Chapter 16

The lac operon in E. coli controls the gene expression of the enzymes that digest lactose in the cell. In the absence of lactose, the lac operon will turn off and gene expression will be inactivated. Place the events of gene regulation by the lac operon in order of their occurrence, from the removal of lactose from the environment to when the cell no longer digests lactose.

1. Lactose scarce in environment. 2. Repressor activated in absence of lactose 3. Repressor binds to the operator 4. RNA polymerase prevented from moving from the promoter 5. Lactose enzyme genes not transcribed.6. The cell no longer digests lactose

What is the function of the promoter in the E. coli lac operon?

A promoter is a type of DNA sequence that activates the expression of the lactose genes by acting as an RNA polymerase binding site for the lac operon.

Which form of gene regulation primarily occurs in eukaryotes? A.) alteration of chromatin structure B.) regulation of mRNA stability C.) All of these forms of regulation occur equally in both prokaryotes and eukaryotes. D.) regulation of transcription E.) regulation of translation

A.) alteration of chromatin structure

The type of transcriptional control in operons in which a regulatory protein is an activator and stimulates transcription is called A.) positive control. B.) repression. C.) negative control. D.) attenuation. E.) induction.

A.) positive control.

Which item is a common DNA‑binding motif? A.) alanine zipper B.) helix‑turn‑helix C.) loop‑helix‑loop D.) ring‑loop‑ring E.) iron‑finger

B.) helix‑turn‑helix

What is the function of the lac operator? A.) It binds RNA polymerase. B.) It produces an initiator RNA molecule. C.) It is bound by the lac repressor protein. D.) It binds lactose. E.) It produces the lac repressor protein.

C.) It is bound by the lac repressor protein.

Which choice is the true inducer of the lac operon? A.) galactose B.) cAMP C.) glucose D.) allolactose E.) lactose

D.) allolactose

Which set of conditions will lead to the highest expression of the lac structural genes? A.) low levels of lactose and low levels of glucose B.) high levels of lactose and high levels of glucose C.) high levels of the lac repressor and high levels of glucose D.) high levels of lactose and low levels of glucose E.) low levels of lactose and high levels of glucose

D.) high levels of lactose and low levels of glucose

Attenuation Prokaryotes commonly use attenuation as a mechanism to control gene expression, but eukaryotes do not. Why do you think that attenuation is more common in prokaryotes than eukaryotes?

In eukaryotes, transcription and translation usually take place in separate physical locations.

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

It encodes a repressor that, in the absence of lactose, binds to the lac operator and blocks expression of the structural genes.

Suppose an operon has the following characteristics: (1) The operon codes for structural proteins that convert compound Q to compound B. (2) The operon is controlled by a constitutively expressed regulatory gene called reg. (3) In wild‑type individuals, the operon is transcribed in the absence of compound B but not in the presence of compound B. (4) In reg− mutants, the operon is constitutively transcribed. Is this operon inducible or repressible? Why?

It is repressible because wild‑type transcription is repressed in the presence of compound B.

Which reaction normally happens in the regulation of the trp operon when high levels of tryptophan are present?

Regions 3 and 4 will pair in the attenuation region in the 5′ UTR of the RNA.

constitutive operon

The amount of gene product is constant

Attenuation What effect on the expression of the trp operon might you expect if there were a deletion of the Shine‑Dalgarno sequence prior to the trpD structural gene?

The entire trp operon will always be transcribed unless the cell is able to obtain tryptophan from the external environment.

Attenuation What effect on the expression of the trp operon might you expect if the stretch of T:A base pairs following region 4 were deleted?

The entire trp operon will be transcribed no matter what the level of tryptophan in the cell.

Attenuation What effect on the expression of the trp operon might you predict if there was a large deletion in region 3 of the 5' UTR of the trp operon?

The entire trp operon will be transcribed no matter what the level of tryptophan in the cell.

Attenuation If the trp operon from E. coli was cloned between a ubiquitous eukaryotic promoter and a strong eukaryotic transcriptional termination sequence, how would this operon function in a eukaryotic cell?

The entire trp operon will be transcribed, but none of the structural genes of the trp operon will be translated no matter what the level of tryptophan in the cell.

Attenuation What effect on the expression of the trp operon might you predict if the Shine‑Dalgarno sequence prior to region 1 of the trp5' UTR were deleted?

The entire trp operon would be transcribed no matter what the level of tryptophan in the cell.

lac operon What is the role of the protein encoded by the lacZ gene?

The lacZ gene encodes an enzyme that converts lactose to allolactose and that also breaks down lactose to glucose and galactose.

lac operon The lac operon is inducible. What does this mean?

The operon is transcriptionally active only in the presence of lactose.

A mutation at the operator site of an operon prevents the repressor from binding. What effect will this mutation have on transcription in a repressible operon?

The operon will always be transcriptionally active.

A mutation at the operator site of an operon prevents the repressor from binding. What effect will this mutation have on transcription in an inducible operon?

The operon will always be transcriptionally active.

Attenuation What effect on the expression of the trp operon might you expect if a deletion occurred in the 5' UTR leader sequence that removed the trp codons in the trp operon?

The structural genes for the trp operon will not be transcribed no matter what the intracellular levels of tryptophan are.

Attenuation What effect on the expression of the trp operon might you expect if the trp codons in the 5' UTR leader sequence of the trpoperon were replaced by codons for isoleucine?

The structural genes for the trp operon will only be transcribed when isoleucine levels are low.

Why is transcription a particularly important level of gene regulation in both bacteria and eukaryotes?

Transcription is the first step in the process of information transfer from DNA to protein. For cellular efficiency, gene expression is often regulated early in the process of protein production.

inducible operon

Transcription is turned on when a specific molecule is present

repressible operon

Transcription stops when the repressor gene product is activated Sufficient amounts of the gene product inhibit further transcription

What is the role of tryptophan in the regulation of the trp operon?

When tryptophan levels are high, tryptophan binds to the inactive trp repressor, making it active.

lac operon Is it possible to get any transcription of the lac operon in the absence of lactose? Why or why not?

Yes, in spite of the presence of the repressor, a low level of basal transcription and translation must take place in order to produce the permease to allow lactose into the cell and in order to produce β‑galactosidase to make allolactose.

lac operon What is the operator?

a DNA sequence in the lac operon to which the lacI repressor protein binds.

lac operon What is a partial diploid?

a bacterium that has at least some common genes on both the chromosome and a plasmid

Which statement describes an operon?

a gene cluster controlled by a single promoter that transcribes to a single mRNA strand

lac operon What best defines an operon?

a group of bacterial genes that are transcribed together on the same mRNA

What is the regulatory segment of an mRNA that binds a small molecule resulting in a change in the production of the protein encoded by this mRNA called?

a riboswitch

lac operon What is the inducer of the lac operon?

allolactose

Proteins that change shape on binding to another molecule are called

allosteric proteins.

A small RNA that can base pair with an mRNA molecule and affects its functioning is called

an antisense RNA.

A mechanism of genetic regulation in which the synthesis of a short RNA is terminated before the structural genes of the operon can be transcribed is called

attenuation.

How do amino acids in DNA-binding proteins interact with DNA? a. By forming covalent bonds with DNA bases b. By forming hydrogen bonds with DNA bases c. By forming covalent bonds with DNA sugars

b. By forming hydrogen bonds with DNA bases

In the trp operon, what happens to the trp repressor in the absence of tryptophan? a. It binds to the operator and represses transcription. b. It cannot bind to the operator, and transcription takes place. c. It binds to the regulator gene and represses transcription. d. It cannot bind to the regulator gene, and transcription takes place.

b. It cannot bind to the operator, and transcription takes place.

What is the effect of high levels of glucose on the lac operon? a. Transcription is stimulated. b. Little transcription takes place. c. Transcription is not affected. d. Transcription may be stimulated or inhibited, depending on the levels of lactose.

b. Little transcription takes place.

In the presence of allolactose, the lac repressor a. binds to the operator. b. binds to the promoter. c. cannot bind to the operator. d. binds to the regulator gene.

c. cannot bind to the operator.

A system of gene control in bacterial operons in which glucose is used preferentially and the metabolism of other sugars is repressed in the presence of glucose is called

catabolite repression

Attenuation results when which regions of the 5′ UTR pair? a. 1 and 3 b. 2 and 3 c. 2 and 4 d. 3 and 4

d. 3 and 4

What is the difference between a structural gene and a regulator gene? a. Structural genes are transcribed into mRNA, but regulator genes aren't. b. Structural genes have complex structures; regulator genes have simple structures. c. Structural genes encode proteins that function in the structure of the cell; regulator genes carry out metabolic reactions. d. Structural genes encode proteins; regulator genes control the transcription of structural genes.

d. Structural genes encode proteins; regulator genes control the transcription of structural genes.

The regulator protein that acts on a negative repressible operon is synthesized as a. an active activator. c. an active repressor. b. an inactive activator. d. an inactive repressor.

d. an inactive repressor.

lac operon What are the structural genes of lac operon?

lacZ, lacY, lacA

A mutation in the lac promoter region that causes this region to no longer function would lead to

lack of expression of the lac structural genes in the presence of lactose.

What is a constitutive gene?

not regulated and is expressed continually.

lac operon When speaking of gene regulation, regulatory elements are often described as cis‑ or trans‑regulatory elements. A cis‑regulatory element is a nucleic acid sequence that regulates the expression of genes located on the same molecule of DNA. In contrast, a trans‑regulatory element is a diffusible factor, usually a protein, that may modify gene expression at distant sites on the same or different molecule of DNA. In the lac operon, which of these is a trans‑regulatory element?

the lacI repressor

lac operon Consider the sequence. P+lacIS...P+ lacO+lacZ+lacY+lacA+ / P+lacI+...P−lacOClacZ+ lacY+lacA+ What would be the result of a partial diploid with a bacterial chromosome containing a super repressor (lacIS) mutation but otherwise wild type, and a plasmid containing a wild‑type lacI gene and lacI promoter but also containing both a nonfunctional lac operon promoter mutation and a lacOC mutation?

β‑galactosidase would not be produced whether or not lactose was present.