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, and also catalyzes the interconversion of other topological isomers of double-stranded DNA rings, including catenanes and knotted rings. Replenishes negative supercoiling downstream of highly transcribed genes to help control overall chromosomal supercoiling density (PubMed:22916023). E.coli makes 15% more negative supercoils in pBR322 plasmid DNA than S.typhimurium; the S.typhimurium GyrB subunit is toxic in E.coli, while the E.coli copy can be expressed in S.typhimurium even though the 2 subunits have 777/804 residues identical (PubMed:17400739). 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. Heterotetramer, composed of two GyrA and two GyrB chains. 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 GyrA/ParC subunit family.