Members of this family are prokaryotic DNA restriction enzymes, exhibiting an alpha/beta structure, with a central region comprising a mixed six-stranded beta-sheet with alpha-helices on each side. A long 'arm' protrudes out of the core of the domain between strands beta2 and beta3 and is mainly involved in the tetramerisation interface of the protein. These restriction enzymes recognise the double-stranded sequence GCCGGC and cleave after G-1 [1].

1: Structure of the tetrameric restriction endonuclease NgoMIV in complex with cleaved DNA. Deibert M, Grazulis S, Sasnauskas G, Siksnys V, Huber R; Nat Struct Biol. 2000;7:792-799. PMID:10966652

2: Sequence, structure and functional diversity of PD-(D/E)XK phosphodiesterase superfamily. Steczkiewicz K, Muszewska A, Knizewski L, Rychlewski L, Ginalski K; Nucleic Acids Res. 2012;40:7016-7045. PMID:22638584