Question

Which of the following subunit of RNA polymerase complex is absent during elongation phase of bacterial transcription?

• A. $\alpha$ subunit
• B. $\beta$ subunit
• C. $\beta'$ subunit
• D. $\sigma$ subunit

Hint:

The sigma ($\sigma$) factor acts as a guide to help RNA polymerase find the promoter start site. Once transcription is initiated and the enzyme moves into the elongation phase, this guide is no longer needed and dissociates from the core enzyme.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Bacterial transcription is carried out by a multi-subunit RNA polymerase enzyme.
The enzyme transitions between two structural forms: the holoenzyme, required for transcription initiation, and the core enzyme, which carries out elongation.

Step 2: Detailed Explanation:
Let us analyze the subunit composition of the bacterial RNA polymerase:
- The core enzyme of RNA polymerase consists of five subunits: $\alpha_2\beta\beta'\omega$.
This core enzyme is catalytically active and carries out the elongation phase of transcription, synthesizing the RNA strand along the DNA template.
- To initiate transcription, the core enzyme must bind to a regulatory subunit known as the sigma ($\sigma$) factor, forming the holoenzyme ($\alpha_2\beta\beta'\omega\sigma$).
- The $\sigma$ subunit is essential for recognizing specific promoter sequences on the DNA template and initiating transcription.
- Once transcription is successfully initiated and the nascent RNA chain reaches approximately 8 to 10 nucleotides in length, the $\sigma$ subunit dissociates from the transcription complex.
- The remaining core enzyme then continues the elongation phase without the $\sigma$ subunit.
Therefore, the $\sigma$ subunit is absent during the elongation phase of transcription.

Step 3: Final Answer:
The subunit of RNA polymerase absent during elongation is the $\sigma$ subunit.