Question

Explain how the addition of lactose in the medium regulates the switching on of the \(\textit{lac}\) operon in bacteria.

Hint:

Include a relevant concept, shortcut, or useful trick related to solving this type of problem. Give useful quick tips for the question. Also properly format the text in quick tip as per latex. \textbf{Quick Tip:} Remember the key players in the \textit{lac} operon: the repressor (active when lactose is absent, inactive when present), the operator (binding site for the repressor), the promoter (binding site for RNA polymerase), and the inducer (allolactose, derived from lactose). The operon is 'off' when the repressor is bound and 'on' when it's not.

Answer 1

The Lac Operon in E. coli

The lac operon in E. coli is a classic example of gene regulation in bacteria. It controls the expression of genes that code for enzymes involved in the metabolism of lactose. The regulation of the lac operon depends on the presence or absence of lactose (specifically, its isomer allolactose).

In the Absence of Lactose: 

• The repressor protein, encoded by the i gene (regulatory gene), is synthesized in its active form.
• This active repressor protein binds to the operator region of the lac operon.
• The binding of the repressor to the operator physically blocks the RNA polymerase from binding to the promoter and transcribing the structural genes (lacZ, lacY, lacA) of the operon.
• As a result, the expression of the lac operon is switched off, and the enzymes for lactose metabolism are not produced.

In the Presence of Lactose:

• When lactose is added to the growth medium, it is transported into the bacterial cell.
• Inside the cell, lactose is converted into its isomer allolactose by a small amount of \( \beta \)-galactosidase (the enzyme encoded by lacZ is present at very low basal levels).
• Allolactose acts as an inducer. It binds to the repressor protein.
• The binding of allolactose to the repressor induces a conformational change in the repressor protein, causing it to become inactive.
• The inactive repressor loses its ability to bind to the operator region of the lac operon.
• With the repressor detached from the operator, RNA polymerase can now bind to the promoter and transcribe the structural genes (lacZ, lacY, lacA).
• These genes are then translated into the corresponding enzymes:
  • (\(\beta\))-galactosidase (breaks down lactose into glucose and galactose),
  • lactose permease (facilitates the transport of lactose into the cell),
  • transacetylase (its exact role in lactose metabolism is not fully understood but is thought to detoxify other sugars that might be transported by permease).
• Thus, the addition of lactose (through allolactose) switches on the lac operon, allowing the bacteria to utilize lactose as a carbon and energy source.
• Once the lactose is metabolized and its concentration decreases, the repressor becomes active again and switches off the operon.

Correct Answer:

Lactose addition leads to the production of allolactose, which binds to the repressor protein, making it inactive and detaching it from the lac operon's operator region. This allows RNA polymerase to transcribe the structural genes, switching on the operon and enabling lactose metabolism.