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#DNA gyrase

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Replication in Prokaryotes

DNA gyrase is a type of

Topoisomerases classified mechanistically by whether cleavage of one versus two DNA strands occurs during catalytic cycle. Type I cuts single strand allowing rotation to change linking number in steps of one. Type II cuts both strands, passes intact duplex through break, alters linking number by two. DNA gyrase and topoisomerase IV are bacterial type II enzymes requiring ATP and Mg2+ forming transient covalent phosphotyrosine intermediates. Gyrase uniquely introduces negative supercoils and removes positive supercoils ahead of replication, while topo IV primarily decatenates chromosomes. Doubl

Ref: Berg et al., Biochemistry, 9th ed., Chapter 27: Type II topoisomerases, DNA gyrase mechanism and classification

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Replication in Prokaryotes

Protein that relieves positive supercoiling is

Unwinding helicase generates positive supercoils ahead of fork increasing helical twist and resisting further strand separation requiring topoisomerase relief. DNA gyrase, type II topoisomerase composed of GyrA and GyrB subunits as A2B2 tetramer, introduces negative supercoils using ATP hydrolysis to counterbalance positive supercoiling essentially removing overwinding stress. It makes transient double-strand cuts, passes another segment through break, and religates. SSB binds single strands but does not resolve topology, ligase seals nicks only. Gyrase thus prevents fork stall and facilitates

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 5: DNA gyrase relieving positive supercoils ahead of fork

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