TOPRIM_TopoIIA_GyrB: topoisomerase-primase (TOPRIM) nucleotidyl transferase/hydrolase domain of the type found in proteins of the type IIA family of DNA topoisomerases similar to the Escherichia coli GyrB subunit. TopoIIA enzymes cut both strands of the duplex DNA to remove (relax) both positive and negative supercoils in DNA. These enzymes covalently attach to the 5' ends of the cut DNA, separate the free ends of the cleaved strands, pass another region of the duplex through this gap, then rejoin the ends. These proteins also catenate/ decatenate duplex rings. DNA gyrase is more effective at relaxing supercoils than decatentating DNA. DNA gyrase in addition inserts negative supercoils in the presence of ATP. The TOPRIM domain has two conserved motifs, one of which centers at a conserved glutamate and the other one at two conserved aspartates (DxD). The conserved glutamate may act as a general base in strand joining and as a general acid in strand cleavage by topisomerases. The DXD motif may co-ordinate Mg2+, a cofactor required for full catalytic function.