DNA gyrase subunit stoichiometry and the covalent attachment of subunit A to DNA during DNA cleavage

Escherichia coli DNA gyrase contains a 1:1 ratio of protomers coded by the genes gyrA and gyrB. This along with previous results shows that the enzyme has two copies of each protomer and thus a molecular weight of 400,000. Abortion of the gyrase reaction results in double-strand breakage of the DNA and covalent attachment of both gyrA protomers to the 5'-cut ends. We conclude that the gyrA protomer contains a critical part of the active site for the concerted breakage and reunion reaction of gyrase, the topoisomerase activity of the enzyme.