Amidase that catalyzes the last step of diphthamide biosynthesis using ammonium and ATP. Diphthamide biosynthesis consists in the conversion of an L-histidine residue in the translation elongation factor eEF-2 (EFT1 or EFT2) to diphthamide. ATP + diphthine-[translation elongation factor 2] + NH4(+) = AMP + diphosphate + diphthamide-[translation elongation factor 2] + H(+) Protein modification; peptidyl-diphthamide biosynthesis. Interacts with elongation factor 2 (eEF-2; EFT1 or EFT2). Resistance to sordarin. Present with 2270 molecules/cell in log phase SD medium. In the C-terminal section; belongs to the RutC family. In the N-terminal section; belongs to the Diphthine--ammonia ligase family.