Pearson: regulation of gene expression in prokaryotes

Use this information to determine the levels of expression for the following genotypes and conditions. Rank the genotypes and conditions from highest to lowest level of trp operon expression.

(Highest - Lowest) trp+/tryptophan absent trp L-/tryptophan Present trp R-/tryptophan present trp+/tryptophan Present trp P- / tryptophan absent

Contrast the role of the repressor in an inducible system and in a repressible system.

-inducible system: repressor interacts with operator and inhibits transcription, it is inactivated by inducer to permit transcription -repressible system: repressor is normally inactive until activated by a corepressor, enabling it to bind to operator and inhibit transcription

Can you match terms related to operons to their definitions?

1. OPERON 2. GENES OF AN OPERON 3. PROMOTER 4. REGULATORY GENE 5. OPERATOR 6. REPRESSOR 7. INDUCER

Now use all the data to match the unknown mutations to specific mutation types.

1. lacZ or lacP 2. lac Is 3. cap-CAMP 4. CAP 5.lacOc 6. lacI-

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

A mutation in the operator sequence

How do we know that the trp operon is a repressible control system, in contrast to the lac operon, which is an inducible control system?

Contrary to the lac system in which lactose in the medium causes the synthesis of lac-utilizing structural proteins, addition of tryptophan to the medium represses the transcription of proteins for the synthesis of tryptophan.

For each of the possible partial diploids, determine whether expression of the lac operon will be inducible (wild type) or constitutive (mutant). Then use this information to identify the mutations In Mutation 5 and Mutation 6.

F' I+ / I- is inducible. Mutation 6 is a lacI- mutation. F' O+ / Oc is constitutive. Mutation 5 is a lacOc mutation.

For each of the partial diploids, determine whether expression of the lac operon will be inducible (wild type) or low expression in the absence of glucose (mutant). Then use this information to identify the mutations in mutant strain 3 and mutation strain 4.

F' cap+ / crp- cap+ shows low expression in the lactose only condition. Mutation 4 is a mutation in the crp gene that produces CAP. F' cap+ / crp+ cap- shows high expression in the lactose only condition. Mutation 3 is a CAP-cAMP binding site mutation.

What is the role of glucose in catabolite repression?

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

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

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

Which of the following statements best describes the trp operon?

It is repressible and under negative control.

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

Oc I-

Parts of an operon that control the expression of both enzyme genes are called "regulatory regions." The genes that produce the enzymes are called "structural genes." Using the data in the table, determine which regions are regulatory regions and which are structural genes.

Regulatory Region(s): B D A Structural Gene(s): C E

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.

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 not be expressed.

How do we know that the lac repressor is a protein?

The lac repressor was shown to be a protein with using a sulfur-containing analog of lactose.

Now that you have determined which are the regulatory regions, identify how this operon is regulated. Is this operon inducible or repressible? Is it under positive control or negative control?

The operon is repressible (the signal represses/stops expression of the enzymes). The operon is under negative control (the regulatory protein is a repressor).

If the UGG codons in Region 1 of trpL were changed to AGG codons, what effect would this have on expression of the trp operon?

The operon would be regulated by arginine levels; high levels of arginine would attenuate expression of trp genes.

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

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

In the wild-type operon (A+ B+ C+ D+ E+), how does the presence of the signal affect expression of the structural enzymes ?

The presence of the signal stops production of the structural enzymes. Without the signal, the structural enzymes are produced.

What is the effect on transcription of the structural and regulatory genes of the lac operon when lactose is present in the environment?

Transcription of the structural genes occurs.

What led researchers to conclude that a repressor molecule regulates the lac operon?

Using a mutant of E. coli that produces relatively large amounts of the repressor, researchers have shown that a repressor binds specifically to DNA containing the operator portion of the lac operon.

How do we know that bacteria regulate the expression of certain genes in response to the environment?

When certain additives are supplied to growth media, organisms respond with the production of certain enzymes.

What evidence established that lactose serves as the inducer of a gene whose product is related to lactose metabolism?

When lactose is added to the medium, E. coli respond with the production of enzymes involved in lactose metabolism. When lactose was removed, such enzymes decreased in concentration.

Based on this information, how is the ath operon most likely controlled?

a) Inactive activator b) cAMP c) Active activator d) Active repressor w/out square e) Athelose f) Inactive repressor w/ square

In positive inducible control, the transcription factor is a(n)________. Binding of the signal molecule to the transcription factor causes transcription to ________.

activator start

In positive repressible control, the transcription factor is a(n) ________. Binding of the signal molecule to the transcription factor causes transcription to _______.

activator stop

The regulatory molecule is

an activator; it simulates transcription

What two important domains within a riboswitch involve the ligand-binding site?

aptamer and expression platform

Genetic regulation in prokaryotes can involve alterations in RNA secondary structure. What phenomenon occurs in the trp operon that involves such alterations?

attenuation

Considering the genotypes in the table below, under what medium conditions would you expect β-galactosidase production and/or green colonies when examined under blue light?

beta: --+----++ green: --+++++--

Is the constitutive mutation in the trans-acting reg element or in the cis-acting o operator element?

cis-acting o operator element

For the lac genotypes shown below, predict whether the structural genes (Z) are constitutive, permanently repressed, or inducible in the presence of lactose.

constitutive: I-O+Z+, I-OcZ+, I-OcZ+/F'O+, I+OcZ+/F'O+ repressed: IsO+Z+, IsO+Z+/F'I+ inducible: I+O+Z+

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

false

Under what conditions do the different stem-loop structures occur, and what effect do they have on transcription of the trp genes?

high: trp not transcribed, region 1, and region 3 low: trp transcribed, region 2

In this system, the presence of oil

induces expression of the operon.

Is this system inducible or repressible?

inducible

Which term is used to describe enzymes that are produced only when specific substrates are present in the environment?

inducible

Determine the expression of the lac operon for each possible partial diploid and for the "unknown" mutation partial diploids (Mutation 1 and Mutation 2).

inducible: F' lac+ /P- F' lac+ /Z- F' lac+ /mutation 1 no expression: F' lac+ /Is F' lac+ /mutation 2

Which of the following mutations could explain the loss of regulation in this subspecies?

lac I- lac Oc

Which of the following mutations would result in higher-than-normal expression of the trp genes in presence of tryptophan?

mutations in region 1 and 3

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

negative control

A mutation occurs in an operon that prevents the transcription factor from binding to its recognition site on the DNA. In which type(s) of gene regulation would this mutation result in constitutive expression of the structural gene?

negative inducible negative repressible

In which type(s) of gene regulation does the signal molecule (either a precursor or the final product) cause inactivation of the transcription factor?

negative inducible positive repressible

A mutation occurs in an operon that prevents the signal molecule (either a precursor or the final product) from binding to the transcription factor. In which type(s) of gene regulation would this mutation result in constitutive expression of the structural gene?

negative repressible positive repressible

The erythritol operon is

negatively regulated by a repressor protein; gene expression is induced by the presence of erythritol.

Sort each mutation into the bin that describes its expression pattern.

no expression: lac P lac Z Is low expression: mutation for CAP CAP-CAMP constituative expression: lac I Oc

Would you consider it likely that in this subspecies the lac operon is "on" or "off"?

on

B is the

operator

What term refers to a contiguous genetic complex that is under coordinated control?

operon

The trp and lac operons are regulated in various ways. How do bacteria regulate transcription of these operons?

operon is not transcribed lac operon: lactose absent trp operon: tryptophan present operon is transcribed, but not sped up through positive control lac operon: lactose present, glucose present trp operon: tryptophan absent operon is transcribed quickly through positive control lac operon: lactose present, glucose absent

Given the following diagram, which type of control, positive or negative, is operating?

positive

Is the operon under positive or negative control?

positive

Which of the following terms best characterizes catabolite repression associated with the lac operon in E. coli?

positive control

D is the

promoter

In negative control of gene expression,

removal of the regulatory molecule from DNA stimulates transcription.

C is the

repressor

. In negative inducible control, the transcription factor is a(n) _________. Binding of the signal molecule to the transcription factor causes transcription to a_______

repressor start

. In negative repressible control, the transcription factor is a(n)__________. Binding of the signal molecule to the transcription factor causes transcription to ________.

repressor stop

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

splitting the beta-linkage of lactose

A is the

structural gene

Which stem-loop structure is responsible for terminating transcription?

the 3-4 stem loop

Which of the following is an example of positive control in operons?

the action of the catabolite activator protein (CAP) in the lac operon

Given only the data in the table, for which part(s) of the operon is it possible to identify the exact region it corresponds to?

the promoter region the gene for enzyme 1 the gene for enzyme 2

In positive control of gene expression,

the regulatory molecule stimulates transcription.

What experimental results would be predicted by your hypothesis? Complete this table with the expected results by dragging the "+" and "-" signs to the appropriate locations.

top row: ++++ bottom row: +-+-

Lactose is an inducer of the lac operon.

true

Regarding the lactose utilization system in E. coli, a constitutive mutant is one in which the three enzymes are produced regardless of the presence or absence of lactose.

true

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

true

The trp operon is typically characterized as being both under negative control and repressible.

true

Under a system of negative control, genetic expression occurs unless such expression is shut off by some form of regulator.

true

Under a system of positive control, transcription does not occur unless a regulator molecule directly stimulates RNA production

true