Poly(U)-directed synthesis of poly(Phe). (a) Twenty different ¹⁴C-labeled amino acids were added, individually or in mixtures, with the rest of the amino acids in unlabeled form, to cell extracts that could synthesize protein in the presence of an added poly(U) template. (b) Only the extracts containing [¹⁴C]phenylalanine produced a significant amount of radioactive polypeptide in the presence of poly(U).