Aspartic acid can be covalently linked to yeast aspartyl-tRNA synthetase and to other proteins, in the absence of tRNA, under conditions where the synthetase activates the amino acid into aspartyl-adenylate, i.e., in the presence of ATP and MgCl2. The linkage between aspartic acid and the protein is acid and alkali resistant; thus it is likely a peptide-like amide bond formed between the activated carboxylate group of aspartic acid and the primary amine function of the side chain of lysine residues.