In the lac operon, which protein acts as a negative regulator by binding to the operator region in the absence of lactose?

Explanation

## The Lac Operon Regulatory Mechanism ### Negative Control in the Lac Operon **Key Point:** The lac repressor protein is the primary negative regulator of the lac operon, functioning as a trans-acting factor that blocks transcription when lactose is absent. ### Structure and Function of Lac Repressor The lac repressor (LacI) is a tetrameric protein with the following characteristics: 1. **Operator Binding Domain** — contains a DNA-binding helix-turn-helix motif 2. **Inducer Binding Domain** — binds allolactose (the true inducer) and lactose metabolites 3. **Oligomerization Domain** — allows tetramer formation for cooperative binding ### Mechanism of Repression **High-Yield:** In the **absence of lactose**, the repressor protein binds tightly to the operator sequence (O~1~, O~2~, O~3~), physically blocking RNA polymerase access to the promoter and preventing transcription of the structural genes (lacZ, lacY, lacA). ### Derepression Upon Lactose Presence When lactose enters the cell: - Lactose is converted to **allolactose** by basal levels of β-galactosidase - Allolactose acts as an **inducer**, binding to the repressor protein - This binding causes a conformational change that weakens operator binding - The repressor releases from the operator - RNA polymerase can now transcribe the operon ### Comparison with Other Regulatory Elements | Component | Role | Type | |-----------|------|------| | Lac repressor | Negative regulation (blocks transcription) | Trans-acting | | Operator | DNA binding site for repressor | Cis-acting | | CAP-cAMP | Positive regulation (enhances transcription) | Trans-acting | | Promoter | RNA polymerase binding site | Cis-acting | **Clinical Pearl:** The lac operon is a classic example of **inducible gene regulation** — genes are normally OFF and turned ON only when substrate (lactose) is available. This is energetically efficient for the cell. **Mnemonic:** **LIAR** — **L**ac **I**s **A** **R**epressor (remembering that LacI = lac repressor protein).