This domain belongs to NPun R1517 which is found in Nostoc punctiforme. Studies indicate that Npun R1517 is encoded by orphan gene 29. Npun R1517 adopts a sleigh-shaped structure with a two-stranded antiparallel beta-sheet forming the floor of the sleigh, a HTH forming the seat and a HTH forming the front of the sleigh [1].

1: Crystal structure of Npun_R1517, a putative negative regulator of heterocyst differentiation from Nostoc punctiforme PCC 73102. Ni S, Benning MM, Smola MJ, Feldmann EA, Kennedy MA; Proteins. 2009;74:794-798. PMID:19089958