BIO Ch11 Study Set
How do activator or repressor proteins function as genetic switches?
Activator or repressor proteins bind to operator sequences in the vicinity of the promoter to regulate transcription.
Which is an example of negative regulation?
An active repressor binds to an operator site.
An operator mutation is a change in the
DNA sequence that is recognized in the lac repressor.
Determine the nature of the expression of the lacZYA genes, represented by L, and label each genotype as constitutive, inducible, or no transcription in the presence of lactose. Assume that all promoters are wild type.
I+O+L+ = inducible I+OcL+ = constitutive I+OcL+F' = constitutive I+O+L+F' = inducible IsO+L+ = no transcription IsO+L+F' = no transcription
The______ gene encodes the lac repressor protein which binds to the lac_________ in the ______ of lactose.
I; operator; absence
Tryptophan synthase is one of the enzymes synthesized from the trp mRNA. In wild‑type E. coli, the trp mRNA has a short half‑life, but the tryptophan synthase half‑life is much longer. To investigate how changes to the stability of the enzyme or its mRNA affect enzyme activity, two strains of E. coli were engineered. Strain A stabilizes the trp mRNA and strain B rapidly degrades tryptophan synthase. The wild‑type, A, and B strains were grown in a medium with glucose as the sole carbon source. After several generations, tryptophan was added to all three cultures and tryptophan synthase activity was measured periodically. Select the statements that describe the expected change in tryptophan synthase activity after the addition of tryptophan.
In strain A, the trp mRNA is stable and tryptophan synthase will continue to be synthesized. However, as the cultures grow, the tryptophan synthase will be diluted and the amount of enzyme activity will slowly decrease. & In the wild‑type strain, trp mRNA is unstable and will be degraded rapidly. As the cells continue to divide, tryptophan synthase will be diluted out and enzyme activity per cell will decrease. & In strain B, since both the trp mRNA and tryptophan synthase are rapidly degraded, the enzyme activity will drastically decrease.
The lac operon in E. coli controls the gene expression of the enzymes that digest lactose in the cell. In the presence of lactose, the lac operon will turn on and the genes will be expressed. Place the events of gene regulation by the lac operon in order of their occurrence, from the introduction of lactose to the environment to when the cell begins to digest lactose.
Lactose is introduced to the environment 1. the repressor is inactivated by allolactose 2. the repressor is removed from the operator 3. RNA polymerase binds to the promoter region 4. lactose enzyme genes are expressed by the operon The cell begins to digest lactose
For lambda bacteriophage, match each deletion of a gene to whether the mutant phage would enter the lytic or lysogenic cycle.
Lytic: cl Lysogenic: Q, N, cro
Suppose Jim isolated four E. coli mutants, mut1 to mut4, that showed abnormal expression of the Lac permease protein. To identify the defect in each mutant, he determined the relative levels of Lac permease protein in each mutant. He also generated F' plasmids that contained the wild‑type (wt) lac operon or the lac operon from each of the four mutants. He determined lac permease protein levels in partial diploids of wild‑type cells containing F' lac from each mutant and mutant cells containing wild‑type F' lac. All measurements were carried out in cultures grown in the presence ( + ) or the absence (-) of lactose and glucose, as shown in the table in cell genotype / F' plasmid genotype format. Identify the defect in each mutant. Not all defects will be placed.
Mutant 1: deletion of operator Mutant 2: deletion of repressor gene Mutant 3: super-repressor Mutant 4: defective CAP-binding site
Suppose a scientist generates random mutations in E. coli and picks four strains to study. The scientist measures the protein level of β‑galactosidase for each strain in the presence of different carbon sources, as shown in the table. Match each type of mutation to the corresponding data.
Mutant 1: operator deletion Mutant 2: repressor gene deletion Mutant 3: superrepressor mutation Mutant 4: defective promoter CAP-cAMP binding site
Match each lambda phage gene to its corresponding function.
O = is required for viral DNA replication cro = promotes the lytic cycle by repressing cl gene expression N = is an antiterminator that promotes downstream gene transcription
Classify the given examples of prokaryotic gene expression as positive or negative gene regulation.
Positive gene regulation: - in the presence of lactose and low glucose, the lac operon is expressed 20-fold higher than in the absence of lactose - in the presence of the sugar arabinose, an activator protein binds the promoter of the genes responsible for processing arabinose and induces their transcription Negative gene regulation: - in the presence of iron, the dtxR repressor protein binds DNA, and the gene that encodes for the diphtheria toxin is not expressed - in the absence of lactose, the lacR repressor protein binds the lac operon - in the presence of excess tryptophan, a repressor protein binds the operator of the trp operon and prevents the operon from being transcribed
What occurs as a result of allolactose binding to a repressor protein?
The repressor falls off the DNA, allowing RNA polymerase to begin transcription.
An experiment was conducted to determine the effects of a novel promoter on the transcription of a gene. The cloned promoter and coding regions were ligated to luciferase, a reporter gene. Several regions of DNA in the promoter region were deleted. Each construct was transformed into cells, and the luciferase activity was measured. The data from the experiment is listed in the table.
Transcription binding sites are not found in region B. & Basal promoters are located in region D because gene expression decreases when it is deleted.
Which is not a key regulatory component of the lactose metabolic system?
a constitutive mutation
Which of the following genes would be expected to work in trans (on a different DNA molecule)?
a gene encoding a DNA-binding protein
What type of protein regulates structural‑gene expression as a result of low glucose level?
activator
In the cytoplasm, some of the lactose is converted into _______ , which binds the _______ site of the lac repressor.
allolactose; allosteric site
What does constitutive mean in regards to the lac operon?
always on
An I- mutation is unable to
bind the operator DNA.
Suppose that you are studying an inducible operon that contains two genes required for the metabolism of compound xyz. Gene A encodes enzyme A, and gene B encodes enzyme B. When compound xyz is absent, the operon is repressed, but when compound xyz is present, the operon is induced. You have isolated four distinct mutants, numbered 1, 2, 3, and 4, which have the effects shown in the table, where + indicates that the enzyme is synthesized and − indicates that the enzyme is not synthesized. Identify the component of the operon that is mutated in each mutant.
gene B operator gene A promoter
Which is not located within a bacterial operon?
genes for different metabolic pathways
If the glucose in the enviroment was then depleted, how would the lac operon mRNA production change?
increased production
Is the lac operon inducible or repressible?
inducible
IS mutant alleles encode lac repressors that are unable to bind to
lactose.
How much lac operon mRNA does the lac operon produce when both lactose and glucose are present?
low mRNA production
Tryptophan is an amino acid necessary for E. coli survival and growth. E. coli contain genes coding for enzymes that synthesize tryptophan. These genes are grouped together on a segment of DNA called the tryptophan (trp) operon. Cells can use these enzymes to synthesize tryptophan when it is not present in the environment. However, when tryptophan is already present in the environment, cellular resources are shifted away from manufacturing the enzymes for tryptophan synthesis. Tryptophan binds with and activates the trp repressor, which then binds to the trp promoter and blocks RNA polymerase. Blocking RNA polymerase decreases the normal transcription rate of the operon. What type of regulation does the trp operon exhibit?
negative regulation
IS mutantations turn _______ the lac operon in the presence of lactose and turn ________the lac operon in the absence of lactose.
off; off
The lac operon in an I-O+Z+Y+ allele will be _______in the presence of lactose and _______ in the absence of lactose.
on; on
What influences the exact pattern of protein binding in an operon?
physiological signals, activators, and repressors
What type of overall gene regulation occurs as a response to low glucose level?
positive
An Oc mutation results in transcription of the lac operon in the _______ of lactose.
presence and absence
Which DNA segment binds to RNA polymerase for transcription?
promoter
ISO+Z+Y+/F'I+ alleles will result in________ of the lac operon in the presence of lactose and _______of the lac operon in the absence of lactose, because the IS mutation is ________ to I+.
repression; repression; dominant
The lac operon consists of a promoter that initiates transcription of the gene i, which encodes a repressor protein. A separate promoter, p, within the operon initiates transcription of z, which encodes β‑galactosidase, and y, which encodes permease. The operator, o, where the repressor protein binds to inhibit transcription, is positioned between the promoter and the permease and β‑galactosidase genes. The superscript u denotes uninducible mutations, whereas superscript c denotes a constitutively active mutant. The table describes the activity of β‑galactosidase and permease for haploid and diploid mutant strains.
strain 1: cis-acting strain 2: trans-acting strain 3: cis-acting strain 4: trans-acting strain 5: cis-acting strain 6: cis-acting
What defines the biological specificity of gene regulation?
the chemical interaction between regulatory proteins and DNA sequences
RNA polymerase cannot transcribe the lac operon when
the lac repressor is bound to the operator.
What is operon induction?
the relief of repression systems
Which observation demonstrates attenuation as a bacterial control mechanism?
the removal of tryptophan increases mRNA expression
The trp operon also undergoes negative regulation by a repressor. Tryptophan is the signal molecule (effector molecule) that binds to the repressor. Determine which events lead to an increase in transcription. The trp operon is transcribed when
the trp repressor dissociates from DNA. & the trp repressor is inactive.
An F' plasmid with an I+ gene is introduced into the I-O+Z+Y+ allele. The I+ gene operates in _______ and is _______ to I-.
trans; dominant
In Escherichia coli (E. coli), the tryptophan operon encodes structural genes, including trpC, trpB, and trpA, that encode the tryptophan synthetase protein complex. A repressor protein encoded by the trpR gene regulates the trp operon. In the presence of high levels of tryptophan, tryptophan molecules bind the repressor protein and allow the repressor to recognize and bind to the operator, called trpO, and repress the expression of the structural tryptophan synthetase genes. In the absence of tryptophan, the repressor is unable to bind the operator and gene expression can occur. Assume any minus symbol represents a null mutant. What is the type of expression of each functional structural gene for the three bacterial strains?
trpR-, trpO+, trpA-, trpB+, trpC+ for trpA: no expression for trpB: constitutive for trpC: constitutive trpR+, trpOc, trpA+, trpB-, trpC- for trpA: constitutive for trpB: no expression for trpC: no expression trpR-, trpO+, trpA-, trpB+, trpC+ I trpR+, trpOc, trpA+, trpB-, trpC- for trpA: repressible for trpB: repressible for trpC: repressible
