Question

Identify the enzyme required for transcribing genes of lac operon of E. coli

• A. RNA polymerase II
• B. RNA polymerase I
• C. RNA polymerase III
• D. RNA polymerase

Hint:

Prokaryotes = One RNA Polymerase for everything.
Eukaryotes = Three (or more) specialized RNA Polymerases.
Don't let eukaryotic nomenclature confuse you when a question specifies a bacterium like E. coli!

Answer 1

The Correct Option is D

Step 1: Understanding the Question:
The question asks for the specific enzyme responsible for the transcription (synthesis of mRNA from DNA) of the lac operon in the bacterium Escherichia coli.

Step 2: Detailed Explanation:

• Prokaryotic vs. Eukaryotic Transcription: A fundamental difference between prokaryotes and eukaryotes is the number of RNA polymerase enzymes.

• Prokaryotic RNA Polymerase: Bacteria like E. coli possess only one type of RNA polymerase that is responsible for transcribing all classes of RNA (mRNA, tRNA, and rRNA).

• Structure: The bacterial enzyme is a multi-subunit complex consisting of a core enzyme ($\alpha_2\beta\beta'\omega$) and a sigma ($\sigma$) factor that grants promoter specificity.

• Analyzing Eukaryotic Options:
- RNA Polymerase I: Transcribes large rRNAs in eukaryotes.
- RNA Polymerase II: Transcribes mRNA (protein-coding genes) in eukaryotes.
- RNA Polymerase III: Transcribes tRNA and 5S rRNA in eukaryotes.

• Conclusion: Since the question specifically mentions E. coli (a prokaryote), the specialized numbered polymerases (I, II, III) do not exist in this organism. The process is carried out by the singular bacterial RNA polymerase.

Step 3: Final Answer:
Transcription in E. coli is performed by a single type of RNA polymerase, making option (D) the only biologically correct choice for a prokaryotic system.