This is the C-terminal domain of the arginine repressor, responsible for arginine binding and multimerization [1,2]. It binds mainly Arg, but also ornithine, Pro and Tyr (Matilla et. al., FEMS Microbiology Reviews, fuab043, 45, 2021, 1. https://doi.org/10.1093/femsre/fuab043).

1: Solution structure of the DNA-binding domain and model for the complex of multifunctional hexameric arginine repressor with DNA. Sunnerhagen M, Nilges M, Otting G, Carey J; Nat Struct Biol 1997;4:819-826. PMID:9334747

2: The structure of AhrC, the arginine repressor/activator protein from Bacillus subtilis. Dennis C CA, Glykos NM, Parsons MR, Phillips SE; Acta Crystallogr D Biol Crystallogr. 2002;58:421-430. PMID:11856827