Which Enzyme Introduces Negative Supercoiling in Prokaryotic DNA?

Topoisomerase II (DNA gyrase) is the enzyme that introduces negative supercoiling in prokaryotic DNA, aiding compaction and processes like replication. This multiple-choice question tests core knowledge of DNA topology in bacteria.

Correct Answer

Topoisomerase II.
In prokaryotes like E. coli, Topoisomerase II—specifically DNA gyrase—uses ATP to introduce negative supercoils, countering positive supercoils from unwinding during replication and transcription. It’s the only enzyme capable of this energy-driven action.

Option Breakdown

Topoisomerase I

Relaxes negative supercoils by nicking one DNA strand, preventing excessive supercoiling but does not introduce it. Common in prokaryotes to balance gyrase activity.

Topoisomerase II

Correct. Known as DNA gyrase in bacteria, it cuts both strands, passes DNA through, and reseals, adding two negative supercoils per ATP hydrolyzed. Essential for bacterial DNA compaction.

Topoisomerase III

Relaxes supercoils and aids decatenation or recombination, but lacks supercoiling introduction. Less prominent in supercoiling regulation.​

Topoisomerase IV

A type II enzyme that decatenates daughter chromosomes post-replication and relaxes supercoils, but does not introduce negative supercoiling like gyrase.