Question

Sigma factor is component of

• A. DNA polymerase III
• B. RNA polymerase
• C. DNA polymerase II
• D. Endonuclease

Hint:

Bacteria produce multiple types of sigma factors that respond to different environmental signals. The primary, housekeeping sigma factor in *E. coli* is $\sigma^{70}$, while others like $\sigma^{32}$ are activated during stress conditions like heat shock.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Transcription in prokaryotes is carried out by a single type of RNA polymerase enzyme.
The bacterial RNA polymerase exists in two structural states: the core enzyme and the holoenzyme.

Step 2: Detailed Explanation:
The bacterial RNA polymerase core enzyme has the subunit composition $\alpha_2\beta\beta'\omega$.
While this core enzyme can synthesize RNA using a DNA template, it cannot recognize promoter sequences or initiate transcription at specific genomic sites.
To initiate transcription accurately, the core enzyme must bind to a regulatory protein called the sigma ($\sigma$) factor.
The assembly of the core enzyme and the sigma factor forms the RNA polymerase holoenzyme ($\alpha_2\beta\beta'\omega\sigma$).
The sigma factor is directly responsible for:
- Recognizing the conserved promoter elements, specifically the $-10$ (Pribnow box) and $-35$ consensus sequences.
- Assisting in melting the double-stranded DNA template around the transcription start site to form an open promoter complex.
Once transcription initiates and a short RNA chain is synthesized (usually around 8-10 nucleotides), the sigma factor dissociates from the core enzyme, allowing the core polymerase to transition into the elongation phase.
The DNA polymerases (I, II, and III) and endonucleases do not contain a sigma factor.

Step 3: Final Answer:
The sigma factor is a component of bacterial RNA polymerase, which corresponds to option (B).