Pyrimidine Moieties in Animals, Plants, and Bacteria 113 



specifically stimulated the incorporation of the amino acid L-phen- 

 ylalanine into a protein resembUng poly-L-phenylalanine. In this 

 system, advantage was taken of the fact that poly-L-phenylalanine 

 is poorly soluble. Hence, it precipitated out of solution and was 

 readily isolated. The obvious implication of the experiments was 

 tliat polyuridylic acid was functioning as a synthetic template — 

 messenger-RNA: Hence, uridylyl-uridylyl-uridylyl (UUU) was 

 probably the nucleotide triplet which coded for the amino acid 

 L-phenylalanine. 



Other synthetic polyribonucleotides were quickly tested in this 

 system by Speyer, Lengyel, Basilio, and Ochoa (67). On the basis 

 of the latter experiments, the nucleotide code letters for the twenty 

 amino acids commonly found in proteins have been identified. 

 The letters for an assumed triplet code are presented in Table 

 XIV. It should be pointed out that the sequence of bases in the 

 triplets is known only for UUU (code letter of L-phenylalanine). 

 The proposed code letters are in excellent agreement with amino 

 acid replacement data on nitrous acid mutants of tobacco mosaic 

 virus (TMV). In experiments with nitrous acid mutuants of 



TABLE XIV 



The Proposed Genetic Code (42) 



Note: During the transcription of genetic infor- 

 mation from DNA to RNA, thymine (T) is 

 replaced by uracil (U). 



