This domain is found in Hpy991 present in Helicobacter pylori. Hpy991 is a beta-beta-alpha-Me restriction endonuclease that recognizes the CGWCG target sequence and cleaves both DNA strands with a stagger that leads to 5'-recessed ends in the cleavage products. This domain is the first of two beta4-alpha2 repeats found after the N-terminal domain. The two repeats have low overall sequence similarity but readily identified by a structural comparison. Both repeats contain contains two CXXC motifs that map to the first beta-hairpin and the first alpha-helix. The four cysteine residues coordinate a structurally bound Zn2+ ion tetrahedrally. The major groove is in contact with the first repeat, with the beta-hairpin 2 inserting deeply into the groove [1].

1: Crystal structure of the beta beta alpha-Me type II restriction endonuclease Hpy99I with target DNA. Sokolowska M, Czapinska H, Bochtler M; Nucleic Acids Res. 2009;37:3799-3810. PMID:19380375