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Architecture of type II topoisomerases. The type IIA topoisomerases topo II, topo IV and gyrase form a dimeric (topo II) or heterotetrameric structure with an N-gate, a DNA-gate and a C-gate. The type IIB topoisomerase topo VI is a heterotetramer that forms only two gates. A: GyrA subunit of gyrase, B: GyrB subunit of gyrase, C: ParC subunit of topo IV, CTD: C-terminal domains of GyrA (gyrase) and ParC (topo IV), E: ParE subunit.
