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Action of a type I topoisomerase

Description
The enzyme breaks one strand and immobilizes the 5’ end by a covalent bond between the DNA phosphate and a tyrosine residue (in E. coli topoisomerase I).

Rotation of the 3’ end is followed by resealing.

The linking number is increased by 1 in the example shown (an underwound DNA).

Action of a type I topoisomerase on overwound DNA would decrease the linking number, by essentially the same mechanism.
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