TOPRIM_OLD: topoisomerase-primase (TOPRIM) nucleotidyl transferase/hydrolase domain of the type found in bacterial and archaeal nucleases of the OLD (overcome lysogenization defect) family. The bacteriophage P2 OLD protein, which has DNase as well as RNase activity, consists of an N-terminal ABC-type ATPase domain and a C-terminal Toprim domain; the nuclease activity of OLD is stimulated by ATP, though the ATPase activity is not DNA-dependent. Functional details on OLD are scant and further experimentation is required to define the relationship between the ATPase and Toprim nuclease domains. 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 acid in strand cleavage by nucleases. The DXD motif may co-ordinate Mg2+, a cofactor required for full catalytic function.