Role of Sigma (σ) Subunit in E. coli RNA Polymerase: Function, Mechanism

11.  σ subunit of E. colt RNA polymerase DOES NOT
(1) initiate transcription and fall off during elongation
(2) increase affinity of the core enzyme to the promoter.
(3) binds to DNA , independent of the core enzyme.
(4) ensures specificity of transcription by interacting with the core enzyme.

The sigma (σ) subunit is a key regulatory component of RNA polymerase in Escherichia coli (E. coli). Understanding its function helps clarify bacterial gene expression mechanisms and is important for competitive exams like CSIR NET, DBT JRF, GATE, etc.

 What is RNA Polymerase in E. coli?

RNA polymerase in E. coli is a multi-subunit enzyme responsible for transcribing DNA into RNA. It consists of:

• Core enzyme: α₂ββ′ω (lacks specificity for promoter regions)

• Holoenzyme: Core enzyme + σ subunit
  (The holoenzyme is competent to initiate transcription.)

 Functions of the σ Subunit:

1. Promoter Recognition

   • The σ subunit enables RNA polymerase to specifically recognize promoter regions (like the -10 and -35 boxes).

   • Without σ, the core enzyme binds DNA randomly.

2. Increases Affinity for Promoter

   • It enhances the binding affinity of RNA polymerase for specific promoter sequences, making transcription initiation more efficient.

3. Initiates and Dissociates After Transcription Begins

   • After transcription begins and RNA synthesis starts, σ subunit dissociates from the core enzyme to allow elongation to proceed efficiently.

4. Provides Specificity

   • There are multiple types of sigma factors (e.g., σ⁷⁰, σ³²) that allow the bacterial cell to transcribe different sets of genes under different conditions (e.g., heat shock, nitrogen limitation).

 Common Misconception (Correct Answer to the MCQ):

(3) binds to DNA, independent of the core enzyme.

This statement is incorrect and is the right choice for the MCQ.

• The σ subunit can bind DNA on its own.

Recap of All Options:

• sigma factor function in prokaryotes

• bacterial RNA polymerase holoenzyme

• sigma subunit role in transcription

• gene expression regulation in E. coli

• difference between core and holoenzyme