catalyze a cleavage and transport cycle to relieve supercoils. Domain III of TopA binds, cleaves,and opens an ssDNA segment. Separation is achieved by domain III lifting away from domains I and IV while remaining tethered to the rest of the protein through the domain II arch. After the other strand passes through the break, domain II reassociates with domains I and IV and religates the cleaved strand. Following religation, domain III again lifts away from the rest of the protein, allowing exit of the segment. Notice strands A and B have switched places. Type IB enzymes such as TopB also cleave a single strand but then allow the protein-bound 3-hydroxyl end of the DNA to swivel around the 5-phosphoryl end. The topoisomerase then reseals the nicked phosphodiester backbone and the DNA is partially unwound.
