Why Is the Lac Operon an Inducible Operon?

Understanding the Correct Answer

The correct answer is (A) Both the Assertion and the Reason are true, and the Reason correctly explains the Assertion. The lac operon is a classical example of an inducible operon because its structural genes are normally maintained in an off or very low-expression state and are induced when an appropriate inducer becomes available.

In the absence of an effective inducer, the lac repressor binds to the operator DNA sequence. The operator is positioned so that repressor binding interferes with productive transcription of the structural genes. Therefore, as long as the functional repressor remains bound to the operator, the lac operon cannot be efficiently induced.

When lactose is available, a small amount is converted into allolactose, which acts as the natural physiological inducer of the lac operon. Allolactose binds to the lac repressor and produces an allosteric conformational change. This change greatly reduces the affinity of the repressor for the operator DNA.

As the repressor leaves the operator, the transcriptional block is relieved and RNA polymerase can transcribe the structural genes more effectively. Therefore, the lac operon is inducible precisely because an inducer can remove the repressor-mediated block that otherwise prevents expression.

Thus, both statements are true, and the Reason correctly explains the regulatory basis for describing the lac operon as an inducible operon.

Analysis of the Assertion

Assertion: Lac Operon Is an Inducible Operon

The Assertion is true. An inducible operon is a regulatory system in which the expression of structural genes is normally absent or maintained at a low basal level but increases in response to a specific inducing signal.

The lac operon controls the expression of genes required for the uptake and metabolism of lactose in bacteria such as Escherichia coli. Producing lactose-metabolizing proteins continuously would waste cellular energy when lactose is absent. Therefore, the bacterial cell keeps the operon largely repressed until lactose becomes available.

When lactose is present, the inducer allolactose interacts with the lac repressor. This interaction reduces the ability of the repressor to remain bound to the operator. The structural genes can then be transcribed more effectively.

Because the presence of an inducer promotes expression of the operon, the lac operon is correctly classified as an inducible operon.

Therefore, Assertion [A] is true.

Analysis of the Reason

Reason: The Lac Operon Is Not Induced When the Repressor Remains Bound to the Operator

The Reason is also true. The lac repressor is a regulatory protein encoded by the lacI gene. In the absence of an effective inducer, the repressor binds strongly to the operator region of the lac operon.

The operator is a regulatory DNA sequence that interacts with the repressor. When the repressor occupies the operator, it prevents efficient transcription of the lac structural genes by interfering with the productive movement or action of RNA polymerase.

Therefore, if the repressor protein remains bound to the operator, the structural genes cannot be efficiently induced. Removal of the repressor from the operator is an essential step in the induction of the lac operon.

The inducer allolactose promotes this removal by binding allosterically to the lac repressor and decreasing its affinity for operator DNA.

Therefore, Reason [R] is true.

Why Does the Reason Correctly Explain the Assertion?

The relationship between the Assertion and the Reason becomes clear when we examine the meaning of an inducible operon. An inducible operon is regulated by a molecular signal that can switch the system from a repressed state to an active state.

In the lac operon, the repressor-bound state represents the non-induced condition. As long as the lac repressor remains attached to the operator, transcription of the structural genes is strongly inhibited.

Induction requires a change in this condition. The inducer binds to the repressor, decreases its affinity for the operator and allows the repressor to dissociate. Once the operator is no longer occupied by the repressor, the operon can be transcribed more efficiently.

Therefore, the statement that the operon is not induced while the repressor remains bound directly explains why the system is inducible: induction occurs through relief of repressor-mediated inhibition.

For this reason, the Reason is not merely another true statement about the lac operon. It provides the correct mechanistic explanation for the Assertion.

What Is the Lac Operon?

The lac operon is one of the best-known models of prokaryotic gene regulation. It controls the expression of genes required for the utilization of lactose as a carbon and energy source.

The system allows a bacterial cell to avoid unnecessary production of lactose-metabolizing proteins when lactose is absent. When lactose becomes available, the regulatory state of the operon changes and the genes required for lactose utilization can be expressed.

The major components associated with lac operon regulation include the lacI regulatory gene, the promoter, the operator, the structural genes lacZ, lacY and lacA, the lac repressor and the inducer.

Together, these components create a highly efficient regulatory system in which environmental nutrient availability can directly influence gene expression.

Major Components of the Lac Operon

The lacI Regulatory Gene

The lacI gene encodes the lac repressor protein. The lacI gene has its own promoter and is not itself one of the structural genes of the lac operon.

The repressor produced by lacI can diffuse through the cell and bind to the operator DNA sequence. Because the repressor is a diffusible protein, the lacI gene product acts in trans.

The Promoter

The promoter is the DNA region where RNA polymerase binds to initiate transcription of the lac structural genes.

For efficient expression of the operon, RNA polymerase must be able to initiate transcription and proceed productively through the transcription unit.

The Operator

The operator is a regulatory DNA sequence that binds the lac repressor. When the repressor occupies the operator, efficient transcription of the structural genes is prevented.

The operator is a cis-acting DNA element because it regulates genes physically linked to it on the same DNA molecule.

The Structural Genes

The lac operon contains three major structural genes: lacZ, lacY and lacA. These genes are transcribed together as a polycistronic mRNA.

The coordinated expression of these genes allows the bacterium to produce several proteins required for efficient lactose utilization.

Functions of the lacZ, lacY and lacA Genes

lacZ and β-Galactosidase

The lacZ gene encodes the enzyme β-galactosidase. This enzyme plays a central role in lactose metabolism by hydrolyzing lactose into glucose and galactose.

β-Galactosidase also converts a small amount of lactose into allolactose. Allolactose is the natural physiological inducer that interacts with the lac repressor.

lacY and Lactose Permease

The lacY gene encodes lactose permease, a membrane transport protein that helps lactose enter the bacterial cell.

Increased production of lactose permease allows the cell to import lactose more efficiently when the sugar is available.

lacA and Thiogalactoside Transacetylase

The lacA gene encodes thiogalactoside transacetylase. Although this enzyme is less central to lactose catabolism than β-galactosidase and lactose permease, it is part of the same polycistronic transcription unit.

The three structural genes demonstrate how an operon allows several functionally related proteins to be regulated together.

What Happens to the Lac Operon When Lactose Is Absent?

When lactose is absent, there is no significant amount of allolactose available to bind the lac repressor. The repressor therefore remains in a conformation with high affinity for the operator DNA sequence.

The repressor binds to the operator and prevents efficient transcription of the lac structural genes. Consequently, the production of β-galactosidase, lactose permease and thiogalactoside transacetylase remains very low.

This regulatory arrangement is metabolically efficient. The bacterium does not waste large amounts of energy and cellular resources producing lactose-utilization proteins when lactose is unavailable.

This is the repressed or non-induced state described in the Reason statement.

What Happens When Lactose Becomes Available?

When lactose enters the bacterial cell, a small amount is converted into allolactose. Allolactose functions as the natural physiological inducer of the lac operon.

Allolactose binds to the lac repressor at an allosteric site. This interaction changes the three-dimensional conformation of the repressor and reduces its affinity for the operator DNA.

The repressor then dissociates from the operator. Removal of the repressor eliminates the major negative regulatory block on transcription.

RNA polymerase can now transcribe the lac structural genes more effectively. The cell produces proteins required for lactose uptake and metabolism.

This transition from the repressed state to the induced state explains why the lac operon is called an inducible operon.

Role of the Repressor in Negative Regulation

The lac operon is a classic example of negative gene regulation because a repressor protein reduces gene expression by binding to a regulatory DNA sequence.

In the absence of the inducer, the lac repressor binds the operator and maintains the operon in a largely repressed state. The repressor therefore acts as a molecular barrier to efficient transcription.

Induction occurs when the inducer changes the activity of the repressor. Instead of directly activating RNA polymerase, the inducer removes a negative regulatory constraint.

This mechanism is often described as negative inducible regulation. The system is negatively controlled by a repressor, and expression is induced when an inducer inactivates the repressor’s DNA-binding function.

How Does Allolactose Act as an Inducer?

Allolactose does not bind directly to the operator DNA sequence. Instead, it binds to the lac repressor protein.

This distinction is essential for understanding lac operon regulation. The operator is the DNA-binding site for the repressor, whereas the inducer is a small molecule that binds allosterically to the repressor.

The regulatory sequence can be represented as follows:

Allolactose binds repressor → repressor changes conformation → repressor affinity for operator decreases → operator becomes free → transcription increases

Therefore, the inducer promotes lac operon expression indirectly by preventing the repressor from remaining bound to the operator.

Why Must the Repressor Leave the Operator for Induction?

The operator occupies a strategically important position in the lac regulatory region. When the repressor is bound to this DNA sequence, it interferes with productive transcription by RNA polymerase.

Therefore, an inducer cannot effectively switch on the operon while the repressor continues to occupy the operator. The regulatory block must first be relieved.

Allolactose accomplishes this by changing the conformation of the repressor. The altered repressor has a much lower affinity for operator DNA and dissociates from the regulatory sequence.

Thus, the statement that the lac operon is not induced while the repressor remains bound is a direct description of the molecular basis of inducibility.

Detailed Explanation of Each Option

Option (A): Both [A] and [R] Are True and [R] Is the Correct Reason for [A]

Option (A) is correct. The Assertion is true because the lac operon is a classical inducible operon. Its expression increases in response to the presence of an inducer.

The Reason is also true because the operon cannot be efficiently induced while the lac repressor remains bound to the operator. Repressor occupancy maintains the negative transcriptional block.

Induction occurs when allolactose binds the repressor, reduces its affinity for the operator and allows the repressor to dissociate. Therefore, the Reason correctly describes the molecular mechanism underlying the inducible nature of the lac operon.

Hence, option (A) is correct.

Option (B): Both [A] and [R] Are True but [R] Is Not the Correct Reason for [A]

Option (B) is incorrect. It is true that both the Assertion and the Reason are factually correct. However, the Reason is not unrelated to the Assertion.

The repressor-bound state is the non-induced state of the lac operon. Induction specifically requires the inducer-mediated reduction of repressor binding to the operator.

Therefore, the Reason provides the mechanistic explanation for why the lac operon behaves as an inducible system. Since the explanatory relationship is valid, option (B) cannot be correct.

Hence, option (B) is incorrect.

Option (C): Both [A] and [R] Are False

Option (C) is incorrect. The lac operon is unquestionably a classical example of an inducible operon, so the Assertion is true.

The Reason is also true because a repressor remaining bound to the operator prevents efficient induction of the structural genes.

Since neither statement is false, option (C) is incorrect.

Hence, option (C) is incorrect.

Option (D): [A] Is True but [R] Is False

Option (D) is incorrect. The Assertion is true, but the Reason is not false.

When the lac repressor remains bound to the operator, it maintains repression and prevents efficient induction. The Reason therefore correctly describes an essential feature of lac operon regulation.

Because both the Assertion and the Reason are true, option (D) is incorrect.

Hence, option (D) is incorrect.

Inducible Operons and Repressible Operons

Inducible Operons

An inducible operon is generally maintained in an off or low-expression state until a specific inducer promotes gene expression. The lac operon is the classic example.

The genes of the lac operon are required mainly when lactose is available. The inducer inactivates the repressor’s operator-binding function and allows increased expression of the structural genes.

Repressible Operons

A repressible operon is generally active until a specific small molecule promotes repression. The trp operon is the classic example.

When tryptophan is abundant, it acts as a corepressor by binding to the trp repressor. The activated repressor can then bind to the operator and reduce transcription of genes involved in tryptophan biosynthesis.

Thus, the lac operon is induced by relief of repression, whereas a classical repressible operon is switched toward repression when its end product becomes abundant.

Basal Expression and Induction of the Lac Operon

The lac operon is often described as being off in the absence of lactose, but this does not mean that absolutely no transcription ever occurs. A very low level of basal or leaky expression can occur.

This basal expression is biologically important because small amounts of lactose permease can help lactose enter the cell, while small amounts of β-galactosidase can convert some lactose into allolactose.

Once allolactose accumulates, it binds to the repressor and promotes induction of the operon. The level of transcription can then increase substantially.

Therefore, the terms “repressed” and “induced” describe major differences in expression level rather than necessarily representing absolute zero transcription and unlimited transcription.

Effect of Glucose on Full Expression of the Lac Operon

Removal of the lac repressor is essential for induction, but the highest level of lac operon expression also depends on the availability of glucose.

When glucose levels are low, intracellular cyclic AMP levels increase. Cyclic AMP binds to the catabolite activator protein, commonly called CAP or CRP. The cAMP-CAP complex binds near the lac promoter and promotes efficient transcription.

Therefore, strong expression of the lac operon generally occurs when lactose is available and glucose is limited. Lactose relieves negative regulation by the repressor, while low glucose favors positive regulation through the cAMP-CAP system.

This additional layer of control does not change the answer to the assertion-reason question. The central point remains that the operon cannot be efficiently induced while the repressor remains bound to the operator.

Why Is the Lac Operon Called a Negative Inducible System?

The term negative refers to the role of the repressor protein. The repressor negatively regulates transcription by binding to the operator.

The term inducible refers to the ability of an inducer to increase gene expression. Allolactose binds to the repressor and decreases its operator-binding ability.

Therefore, the lac operon combines two regulatory features:

Negative control: a repressor inhibits transcription

Inducible control: an inducer removes the inhibitory effect of the repressor

This is why the lac operon is described as a classical negative inducible operon.

Final Answer

Correct Answer: (A) Both [A] and [R] are true and [R] is the correct reason for [A]

The Assertion is true because the lac operon is a classical inducible operon. Its structural genes are maintained in a repressed or low-expression state and are induced when an appropriate inducer becomes available.

The Reason is also true because the lac operon cannot be efficiently induced while the lac repressor remains bound to the operator DNA sequence. Repressor binding maintains the transcriptional block.

Allolactose binds to the repressor, causes an allosteric conformational change and decreases the repressor’s affinity for the operator. The repressor then dissociates, allowing increased transcription of the lac structural genes.

Therefore, the Reason correctly explains the molecular basis of the inducible nature of the lac operon.

Final Answer: (A)