Catalysis of transcription: Which enzyme catalyzes the synthesis of RNA from a DNA template during transcription?

Introduction / Context:
Transcription converts genetic information stored in DNA into RNA. The catalytic engine of this process is RNA polymerase, an enzyme conserved across bacteria, archaea, and eukaryotes, albeit with differences in subunit composition and accessory factors.

Given Data / Assumptions:

• The reaction is template-directed polymerization of ribonucleotides into RNA.
• DNA remains as the template; product is RNA, not DNA.
• Accessory enzymes like topoisomerases may assist but do not catalyze RNA synthesis.

Concept / Approach:
RNA polymerase binds promoters, melts a transcription bubble, and elongates the RNA chain in the 5' to 3' direction by forming phosphodiester bonds. DNA gyrase and topoisomerase I modulate DNA supercoiling ahead of and behind the transcription complex, and DNA ligase seals DNA nicks in replication or repair, but none of these synthesize RNA.

Step-by-Step Solution:
Define the biochemical task: DNA-templated RNA synthesis. Identify the enzyme class performing this reaction: RNA polymerase. Eliminate enzymes with different functions (gyrase, ligase, DNA polymerase). Choose RNA polymerase.

Verification / Alternative check:
Inhibitors like rifampicin block bacterial RNA polymerase and halt RNA synthesis, providing functional proof that RNA polymerase is the catalytic enzyme for transcription.

Why Other Options Are Wrong:

• DNA polymerase: Catalyzes DNA synthesis, not RNA.
• DNA ligase: Joins DNA fragments; no polymerization from nucleoside triphosphates into RNA.
• DNA gyrase / Topoisomerase I: Change DNA topology; do not form RNA chains.

Common Pitfalls:
Assuming any nucleic acid enzyme can make RNA; catalytic specificities are distinct and highly regulated.

Final Answer:
RNA polymerase.