This is the HNH endonuclease domain found in a set of group II intron proteins, including AIM1/2 from yeast and its orthologue from S. pombe. This domain is also found in related bacterial sequences [1].

1: Structure of a group II intron in complex with its reverse transcriptase. Qu G, Kaushal PS, Wang J, Shigematsu H, Piazza CL, Agrawal RK, Belfort M, Wang HW; Nat Struct Mol Biol. 2016;23:549-557. PMID:27136327