Topoisomerase IIIα and RMI1 reconstitute a DNA decatenase. (A) Structure of the E. coli TopoI relaxase (PDB ID 1ECL) and the E. coli TopoIII decatenase (PDB ID 1D6M). TopoIII contains specific insertions lining the pore of the topoisomerase toroid which are critical for efficient decatenation. (B) Linear and three dimensional structure of RMI1 and RMI2 (PDB IDs 3NBI and 4DAY). The decatenation loop of RMI1 is highlighted in red. (C) The N-terminal domain of RMI1 contributes to TopoIIIα a decatenation loop in trans (marked in red, PDB ID 4CGY).