Supercoils relaxed DNA in an ATP-dependent manner (PubMed:8574396). A type II topoisomerase that negatively supercoils closed circular double-stranded (ds) DNA in an ATP-dependent manner to modulate DNA topology and maintain chromosomes in an underwound state, also catalyzes the interconversion of other topological isomers of double-stranded DNA rings, including catenanes (PubMed:12000834). At comparable concentrations has a stronger decatenation activity than E.coli, which is inhibited by ciprofloxacin and novobiocin (PubMed:12000834). Cleaves dsDNA at the sequence 5'-AT/GGCC-3', leaving a 4 base overhang (PubMed:12000834). Relaxes negatively supercoiled DNA in an ATP-independent manner (PubMed:12000834). Negative supercoiling favors strand separation, and DNA replication, transcription, recombination and repair, all of which involve strand separation. Type II topoisomerases break and join 2 DNA strands simultaneously in an ATP-dependent manner. ATP-dependent breakage, passage and rejoining of double-stranded DNA. Binds two Mg(2+) per subunit. The magnesium ions form salt bridges with both the protein and the DNA. Can also accept other divalent metal cations, such as Mn(2+) or Ca(2+). DNA supercoiling is inhibited by the coumarin antibiotic novobiocin (PubMed:8574396). Also inhibited by the fluoroquinolones ciprofloxacin and moxifloxacin (PubMed:12000834). Heterotetramer, composed of two GyrA and two GyrB chains (PubMed:12000834, PubMed:8574396). In the heterotetramer, GyrA contains the active site tyrosine that forms a transient covalent intermediate with DNA, while GyrB binds cofactors and catalyzes ATP hydrolysis. Few gyrases are as efficient as E.coli at forming negative supercoils. Not all organisms have 2 type II topoisomerases; in organisms with a single type II topoisomerase this enzyme also has to decatenate newly replicated chromosomes. Belongs to the type II topoisomerase GyrB family. Extended N-terminus.