In E. coli, which DNA polymerase is responsible for bulk DNA replication?

In E. coli, DNA polymerase III is responsible for bulk DNA replication. Its multi-subunit holoenzyme is the bacterium’s main chromosomal replicase, extending new DNA strands rapidly and accurately at replication forks.

The polymerase works with a sliding clamp, called the beta clamp, which keeps the enzyme attached to DNA and gives it high processivity. Its alpha subunit performs DNA synthesis, while the epsilon subunit provides 3′-to-5′ proofreading exonuclease activity. This teamwork helps copy the roughly 4.6-million-base-pair E. coli chromosome efficiently.

A common mix-up is DNA polymerase I. Arthur Kornberg discovered polymerase I first, but it is not the main replicative enzyme. Polymerase I removes RNA primers, fills short gaps, and participates in repair. Polymerase II is mainly associated with repair and damage tolerance, while polymerases IV and V are specialized translesion polymerases that help cells copy damaged DNA.

DNA polymerase III was identified as the primary replicase through genetic studies: temperature-sensitive mutants lacking functional polymerase III could not replicate their DNA under restrictive conditions. The enzyme is therefore the canonical answer for bulk chromosomal replication in E. coli.