This is the N-terminal (nuclease) domain of the BpuSI restriction endonuclease [1].

1: Characterization and crystal structure of the type IIG restriction endonuclease RM.BpuSI. Shen BW, Xu D, Chan SH, Zheng Y, Zhu Z, Xu SY, Stoddard BL; Nucleic Acids Res. 2011;39:8223-8236. PMID:21724614

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