Regulation of Gene Expression — Lac Operon

NCERT grounding

NCERT Class 12 Biology, Chapter 5, places this topic in section 5.8, Regulation of Gene Expression. The text first establishes that gene expression — which results in the formation of a polypeptide — can be regulated at several levels: transcriptional, processing, transport of mRNA, and translational. It then states the decisive point for bacteria: in prokaryotes, control of the rate of transcriptional initiation is the predominant site for control of gene expression. The activity of RNA polymerase at a promoter is regulated by accessory proteins that act positively (activators) or negatively (repressors).

Section 5.8.1 then introduces the worked example: the lac operon, elucidated by the geneticist François Jacob and the biochemist Jacques Monod, who were the first to elucidate a transcriptionally regulated system. The NIOS supplement (Chapter 23) reinforces this, describing the lac operon as an inducible system switched on in the presence of the substrate lactose. Both texts agree on the genes, the products and the logic — this page goes deeper on each.

“The operator region is adjacent to the promoter elements in most operons and in most cases the sequences of the operator bind a repressor protein. Each operon has its specific operator and specific repressor.” — NCERT Class 12 Biology, Section 5.8.

The lac operon — components and switching logic

An operon is a cluster of genes in bacteria in which a polycistronic structural gene is regulated by a common promoter and regulatory genes. This arrangement is very common in bacteria — examples include the lac operon, trp operon, ara operon, his operon and val operon. The lac operon (here lac refers to lactose) controls the genes responsible for the metabolism of the disaccharide lactose in Escherichia coli. Because all the genes needed for one metabolic task are transcribed together as a single unit, the bacterium can switch an entire pathway on or off with one molecular decision.

The lac operon consists of one regulatory gene (the i gene) and three structural genes (z, y and a), flanked and controlled by a promoter and an operator. The i gene lies a little upstream and carries its own promoter; the structural genes z, y and a lie in sequence downstream of the operator. Understanding each component in isolation is the foundation for understanding how the switch works.

The regulatory machinery — i gene, promoter, operator

The i gene is the regulatory gene. NCERT is explicit that here the term i does not refer to inducer — it is derived from the word inhibitor. The i gene codes for the repressor of the lac operon. The repressor is a protein that, when active, prevents transcription. The promoter (p) is the DNA sequence that provides the binding site for RNA polymerase; it is the landing strip the enzyme must occupy before it can begin transcription. The operator (o) is a short DNA sequence located adjacent to the promoter; it is the specific site that binds the repressor protein. NCERT stresses that the lac operator is present only in the lac operon and interacts specifically with the lac repressor only — each operon has its own matched operator–repressor pair.

The structural genes — z, y and a

The three structural genes encode the proteins that actually do the work of lactose metabolism. NCERT notes that all three gene products in the lac operon are required for the metabolism of lactose, and that in most operons the genes function in the same or a related metabolic pathway.

Notice the dual role of lactose. It is the substrate of β-galactosidase, the enzyme that breaks it apart. It is also the inducer — the molecule that regulates switching the operon on and off. NCERT phrases this elegantly: regulation of the lac operon can be visualised as regulation of enzyme synthesis by its substrate. The cell makes the lactose-digesting enzymes only when lactose is around to digest.

The OFF state — operon repressed

The repressor of the operon is synthesised all the time — constitutively — from the i gene. In the absence of lactose, this repressor protein binds the operator region of the operon. With the repressor sitting on the operator, RNA polymerase is physically blocked from transcribing the operon. No functional polycistronic mRNA is made, so no β-galactosidase, permease or transacetylase is produced. The operon is OFF. This is the default, resting state: with no lactose to metabolise, the bacterium does not waste energy making enzymes it cannot use.

The ON state — operon induced

When a preferred carbon source such as glucose is absent and lactose is supplied in the growth medium, lactose is transported into the cell through the action of permease. NCERT inserts an important caveat here: a very low level of expression of the lac operon must be present in the cell all the time, otherwise lactose cannot enter the cells at all. That basal trickle of permease admits the first lactose molecules. Once inside, lactose — or its isomer allolactose — acts as the inducer. The inducer binds the repressor protein and inactivates it by interaction. The inactivated repressor changes shape and can no longer hold the operator.

With the operator now free, RNA polymerase gains access to the promoter and transcription proceeds. The polycistronic mRNA for z, y and a is produced, the three enzymes are synthesised, and lactose metabolism begins. The operon is ON. Because the regulatory protein here is a repressor whose default action is to switch the operon off, NCERT classifies this as negative regulation — the operon turns on only when the negative control is lifted.

The switch is reversible. As lactose is consumed and broken down by β-galactosidase, the level of inducer falls. With little or no lactose left, free active repressor again binds the operator, RNA polymerase is blocked once more, and the operon shuts back down. This is the answer to the classic NCERT question of why the lac operon shuts down some time after lactose is added: the operon stays on only as long as the inducer is present.