2F4Q:
reference: Crystal structure of a bacterial type IB DNA topoisomerase reveals a preassembled active site in the absence of DNA., Patel A, Shuman S, Mondragon A, J Biol Chem. 2006 Mar 3;281(9):6030-7. Epub 2005 Dec 19. PMID: 16368685

This OCA Structure page
uses Jmol, developed by the Jmol Development Team (documentation).