Question
What is the lac operon? Explain how gene expression is regulated in E. coli using the lac operon as an example.
Solution — Step by Step
An operon is a unit of gene expression in prokaryotes, consisting of:
- Structural genes: genes that code for functional proteins (enzymes)
- Operator: a DNA sequence between the promoter and structural genes; the repressor binds here
- Promoter: the region where RNA polymerase binds to begin transcription
The entire operon is controlled as a single unit — all structural genes are transcribed together into one mRNA (polycistronic mRNA).
The lac operon concept was proposed by Jacob and Monod (1961), for which they received the Nobel Prize in 1965.
The lac operon controls the metabolism of lactose in E. coli.
Structural genes:
- lacZ: codes for β-galactosidase (breaks lactose into glucose + galactose)
- lacY: codes for permease (transports lactose into the cell)
- lacA: codes for transacetylase (minor role)
Regulatory elements:
- Promoter (P): where RNA polymerase binds
- Operator (O): where the lac repressor binds
- CAP site: where the CAP-cAMP complex binds (positive regulation)
Separate from the operon: the lacI gene (regulator gene) continuously produces the lac repressor protein.
When lactose is not present in the medium:
- The lacI gene continuously produces the lac repressor protein (active form)
- The repressor binds to the operator sequence on DNA
- This physically blocks RNA polymerase from transcribing the structural genes
- Result: No β-galactosidase is made — no need for it since there’s no lactose to digest
This is an efficient economy: don’t make enzymes you don’t need.
When lactose enters the cell:
- Lactose is converted to allolactose (the actual inducer)
- Allolactose binds to the lac repressor, changing its shape (allosteric change)
- The altered repressor cannot bind to the operator
- RNA polymerase proceeds to transcribe lacZ, lacY, lacA
- Result: β-galactosidase and permease are produced — the cell can now metabolize lactose
This is negative regulation (repressor normally OFF, relieved by inducer).
The term inducible operon: the operon is “on” only when induced by the substrate (lactose/allolactose).
Why This Works
The lac operon is a beautiful example of feedback regulation: the substrate (lactose) induces production of the very enzymes needed to metabolize it. When lactose is available, the enzymes are made. When it runs out, repressor re-binds the operator, transcription stops, and enzyme production halts.
This prevents wasteful enzyme synthesis when the substrate is absent — conserving cellular resources.
Alternative — Glucose and the CAP Site (Positive Regulation)
When glucose IS present (preferred energy source), the lac operon is kept OFF even if lactose is present — through catabolite repression:
- High glucose → low cAMP → CAP-cAMP complex cannot form → weak promoter binding → low transcription
When glucose is absent: high cAMP → CAP-cAMP forms → binds CAP site → enhances RNA polymerase binding → strong transcription
Common Mistake
Students write that “lactose directly inactivates the repressor.” More precisely, it’s the metabolic derivative allolactose (not lactose itself) that binds to the repressor and inactivates it. β-galactosidase converts small amounts of lactose to allolactose. This distinction appears in NEET MCQs — “What is the inducer of the lac operon? Allolactose (not lactose).”