This domain is found at the C-terminal end of CRISPR-associated nuclease/helicase Cas3 subtype I-F/YPEST from Pseudomonas aeruginosa. Cas3 contains an HD nuclease domain (Pfam:PF18019), RecA1, RecA2, a long linker region, and this C-terminal domain. This domain shows two alpha-helices and two six-stranded beta-blades [1,2].

1: Structural basis of Cas3 inhibition by the bacteriophage protein AcrF3. Wang X, Yao D, Xu JG, Li AR, Xu J, Fu P, Zhou Y, Zhu Y; Nat Struct Mol Biol. 2016;23:868-870. PMID:27455460

2: A CRISPR evolutionary arms race: structural insights into viral anti-CRISPR/Cas responses. Wang J, Ma J, Cheng Z, Meng X, You L, Wang M, Zhang X, Wang Y; Cell Res. 2016;26:1165-1168. PMID:27585537