes would not be blocked.

Which of the following statements about AraC's regulation of the ara operon is true?

Increasing the concentration of arabinose would increase the production of the AraB protein. (The ara operon, consisting of araB, araA and araD genes, is regulated by the AraC protein. Binding of arabinose to AraC results in positive regulation of transcription of the ara operon through binding of AraC to the RNA polymerase to initiate transcription. Increasing the concentration of arabinose would increase the production of the AraB protein. Increasing the concentration of glucose would have no effect on the production of AraB. Increasing the concentration of glucose would result in an increase in glucose in the cells, but this would have no effect on the transcription of the ara operon.)

Which of the following is true about transcriptional control?

It is relatively slow but efficient in resource use.

Which molecule acts as an inducer of lac operon transcription?

Lactose

What happens when the interaction of the repressor protein with the DNA of the lac operator is disrupted?

Lactose binds the repressor and causes the protein to change shape and fall off the DNA.

Which of the following conditions would result in the highest levels of transcription of the lac operon?

Low levels of glucose and high levels of lactose in the cell

Which of the following experiments would help you to determine whether the β-galactosidase gene is induced by lactose or glucose?

Measure the amount of β-galactosidase produced by E. coli grown on a glucose plate, a lactose plate, and a glucose + lactose plate.

An E. coli cell contains a mutation in lacI gene such that the encoded protein is able to bind the lac operator but is unable to bind lactose, the inducer. Would you expect the transcription of the lacZ and lacY genes to increase or decrease when this cell is exposed to high levels of lactose?

No change in transcription of both genes There would be no change in transcription of both genes. E. coli cells that contain a mutation in lacI gene such that the encoded protein is able to bind the lac operator, but is unable to bind lactose, would be expected to have low levels of lacZ and lacY transcription when exposed to high levels of lactose. Lactose would be unable to bind this repressor so the repressor would not release from the operator and transcription of these genes would still be blocked.

Which of the following statements best summarizes how the lac operon is regulated?

Positive and negative control elements regulate the lac operon.

How would you use replica plating and indicator plates to test for strains of E. coli that are susceptible to the effects of the antibiotic Cipro?

Press a velvet block onto a master plate without Cipro, and then plate the replica plate on medium with Cipro.

For an operon under negative control to be transcribed, which of the following must occur?

RNA polymerase must bind to the promoter, and the repressor must be inactive. (For an operon under negative control to be transcribed, RNA polymerase must bind to the promoter, and the repressor must be inactive. Transcription of an operon under negative control involves the ability of the RNA polymerase to bind to the promoter and have no impediment in transcribing the DNA into mRNA. The repressor protein, which normally impedes the ability of RNA polymerase to transcribe the DNA, would also need to be inactivated for transcription to occur. If an RNA polymerase is present with an active repressor protein, no transcription would occur. If the RNA polymerase does not occupy the promoter, no transcription will occur. The repressor must be inactive and the RNA polymerase must be present for transcription to occur.)

What effects will growth in glucose and lactose have on E. coli sugar metabolism and gene expression?

The bacteria will use glucose as their primary sugar source until it is used up and then switch to lactose.

In a mouse experiment, researchers used the lac operon to control the expression of the tyrosinase gene. If the tyrosinase gene contained the protein-coding sequence and the normal mouse regulatory sequences, these regulatory sequences would provide positive control of tyrosinase transcription. If nothing else was done to this DNA and it was introduced into a mouse, tyrosinase would always be expressed. Because the goal was to regulate expression of tyrosinase, what portion of the lac operon should be added to this mouse gene?

The lac operator (Addition of the lac operator regulatory DNA sequence could allow for additional regulation of the tyrosinase gene to be imposed. However, additional sequences would also need to be added so that the operator sequence could be used as a regulatory element in a eukaryote like a mouse. The addition of the lacI gene by itself to the mouse DNA would not allow regulation of tyrosinase activity. The introduction of a regulatory sequence of DNA such as the operator sequence would be required to regulate expression of tyrosinase. The addition of the lacY gene, encoding the lactose permase, would not regulate the expression of the tyrosinase unless lactose was required as an inducer for a repressor protein. Furthermore, if a repressor protein were present, it would require a DNA element to bind to for regulation of the tyrosinase gene. The lacZ gene, encoding the beta-galactosidase enzyme, would have no effect on the expression of the tyrosinase gene.)

Which of the following is true when mutant cells containing a mutation that eliminates lacI transcription are grown in high lactose and high glucose?

The lac operon will be transcribed at low levels, despite the presence of glucose.

In a mouse experiment, researchers used the lac operon to control the expression of the tyrosinase gene. Specifically, they attached the lac operon to the mouse tyrosinase gene. Introducing lac operon/mouse tyrosinase DNA into mice is only half of the equation for regulating tyrosinase. In addition to lac operon/mouse tyrosinase DNA, what other gene must be added to mice to regulate tyrosinase expression?

The lacI gene In addition to lac operon/mouse tyrosinase DNA, the lacI gene would also need to be added to regulate the tyrosinase gene. Addition of lacI would allow repressor binding to the lac operon to regulate tyrosinase production. The lac operon/mouse tyrosinase DNA already has the lac operator present. The addition of another gene that could bind and regulate the lac operon would be required to regulate the lac operon/mouse tyrosinase DNA. The addition of the lacY gene, encoding the lactose permease, would have no effect on the lac operon/tyrosinase DNA for expression of tyrosinase. The addition of the lacZ gene, encoding the beta-galactosidase, would have no effect on the lac operon/tyrosinase DNA for the expression of tyrosinase.

Why is the lacI mutant a constitutive mutant?

The mutant cells do not need an inducer to express β-galactosidase.

Which of the following is true of a mutant cell containing only mutant, non-functional LacY proteins and growing in high-lactose, low-glucose conditions?

These cells will be unable to utilize lactose as an energy source.

What is the mutant phenotype of bacteria that lack a functional lacY gene?

They do not accumulate lactose inside the cell.

What is the function of the promoter of the lac operon?

To bind RNA polymerase, initiating transcription of the lac operon The function of the promoter of the lac operon is to bind RNA polymerase, initiating transcription of the lac operon. Beta-galactosidase cleaves the dissacharide lactose, found in milk, into galactose and glucose. Lactose, the inducer, binds with the repressor releasing it from the operator. When the repressor is bound to the operator, transcription is blocked.

What is the function of the lac repressor?

To bind the lac operator, preventing transcription of the lac operon The function of the lac repressor is to bind the lac operator, preventing transcription of the lac operon. This prevents the expression of the lac operon genes when the inducer lactose is absent from the cell. The lac promoter binds RNA polymerase, initiating transcription of the lac operon. Beta-galactosidase cleaves the dissacharide lactose, found in milk, into galactose and glucose. The galactoside permease (encoded by the lacY gene) transports lactose into the cell allowing it to induce the operon by binding the lac repressor.

What is the function of the lac operator?

To bind the lac repressor, preventing transcription of the lac operon The function of the lac operator is to bind the lac repressor, preventing transcription of the lac operon. The lac promoter binds RNA polymerase initiating transcription of the lac operon. Beta-galactosidase cleaves the dissacharide lactose, found in milk, into galactose and glucose. The galactoside permease (encoded by the lacY gene) transports lactose into the cell allowing it to induce the operon by binding the lac repressor.

What is the function of Beta-galactosidase?

To cleave lactose into glucose and galactose The function of Beta-galactosidase is to cleave lactose, into glucose and galactose. Beta-galactosidase is not normally expressed in the absence of lactose or when the levels of glucose, E. coli's preferred carbon source, are high. Lactose, the inducer, binds with the repressor releasing it from the operator. RNA polymerase binds to the lac promoter initiating transcription of the lac operon genes.

Which type of regulation is not involved in normal induction of the lac operon by lactose?

Translational control Translational control is not involved in the induction of the lac operon by lactose. Translational control occurs when regulatory molecules alter the length of time an mRNA survives, or affect translation initiation or elongation. This does not occur in the lac operon. Negative control of transcription is released when lactose interacts with the repressor that is blocking transcription. Negative control occurs when a regulatory protein called a repressor binds to DNA and shuts down transcription. Allosteric regulation is regulation of a protein's function by the binding of a regulatory molecule. The binding of the lac inducer, lactose, alters the function of the lac repressor, releasing it from the operator. This is an example of post-translational control. Positive control of transcription occurs in the binding of an activator protein called CAP to a regulatory sequence just upstream of the lac operon promoter. This increases the frequency of transcriptional initiation and is required for efficient induction of the lac operon.

How can the AraC protein act as both a transcriptional activator and inhibitor for the ara operon?

When AraC is bound to arabinose, it stimulates transcription. In the absence of arabinose, AraC has a different conformation that results in transcriptional repression of this operon.

The change in shape of AraC protein with the binding of arabinose is an example of __________.

allosteric regulation The change in shape of AraC protein with the binding of arabinose is an example of allosteric regulation. In allosteric regulation, a small molecule binds to a protein and causes it to change its shape and activity. The shape change that results with binding of arabinose changes how and where the AraC dimers bind the DNA. Negative control of transcription occurs when a regulatory protein called a repressor binds to DNA and shuts down transcription. Positive control of transcription occurs when a regulatory protein called an activator binds to DNA and triggers transcription. Both occur in the arabinose operon but they are not examples of the specific interaction in the question. Indeed the allosteric regulation of AraC allows this protein to act either as a repressor or an activator. Translational control occurs when regulatory molecules alter the length of time an mRNA survives, or affect translation initiation or elongation. This does not occur in the arabinose operon.

A mutation that inactivates the regulatory gene for an operon under negative control in E. coli would result in __________.

continuous transcription of the genes controlled by that regulator

In a mouse experiment, researchers used the lac operon to control the expression of the tyrosinase gene. The experimental control of pigment production using the lactose operon in mice suggests that this same genetic system can potentially be used to __________.

control the expression of almost any gene The experimental control of pigment production using the lactose operon in mice suggests that this same genetic system can potentially be used to control the expression of almost any gene. In fact, the process of negative regulation is widely used in bacteria and could be used universally in prokaryotes and eukaryotes. The experimental control of pigment production using the lactose operon has nothing to do with the sex determination of a mouse. The experimental control of pigment production using the lactose operon in mice would not be the most effective way to control hair loss in mice and humans because it provides negative regulation of transcription. The experimental control of pigment production using the lactose operon in mice does not necessarily lead to a cure for cancers.

An E. coli cell with a mutation in the CAP gene such that the CAP protein is unable to bind to regulatory sequence upstream of the lac promoter would be expected to __________.

have lower levels of transcription of the lac operon genes than a wild-type E. coli cell An E. coli cell with a mutation in the CAP gene such that the CAP protein is unable to bind to regulatory sequence upstream of the lac promoter would be expected to have lower levels of transcription of the lac operon genes than a wild-type E. coli cell. There is strong evidence that binding of CAP to a regulatory sequence just upstream of the promoter increases the frequency of transcriptional initiation and is important for efficient transcription of the lac operon.

The binding of arabinose to AraC __________.

increases the transcription of the araB, araA, and araD genes The binding of arabinose to AraC increases the transcription of the araB, araA, and araD genes. Binding of the regulatory molecule, arabinose, to AraC alters the function of this protein, changing it from a repressor to an an activator of the transcription of these genes. In the absence of arabinose, araC represses transcription of all the arabinose genes including araC.

The presence of glucose reduces lactose induction of the transcription of the lac operon genes by __________.

inhibiting the transport of lactose into the cell by galactoside permease The presence of glucose reduces lactose induction of the transcription of the lac operon genes by inhibiting the transport of lactose into the cell by galactoside permease. Because lactose does not accumulate in the cytoplasm, the repressor remains bound to the operator. This mechanism is known as inducer exclusion. Beta-galactosidase cleaves the dissacharide lactose, found in milk, into galactose and glucose. Glucose does not interfere with the binding of lactose to the lac repressor or the binding of RNA polymerase with the lac promoter.

When lactose levels are low, which statement is true about the levels of transcription from the lacI gene?

lacI transcription is unaffected.

Which of the following mutants would be predicted to produce Beta-galactosidase even in the absence of lactose?

lacI- lacI- mutants produce Beta-galactosidase even in the absence of lactose. These constitutive mutants lack the functional repressor and transcription of the lac operon is not blocked in the absence of lactose. lacZ- mutants have a defective gene for Beta-galactosidase, which would not be produced in the presence or absence of the inducer lactose. lacY- and lacA- mutants have defective genes for galactoside permease and transacetylase, respectively, and would not affect regulation of Beta-galactosidase production.

The lactose operon is likely to be transcribed when __________.

lactose levels are high within the cell (The lactose operon is likely to be transcribed when lactose levels are high within the cell. The presence of lactose in the cell increases as the lac operon is turned on due to the increased production of the lactose permease. The presence of high levels of lactose would result in release of the repressor protein (the product of the lacI gene) from the operator DNA sequence and transcription of the lacZ, lacY, and lacA genes. When lactose levels are low, the lac operon is not likely to be transcribed due to binding of the repressor protein to the operator sequence. The presence of glucose and no lactose in the cell would result in tight binding of the repressor to the operator sequence and inhibition of transcription of the lac operon. Glucose inhibits the lactose transport activity of galactoside permease. When both glucose and lactose are present in the environment, the transport of lactose into the cell is inhibited. Because lactose does not accumulate in the cytoplasm, the repressor remains bound to the operator.)

A bacterial cell that lacked the araC gene and, therefore, made no AraC protein would be expected to have __________.

low constant levels of transcription of the araB, araA, and araD genes, whether arabinose is present or not A bacterial cell which lacked the araC gene and, therefore, made no AraC protein would be expected to have low constant levels of transcription of the araB, araA, and araD genes, whether arabinose is present or not. Without AraC to act as a repressor (preventing transcription) or an activator (accelerating initiation of transcription by RNA polymerase), regulation of the genes would be lost and the genes would be transcribed at low levels at all times.

What would you expect the levels of transcription of the ToxR regulon genes to be in a Vibrio cholera cell containing a frameshift mutation in the gene for the common activator protein?

low levels of transcription, whether the cell is in a human gut or not The levels of transcription of the ToxR regulon genes in a Vibrio cholera cell containing a frameshift mutation in the gene for the common activator protein would be expected to be low whether the cell is in a human gut or not. These genes are typically stimulated by the activation of the activator protein when the bacteria from contaminated drinking water encounter the human gut. Frameshift mutations add or delete bases in the coding sequence of the gene that shifts the reading frame of the mRNA. The activator protein would likely therefore be inactive. This inactive activator would result in no stimulation of the transcription of these genes so the levels would be low.

When arabinose is abundant in the surroundings of E. coli, the transcription of araB, araA, and araD is up-regulated by the interaction of the AraC protein with the initiator. This is an example of what type of regulation?

positive control

Regulation of the activity of the LacI protein by binding to lactose is an example of gene regulation at the level of __________.

post-translational modification

Gene expression is __________.

the process of converting information in DNA into molecules that the cell needs to function Gene expression is the process of converting information in DNA into molecules that the cell needs to function. Gene expression does not involve altering the amount of DNA in a cell. Gene expression can be controlled at multiple levels including gene transcription, mRNA translation, and post-translational control of protein activity. Gene expression in E. coli is regulated by both positive and negative control.

A researcher has noticed that the production of protein X is reduced in the presence of substance Z. However, the amount of mRNA of the gene encoding protein X is the same in cells exposed or not exposed to substance Z. Substance Z is likely interfering with __________.

translation of the mRNA for protein X A researcher has noticed that the production of protein X is reduced in the presence of substance Z. However the amount of mRNA of the gene encoding protein X is the same in cells exposed or not exposed to substance Z. Substance Z is likely interfering with translation of the mRNA for protein X. The same amount of mRNA results in lower amounts of translated protein in the presence of substance Z. Interfering with the transcription of the gene would lower the amount of mRNA in the presence of substance Z. Interfering with the replication of the gene would result in the loss of the gene from descendent cells and, therefore, no gene to transcribe into mRNA in the presence of substance Z. Interfering with chemical modification of the protein would have no effects on the amounts of mRNA or protein in the presence of substance Z but would be expected to change the activity of the protein.

If cells that are in competition for space and nutrients had no regulation of gene expression, then __________.

uncontrolled transcription would result in inefficient use of resources and eventually cell death If cells that are in competition for space and nutrients had no regulation of gene expression, then uncontrolled transcription would result in inefficient use of resources and eventually cell death. Cells compete for limited resources. A cell that synthesizes all mRNAs and, therefore, all proteins, not just the ones it needs, has fewer resources to devote to making the proteins it does need. This cell is a loser—it would compete less successfully for resources that are required to produce offspring.

The positive effect of arabinose on the expression of the genes required to utilize arabinose is logical because __________.

when arabinose is present, bacteria need to utilize arabinose The positive effect of arabinose on the expression of the genes required to utilize arabinose is logical because when arabinose is present, bacteria need to utilize arabinose. Expressing these genes only when their function is required conserves energy and other resources. The function of the encoded enzymes is not to make arabinose but to utilize arabinose for energy.