chapter 18

How many lac operator sites are there?

3

The RNA leader sequence is a region at the _______.

5' end of the mRNA which is untranslated

The lac operon genes are transcribed together in what order?

5'-lacZ-lacY-lacA-3'

Regulatory sRNAs are typically ______ nucleotides long.

50-400

What effect would either a loss-of-function mutation in the lacI gene or a mutation in the operator that prevents repressor binding have?

Both result in constitutive expression of the lac operon.

The somewhat imprecise term that describes how transcriptional regulation is influenced by glucose is

Catabolite repression

An cis-acting element is typically a ______.

DNA segment that can influence only the expression of adjacent genes

The helix-turn-helix motif is commonly found in _____.

DNA-binding proteins

What happens when tryptophan is removed from the medium in which a trpR— mutant is growing?

Expression of the trp genes increases about three-fold.

Judging from this figure, which of the following statements is most likely true about gene X?

It is induced by the presence of lactose.

Scientists are currently exploring the possibility of targeting the quorum-sensing mechanism of pathogenic bacteria in the development of new antibiotics. What are advantages of this approach?

It might be more difficult for bacteria to develop resistance to such drugs. Quorum-sensing proteins do not resemble any human proteins.

What is the function of the operator site in the lac operon?

It serves as a binding site for the repressor.

Which of the following proteins may function as a dimer or tetramer?

Lac repressor

This figure shows the results of a set of experiments conducted as part of the PaJaMo study. Which of the following statements is the best explanation of these results?

LacI+ acts in trans

Bacteria can adjust their behavior according to their population density by using a communication system called

Quorum sensing

______ is a bacterial communication system that allows them to adjust their behavior according to their population density.

Quorum sensing

The enzyme that transcribes a gene's DNA into RNA is called

RNA polymerase

Transcription is the work of ______.

RNA polymerase

______ are allosteric RNA leaders that bind small effector molecules to control gene expression.

Riboswitches

How are the two sigma factors σ70 and σ32 in E. coli different from each other?

They bind to different promoter sequences. They denature at different temperatures.

When purified, the Lac repressor is ______.

a dimer or a tetramer

An operon is a group of two or more genes under the transcriptional control of ______.

a single promoter

The product of the lacA gene adds ________ groups to lactose and other sugars.

acetyl

In Vibrio fischeri, the quorum-sensing genes are luxR, which encodes a transcriptional _________________ protein and luxI, which encodes a enzyme that is used to make a molecule called a(n) _________________

activator ; autoinducer

The transport of glucose into a bacterial cell causes the intracellular level of cyclic-AMP to fall because the enzyme ________________ is inhibited.

adenylyl cyclase

RNA leader sequences are _______________ in that they can alter their stem-loop structures and thus their function in response to a wide variety of environmental cues.

allosteric

Regulatory RNAs that are transcribed from the opposite DNA strand to that of the mRNA are called

antisense RNAs

Transcription begins but terminates before the entire mRNA is made during

attenuation

In this figure, the red structures represent a molecule called a(n)

autoinducer

Because it is easy to measure the levels of ___________ , the lacZ gene which encodes this enzyme can be used as a(n) ___________ gene to detect the amount of transcription that occurs in response to any specific regulatory element.

beta galactosidase ; reporter

The lac operon contains three genes; lacZ, which encodes ___________ , lacY, which encodes Lac ____________ , and lacA, which encodes an enzyme called ______________

beta-galactosidase ; permease ; transacetylase

An effector is a small molecule that _____.

binds to a protein and changes its conformation

The positive regulator of the lac operon is the

cAMP receptor protein (CRP)

Pathways that break down complex molecules into smaller units are termed ___________ , while pathways that construct end product molecules from simpler units are called __________

catabolic ; anabolic

Breakdown of sugars occurs via ______ pathways, whereas the synthesis of amino acids occurs via ______ pathways.

catabolic; anabolic

A DNA segment that must be adjacent to the gene(s) it regulates is called a

cis-acting element.

Antisense RNA is an RNA strand that is ________ to an mRNA molecule.

complementary

mutant is an organism that synthesizes certain proteins all the time, regardless of environmental conditions.

constitutive

A mutation in the operator that renders it nonfunctional will cause the ______.

constitutive expression of the lac operon

The small nucleotide _______________________ (cAMP) binds to a protein called CRP, and the complex can positively regulate the lac operon.

cyclic adenosine monophosphate

The lacI+ allele is ______ to lacI— in ______.

dominant; trans

The lacIs allele is ______ to lacI+ in ______.

dominant; trans

A small molecule that binds to an allosteric protein or an RNA molecule and causes a conformational change is called a(n)

effector

Cyclic AMP is a(n) ______.

effector molecule that is involved in the positive control of the lac operon

Positive regulation and negative regulation typically refer to mechanisms that _____.

enhance or inhibit RNA polymerase activity, respectively

The genes in the trp operon encode ______.

enzymes used in tryptophan biosynthesis

If the transcription of the trp operon is regulated by the action of the the repressor (TrpR) only, we would expect a trpR— mutant to ______.

exhibit constitutive expression of the trp genes at the same level regardless of the presence of tryptophan in the medium

The cAMP receptor protein (CRP) _______ of the lac operon.

functions as an activator

In order to use lacZ as a reporter gene, scientists use recombinant DNA technology to create a ______________ gene which contains the coding region of lacZ and the cis-acting regulatory regions of a different gene.

fusion

The synthesis of RNAs and proteins according to the instructions encoded in DNA is a process called ______.

gene expression

In catabolite repression, transcription is influenced by the presence of _____.

glucose

The transport of ______ into a cell causes a(n) _______ in the concentration of cyclic-AMP by inhibiting adenylate cyclase.

glucose; decrease

The DNA-binding domain of the Lac repressor protein has a characteristic three dimensional structure known as the

helix turn helix motif

The binding of the ___________ to the repressor causes an allosteric effects that abolishes the repressor's ability to bind to the operator

inducer

A lacIS mutation will affect codons for amino acids in the _____.

inducer-binding domain of lacI

The process by which a specific molecule stimulates synthesis of a given protein is called

induction

In attenuation, transcription ______.

is stopped prematurely

This graph shows the results of the PaJaMo experiment. It appears that the β-galactosidase was synthesized initially from the introduced ________________ allele, but then stopped as the ______________ , which is encoded by the _______________ allele, accumulated.

lacZ+ ; repressor ; lacI+

The lacA gene encodes a transacetylase, enzyme that covalently modifies _____.

lactose

Bacterial mRNAs always begin with a region that is not translated. This 5' untranslated region (5' UTR) is also referred to as the RNA

leader sequence

When the tryptophan level in the cell is ______, the ribosome pauses in region 1 of the trp mRNA leader, causing region 2 to bind to region 3 and transcription to continue.

low

The inhibition of RNA polymerase activity is termed _____________ regulation, whereas the enhancement of RNA polymerase activity is known as __________________ regulation

negative ; positive

The lac operon control region can help produce protein drugs in bacteria. The first step in this process is the creation of a recombinant plasmid in which the lac regulatory region is fused to the __________________ encoding the protein to be expressed.

open reading frame

Constitutive mutants ______.

synthesize certain proteins all the time

The conversion of a riboswitch from one conformation to another is due to _____.

the binding of a small molecule

When lactose is depleted from the environment, ______.

the repressor reverts to its DNA-binding shape

DNA footprinting experiments have revealed that the lac operon has a total of __________ operator sites.

three

Each subunit of the Lac repressor protein contains

three domains

If a mutation is _________ -acting, whichever allele of the gene is dominant will override any other allele of that gene in the cell.

trans

A protein that can diffuse through the cytoplasm and act at DNA target sites elsewhere in the genome is called a

trans -acting element.

The product of the lacI gene is a(n) _________________ The product of the lacI gene is a(n) _____________ of a lac operon regardless of its chromosomal location.

trans ; operator

During gene expression, the information in DNA is __________ into RNA, and then the mRNA message is _____________ into a polypeptide

transcribed ; translated

The base sequences of all the RNAs that a cell produces under a particular set of conditions is known as the

transcriptome

The entire set of RNAs produced by a cell is called its ______.

transcriptome

Small RNA (sRNA) molecules regulate _____.

translation in trans by base-pairing with mRNAs

Select the genes in the trp operon.

trpE trpC trpA trpD trpB

The ribosome-binding site is a special initiation site located ______.

upstream (near the 5' end) of the open reading frame

lacZ lacY lacA

β-galactosidase Lac permease Transacetylase

The normal housekeeping sigma factor is _______.

σ70

Why does transcription of the trp operon continue when tryptophan levels are low?

The ribosome pauses in region 1, preventing the 3-4 stem loop from forming.

The addition of lactose to a bacterial medium causes a 1000-fold increase in the synthesis of the two proteins Lac permease and β-galactosidase. This phenomenon is termed _______.

induction

A mutation that changes the nucleotide sequence of the ________ , so that the repressor is no longer able to bind to the site, will cause a constitutive synthesis of the lactose-utilizing proteins.

operator

In the lac operon, the repressor binds to a DNA sequence called the ________ site. This site is near the lac promoter.

operator

An RNA leader whose secondary structure, and thus its effect on gene expression, is determined by whether or not it's bound to a small molecule effector is called a(n)

riboswitch

In attenuation, the level of an amino acid governs the conformation of the leader indirectly. However, leaders that act as ____________ change conformation by binding a particular effector directly.mation

riboswitches

RNA leader sequences can form secondary structures called ________________ . These can terminate transcription of the rest of the mRNA prematurely.

stem loops

Because there is an overlap between the sequences of the lac _____________ and lac _____________ , binding of the repressor to the DNA will inhibit RNA polymerase from binding. As a result, the expression of the lac operon genes is blocked.

operator ; promoter

A loss-of-function mutation in the lacI gene can have the same effect as a mutation in the _______.

operator site

is a group of two or more genes under the transcriptional control of a single promoter.

operon

The lac operon structural genes are transcribed as _____.

part of a single polycistronic mRNA with three independent ribosome binding sites

In the lac operon, some of the nucleotides of the operator are also part of the ______.

promoter

An element that acts in trans is typically a ______.

protein that can diffuse through the cytoplasm and act on any DNA target in the cell

Mutations in trans-acting elements typically occur in ______, while mutations in cis-acting elements occur in ______.

protein-encoding genes; protein-binding sites Reason: If a mutation is trans-acting, whichever allele of the gene is dominant will override any other allele of that gene in the cell. If a mutation is cis-acting, it affects only the expression of adjacent genes on the same DNA molecule, such as a protein binding site.

Consider a bacterial cell with the genotype: lacI— lacZ+ lacY—/F' (lacI+ lacZ— lacY+). The expression of the genes lacZ+ and lacY+ is _____.

repressible in the absence of lactose and inducible in its presence

Consider a bacterial cell with the genotype lacI+ o+ lacZ+ lacY— that was transformed with an F' plasmid carrying lacIs o+ lacZ— lacY+. The expression of the genes lacZ+ and lacY+ in the merodiploid bacterium is ______.

repressible in the absence or presence of lactose

In this graphic representation of the PaJaMo study, the addition of lactose to the medium resulted in the

resumption of the synthesis of β-galactosidase.

To initiate translation, ribosomes attach to the mRNA at a site called the _______________

ribosome binding site