Genetic Control of Protein Synthesis 67 



The implication of the Nirenberg experiment is that polyuridylic 

 acid is the messenger RNA for polyphenylalanine and that a 

 sequence of U is the messenger RNA code for plienylalanine (or a 

 sequence of A is the DNA code). The "synthetic" and "normal" 

 systems are compared in Figure 3. Since there exists an enzyme, 

 discovered by Ochoa and Grunberg-Manago (15), which will 

 catalyze the random synthesis of ribonucleotides into a polymer, 

 there is now a very powerful tool available for investigating the 

 genetic code. For example, a mixed polymer of A and U provides 

 for sequences of AAA, AAU, AUA, UAA, AUU, UAU, UUA, 

 and UUU, choosing only triplets for purposes of illustration, 

 (It should be noted tiiat at least three bases per amino acid are 

 required if four bases are to specify twenty amino acids.) If poly 

 AU is added as a synthetic messenger, then amino acids coded by 

 the above triplets will be incorporated into a polypeptide chain. 

 Even if some of the triplets are "nonsense" in that they do not 

 specify an amino acid, by using a large excess of U some poly- 

 peptide formation can be assured by providing a polyphenyl- 

 alanine 'handle" so that those triplets which spell an amino acid 

 will not be lost. 



This approach has been pursued very successfully by the Ochoa 

 and Nirenberg groups to describe the most probable code letter 

 for fourteen of the twenty amino acids (16, 17). To carry out the 

 synthetic messenger experiment, the coli extract is first treated 

 ("preincubated") to remove existing messenger RNA. Existing 

 DNA is removed by the enzyme deoxyribonuclease so that new 

 messenger RNA cannot be synthesized. Then synthetic messenger 

 RNA is added, and the amount of C^^ amino acid incorporated 

 into protein-like material (insoluble in trichloroacetic acid) is deter- 

 mined by radioactivity measurements. Any significant incorpora- 

 tion of a CI'^ amino acid, using the UA polymer as the messenger 

 RNA, implies that the code for that amino acid consists of some 

 combination of A and U or of a sequence of A. One can then 

 hope to separate a 2U1A from a 1U2A or a 3A code by deter- 

 mining the ratio of the observed incorporation of a given amino 

 acid to that of phenylalanine and comparing this ratio with that 

 expected for the calculated number of 3U, 2U1A, 1U2A, and 

 3x\ sequences (using a polymer with U in large excess so that the 



