Arginine hydroxylase involved in the assembly of mitochondrial NADH:ubiquinone oxidoreductase complex (complex I, MT-ND1) at early stages. Acts by mediating hydroxylation of 'Arg-111' of ndufs7. May also have methyltransferase activity. Peripherally localized on the matrix face of the mitochondrial inner membrane. Belongs to the methyltransferase superfamily.