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 (eft201 or eft202) to diphthamide (By similarity). Has a role in meiosis. ATP + diphthine-[translation elongation factor 2] + NH4(+) = AMP + diphosphate + diphthamide-[translation elongation factor 2] + H(+) Protein modification; peptidyl-diphthamide biosynthesis. In the N-terminal section; belongs to the Diphthine--ammonia ligase family. In the C-terminal section; belongs to the RutC family.