- A scientist compares two distinct bacterial operons, Operon A and Operon B, both regulated by a repressor protein. A mutation that prevents the repressor from binding its small-molecule effector results in permanent repression for Operon A, but constitutive expression for Operon B. Which of the following is the most valid conclusion?
Answer: The repressor for Operon A binds to the operator in its default state, while the repressor for Operon B does not.
For Operon A, if preventing effector binding leads to permanent repression, it means the repressor is 'stuck' on the operator. This describes a super-repressor in an inducible system (e.g., ∗lacIs*), where the repressor is active by default. For Operon B, if preventing effector binding leads to constitutive expression, it means the repressor can never bind the operator. This describes a repressor in a repressible system (e.g., trpR) which is inactive by default and requires a corepressor to bind the operator. Therefore, the fundamental difference is that Repressor A is active by default and Repressor B is inactive by default. - A bacterium has a repressible operon for synthesizing isoleucine. The cell experiences a mutation that inactivates the primary enzyme responsible for degrading isoleucine when it is in excess. Assuming the regulatory system of the operon is wild-type, what would be the long-term consequence of this mutation on the operon's expression?
Answer: The operon will become permanently repressed due to the chronic accumulation of isoleucine.
Questions about repressible operons test your understanding of negative feedback regulation in gene expression. When you encounter these problems, focus on how the end product of the pathway controls its own production.
In a repressible operon like the isoleucine synthesis system, the operon is normally "on" (actively transcribing genes) when the cell needs isoleucine. When isoleucine accumulates to sufficient levels, it acts as a corepressor, binding to the repressor protein and shutting down the operon. This creates a balanced system where synthesis stops when the amino acid is abundant and resumes when levels drop.
The mutation described eliminates the enzyme that degrades excess isoleucine. This means that once isoleucine builds up in the cell, it cannot be broken down and removed. The persistent high concentration of isoleucine will continuously bind to the repressor protein, keeping the operon permanently shut off. Answer A correctly identifies this chronic repression.
Answer B is incorrect because the cycling would actually slow down or stop entirely, not speed up, due to the inability to clear isoleucine. Answer C misses the regulatory mechanism entirely—the cell doesn't "need" more isoleucine when it already has excess that can't be degraded. Answer D ignores the fundamental principle that synthesis and degradation work together to maintain cellular homeostasis.
Remember: in repressible operons, accumulation of the end product = repression. If you can't clear the end product, you get permanent repression. Always trace through the entire regulatory cycle when analyzing operon mutations.
- An operon's transcription is controlled by a regulatory protein. When a small molecule, 'M', is added to the culture, transcription ceases. A loss-of-function mutation in the gene encoding the regulatory protein results in constitutive transcription of the operon, regardless of the presence of 'M'. What is the role of molecule 'M'?
Answer: M is a corepressor that binds to an aporepressor protein.
The operon is on by default and turned off by molecule 'M'. This defines a repressible system. The loss-of-function mutation in the regulatory protein causes constitutive expression, meaning the protein is a repressor (it's needed to turn the operon off). Since the system is repressible, the repressor protein is an aporepressor, which is inactive on its own. It requires the small molecule 'M' to bind to it and activate it. A small molecule that activates an aporepressor is called a corepressor. Thus, M is a corepressor.
- The results of the experiments are shown in the table. Based on these data, what is the most likely mode of regulation for the arg operon?
Answer: Negative repressible, where arginine is a corepressor and ArgR is an aporepressor.
In the wild-type strain, the presence of arginine represses gene expression, indicating a repressible system where arginine acts as a corepressor. In the argR mutant, expression is high (constitutive) regardless of arginine's presence. This means the ArgR protein is necessary for repression. Therefore, ArgR is a repressor. Since it requires a corepressor (arginine) to function, it is specifically an aporepressor. This matches the definition of a negative repressible system.
- The his operon, which synthesizes histidine, is regulated by a negative repressible mechanism. A merodiploid strain of bacteria has the genotype *R+ Oc E− / F' R− O+ E+*, where R is the aporepressor gene, O is the operator, and E is a structural gene. How will the synthesis of enzyme E be regulated in this cell when histidine is added to the medium?
Answer: Synthesis of enzyme E will be repressed, similar to a wild-type cell.
The functional repressor protein (R+) is trans-acting, meaning the protein produced from the chromosome can act on the plasmid's operator. The functional enzyme gene (E+) is on the plasmid, linked to a wild-type operator (O+). The chromosomal DNA has a constitutive operator (Oc), but its linked enzyme gene (E-) is non-functional. Therefore, the functional repressor from the chromosome will bind to the wild-type operator on the plasmid in the presence of the corepressor (histidine), repressing the synthesis of enzyme E. The system's regulation of functional enzyme E will appear wild-type. - A key distinction between negative inducible and negative repressible operons lies in the default state of their respective repressor proteins. Which statement accurately describes this difference?
Answer: In inducible systems, the repressor is synthesized in an active form that binds the operator; in repressible systems, it is synthesized in an inactive form that cannot.
When analyzing gene regulation systems, you need to understand how repressor proteins behave in their "default" state—the form they're synthesized in before any regulatory molecules interact with them.
In negative inducible operons (like lac), the repressor protein is made in an active form that immediately binds to the operator sequence, blocking transcription. The inducer molecule (lactose) then binds to this active repressor, causing a conformational change that makes it release from the operator, allowing transcription to proceed. In contrast, negative repressible operons (like trp) synthesize their repressor in an inactive form that cannot bind DNA on its own. Only when the corepressor (tryptophan) binds to this inactive repressor does it become active and capable of binding the operator to block transcription.
Answer D correctly captures this fundamental difference: inducible repressors start active and get inactivated, while repressible repressors start inactive and get activated.
Answer A reverses the inducible mechanism—the repressor doesn't need activation by the inducer; it needs inactivation. Answer B misunderstands repressible systems, suggesting the corepressor inactivates rather than activates the repressor. Answer C incorrectly states both repressors start active and wrongly claims only repressible repressors can be inactivated.
Remember this pattern: inducible systems are "normally off" (active repressor blocks transcription until inducer turns it on), while repressible systems are "normally on" (inactive repressor allows transcription until corepressor turns it off). The repressor's default state determines the operon's default activity level.
- You are studying a novel operon that is clearly under negative control by a repressor protein. Which of the following experimental findings would be necessary to classify the system as repressible rather than inducible?
Answer: The operon is transcribed at a high level when the repressor's small-molecule ligand is absent.
The core difference is the default state. A repressible operon is normally ON and is turned OFF by the repressor + corepressor. An inducible operon is normally OFF and is turned ON by the inducer removing the repressor. Therefore, finding that the operon is actively transcribed when the small-molecule ligand is absent demonstrates the default state is ON, which is the defining characteristic of a repressible system. Choices A and B are true for both negative inducible and negative repressible systems, so they cannot be used to distinguish between them. Choice D is also insufficient, as it doesn't specify whether a ligand is required for that binding.
- A bacterial operon is regulated by an activator protein that promotes transcription. The activator is only able to bind to the DNA when a small molecule, 'Q', is NOT bound to it. When 'Q' binds to the activator, the activator detaches from the DNA and transcription halts. Which term best describes this mode of regulation?
Answer: Positive repressible regulation.
When analyzing operon regulation, you need to identify two key factors: whether the regulatory protein is an activator or repressor, and whether the system is inducible or repressible based on the small molecule's effect.
In this scenario, the regulatory protein is an activator that promotes transcription when bound to DNA. The small molecule 'Q' acts as a co-repressor - when present, it binds to the activator, causing it to detach from DNA and halt transcription. This means 'Q' represses the operon by preventing the activator from functioning.
Since transcription stops when 'Q' is present, this is repressible regulation. The operon is "turned off" by the presence of 'Q', typically because the gene products are no longer needed (like when end-product molecules accumulate). Because the regulatory protein is an activator, this is positive regulation.
Option A is incorrect because this isn't inducible - 'Q' doesn't induce transcription, it stops it. Option B is wrong because the regulatory protein is an activator, not a repressor, so this isn't negative regulation. Option C fails because again, this involves an activator (positive regulation), not a repressor (negative regulation).
The correct answer is D: positive repressible regulation. The activator protein provides positive regulation, and molecule 'Q' makes the system repressible by shutting down transcription.
Study tip: Remember that "positive" vs "negative" refers to the type of regulatory protein (activator vs repressor), while "inducible" vs "repressible" describes what the small molecule does to gene expression.
- The expression of a hypothetical bacterial operon is summarized by the diagram. The regulatory protein is a repressor. Based on the diagram, which statement is the most accurate conclusion?
Answer: The system is inducible, and Molecule X is an inducer.
The diagram shows two states. In the top panel ('- Molecule X'), the repressor protein is shown bound to the operator, and an arrow indicates 'NO TRANSCRIPTION'. This means the default state of the operon is OFF. In the bottom panel ('+ Molecule X'), Molecule X is shown binding to the repressor, causing it to change shape and release from the operator, which allows for transcription. A system that is normally OFF and is turned ON by a small molecule is an inducible system. The small molecule that causes transcription to start is an inducer. Therefore, the system is inducible, and Molecule X is the inducer.
- Consider a negative inducible operon for metabolizing compound 'A'. Which of the following mutations would lead to a constitutive phenotype?
Answer: A mutation in the repressor protein that prevents it from binding to the operator DNA.
A constitutive phenotype means the operon is always ON. In a negative inducible system, the operon is kept OFF by the repressor binding to the operator. If a mutation prevents the repressor from binding to the operator, then transcription will never be blocked, leading to constitutive expression. Choice A describes a super-repressor, which would be permanently OFF. Choice B would make the operon permanently OFF. Choice C would lead to a stronger OFF state, making it harder to induce.
- For a negatively inducible operon, a mutation in the repressor gene's inducer-binding site results in a super-repressor phenotype (permanent repression). What would be the analogous mutation in a negative repressible operon that also results in permanent repression?
Answer: A mutation that causes the aporepressor to bind the operator without needing the corepressor.
When analyzing operon regulation, you need to understand how different types of operons achieve permanent repression through analogous mutations. Both negatively inducible and negatively repressible operons use repressor proteins, but they function oppositely.
In a negatively inducible operon (like lac), the repressor normally binds the operator and blocks transcription. The inducer molecule causes the repressor to release from the operator, allowing transcription. A super-repressor mutation prevents inducer binding, so the repressor never releases—causing permanent repression.
For a negatively repressible operon (like trp) to achieve the same permanent repression, you need a mutation that keeps the repressor constantly bound to the operator. Normally, the aporepressor (inactive repressor) only binds the operator when complexed with its corepressor molecule. Answer A describes exactly this scenario: if the aporepressor can bind the operator without needing the corepressor, transcription will be permanently blocked.
Answer B is incorrect because while increased operator affinity might enhance repression, it doesn't guarantee permanent repression—the repressor could still be regulated by corepressor availability. Answer C is wrong because a mutation in the corepressor-binding site would likely prevent repressor activation, leading to permanent transcription, not repression. Answer D is incorrect because structural gene mutations affect the gene products, not the regulation mechanism itself.
Remember: analogous mutations in different operon types produce similar phenotypes through opposite mechanisms—one prevents deactivation of repression, the other prevents the requirement for activation of repression.
- The sugar L-arabinose can act as an inducer for the araBAD operon (catabolism) and is also a precursor for the synthesis of the vitamin pentathenate, a pathway controlled by the pen operon. If both operons are regulated efficiently to conserve cellular resources, what is the most likely regulatory effect of high intracellular pentathenate levels?
Answer: Repression of the pen operon and no effect on the araBAD operon.
The pen operon is for an anabolic pathway (synthesis of pentathenate). In an efficiently regulated system, the end product of an anabolic pathway acts as a corepressor. Therefore, high levels of pentathenate should repress the pen operon. The araBAD operon is catabolic and is induced by arabinose. The level of pentathenate, a downstream product, would not be expected to directly regulate the catabolism of the initial precursor, as that would be an inefficient feedback mechanism. Thus, high pentathenate represses its own synthesis but does not affect the unrelated catabolic operon.
- A microbiologist investigates the sor operon, which metabolizes sorbitol. They grow bacteria in a medium containing glycerol and measure the concentration of the enzyme SorA. At t=30 min, sorbitol is added. At t=60 min, glucose is added. The results are shown in the graph. What is the primary reason for the sharp decline in [SorA] after t=60 min?
Answer: The addition of glucose has triggered catabolite repression, which inhibits the expression of operons for secondary sugars.
The initial rise after t=30 shows that sorbitol induces the operon. The sharp decline after glucose is added at t=60 is a classic example of catabolite repression (also known as the glucose effect). Bacteria preferentially metabolize glucose. When glucose is present, it triggers a signaling cascade that reduces cAMP levels, which in turn prevents the CAP activator protein from binding to the promoter of operons for other sugars like sorbitol. This leads to a sharp drop in transcription. While sorbitol depletion (A) could eventually cause this, the immediate and sharp drop upon glucose addition points specifically to catabolite repression.
- In a negative repressible operon, the aporepressor protein has a DNA-binding domain and a corepressor-binding domain. How would a mutation that selectively inactivates the DNA-binding domain differ phenotypically from a mutation that selectively inactivates the corepressor-binding domain?
Answer: Both mutations would result in a non-functional repressor, leading to constitutive expression of the operon.
In a negative repressible system, the aporepressor is inactive by default. It must bind the corepressor to become active and bind DNA. If the DNA-binding domain is mutated, the repressor can never bind the operator, even if it binds the corepressor. This leads to constitutive expression. If the corepressor-binding domain is mutated, the aporepressor can never be activated. It will not bind the operator. This also leads to constitutive expression. Therefore, both mutations prevent the repressor from performing its function, resulting in the same constitutive phenotype.
- Consider a hypothetical merodiploid bacterial strain with the following genotype for the lac operon: I+OcZ−Y+/F′IsO+Z+Y− . The Is allele produces a 'super-repressor' that cannot bind the inducer. The Oc allele is a constitutive operator. Which statement best predicts the phenotype of this cell regarding lactose metabolism?
Answer: The cell will produce functional permease constitutively but will not produce functional β-galactosidase under any condition.
This requires analyzing each DNA molecule separately. On the chromosome (I+OcZ−Y+), the Oc operator is constitutive and cis-acting. It controls the adjacent Z− and Y+ genes. Therefore, functional permease (from Y+) will be produced constitutively. No functional β-galactosidase (from Z−) is made. On the plasmid (F′IsO+Z+Y−), the Is super-repressor is produced. This protein is trans-acting and dominant. It will bind to the wild-type operator (O+) on the plasmid and cannot be removed by the inducer. This will permanently repress the transcription of the plasmid's genes, so no functional β-galactosidase (from Z+) is made. The Is cannot bind to the Oc on the chromosome. Thus, the final phenotype is constitutive permease production but no β-galactosidase production. - In comparing inducible and repressible systems, which statement regarding the physical state of the regulatory protein and DNA is correct for an uninduced inducible system versus a repressed repressible system?
Answer: In an uninduced inducible system the protein is bound to the operator; in a repressed repressible system it is also bound, but is complexed with its corepressor.
When analyzing gene regulation systems, you need to understand how regulatory proteins behave differently in inducible versus repressible operons, and how small-molecule effectors influence protein-DNA binding.
In an uninduced inducible system (like lac operon without lactose), the repressor protein is naturally bound to the operator, blocking transcription. The system remains "off" until an inducer molecule arrives to remove the repressor. In a repressed repressible system (like trp operon with tryptophan present), the regulatory protein is also bound to the operator, but only after forming a complex with its corepressor molecule. The corepressor enables the repressor to bind and shut down transcription.
Answer A correctly describes both states: the regulatory protein is bound to the operator in both cases, and in the repressible system, it's complexed with its corepressor.
Answer B is wrong because it states the small-molecule effector is bound in both cases, but in uninduced inducible systems, the inducer is absent. Answer C incorrectly claims the effector is absent in both systems, but repressed repressible systems require the corepressor to be present and bound. Answer D is incorrect because it states the protein isn't bound in uninduced inducible systems, when actually the repressor is bound and blocking transcription.
Remember this key pattern: both uninduced inducible and repressed repressible systems have their genes "turned off" with regulatory proteins bound to operators, but they differ in whether small-molecule effectors are required for that binding.
- A bacterium lives in an environment where the sugar maltose is only available intermittently and unpredictably. The bacterium possesses a maltose-catabolizing operon. To maximize fitness by conserving resources, which regulatory strategy would be most advantageous for controlling this operon?
Answer: An inducible system where maltose or one of its metabolites acts as an inducer to activate transcription.
The operon's function is catabolic (breaking down maltose). Energy is best conserved by only producing these enzymes when the substrate (maltose) is present. An inducible system accomplishes this perfectly. The operon is kept off by a repressor. When maltose appears, it (or a derivative) acts as an inducer, removing the repressor and turning the operon on. A repressible system is suited for anabolic pathways, and constitutive expression would be wasteful since maltose is not always present.
- Which of the following scenarios would result in a phenotype that is phenotypically indistinguishable from a wild-type repressible operon?
Answer: A merodiploid with genotype R−O+E+/F′R+OcE− where R is the aporepressor.
When analyzing repressible operons, you need to understand how complementation works in merodiploids (partial diploids with extra genes on a plasmid). A wild-type repressible operon is normally "on" but gets turned "off" when the corepressor binds to the aporepressor, forming an active repressor complex.
The correct answer is D because this merodiploid can produce functional repressor protein. The chromosome has R−O+E+ (no functional aporepressor), while the plasmid has R+OcE− (functional aporepressor but constitutive operator). The R+ gene on the plasmid produces aporepressor protein that can act in trans, diffusing through the cell to bind the normal O+ operator on the chromosome when corepressor is present. This restores normal repressible regulation, making it phenotypically identical to wild-type.
Option A is wrong because the chromosome lacks functional enzyme (E−), so even though the plasmid has E+, the constitutive Oc operator means this gene can't be repressed, creating abnormal expression patterns. Option B produces a leaky phenotype with some basal transcription even when repressed, unlike true wild-type behavior. Option C prevents repression entirely because without intracellular corepressor, the aporepressor cannot bind DNA, leaving the operon constitutively active.
Remember: in merodiploid problems, focus on whether all components (functional repressor, normal operator, and enzyme) can work together to restore wild-type regulation through complementation. - A bacterial operon, the xyl operon, is required for the catabolism of the sugar xylose. Its structural genes are expressed at high levels only when xylose is present and a preferred carbon source is absent. A mutation in the operator sequence, xylO^c, prevents the binding of any regulatory protein to it. What is the most likely phenotype of this xylO^c mutant?
Answer: Constitutive expression of the xyl operon, regardless of the presence of xylose.
A catabolic pathway like this is typically under negative inducible control. The repressor protein is normally bound to the operator, preventing transcription. The inducer (xylose) binds the repressor, causing it to detach. The xylO^c mutation prevents the repressor from binding to the operator. Therefore, RNA polymerase will have continuous access to the promoter, leading to constitutive expression, regardless of whether the inducer (xylose) is present.
- An operon in a newly discovered bacterium encodes enzymes for the synthesis of the amino acid fictamine. The operon is transcriptionally active only when fictamine is absent from the growth medium. A loss-of-function mutation in a separate regulatory gene, ficR, causes constitutive expression of the operon regardless of fictamine levels. Which of the following statements accurately describes this regulatory system?
Answer: The system is repressible, with fictamine functioning as a corepressor that binds to the FicR aporepressor.
The operon is for an anabolic pathway (synthesis), which is typically repressible. The end product, fictamine, stops transcription, meaning it's a corepressor. The operon is normally ON and is turned OFF by the corepressor. The regulatory protein, FicR, must be the aporepressor that binds fictamine to become active. A loss-of-function mutation in ficR means the aporepressor is non-functional and cannot repress the operon, leading to constitutive expression. This matches the description of a negative repressible system.
