POSTSCRIPT 399 



II. PROTEIN SYNTHESIS AND THE GENETIC CODE. 



The genetic coding problem was first formulated by George Gamow 

 who proposed a triplet code in which the translation of genetic informa- 

 tion from the four-letter nucleotide alphabet to the 20 letter amino 

 acid alphabet would require a minimum coding ratio of three nucleotides 

 to one amino acid. Within the past year this proposal has not only been 

 tested experimentally, but an actual translation of the code has been 

 achieved. "Breaking the code" was made technically feasible by the 

 development of a reliable in vitro system in which the incorporation of 

 amino acids into polypeptides could be studied. 



The system developed by Nirenberg and Matthaei with cell fractions 

 from E. coli involves the known constituents: ribosomes, cell supernatant 

 containing the amino acid activating enzymes, the s-RNA's, ATP and an 

 ATP-generating system, and the 20 amino acids for the continuing syn- 

 thesis of the 20 amino acid-s-RNA complexes. When such a system is 

 incubated with radioactive amino acids, a measurable incorporation of 

 label into protein occurs. Nirenberg found that the rate and extent of 

 incorporation could be greatly increased by adding any one of a variety 

 of phenol-extracted RNA preparations. These results led him to test the 

 response of the system to synthetic polynucleotides and in so doing he 

 established a method for deciphering the genetic code. He found that the 

 addition to the system of polyuridylic acid (poly U) led to a great increase 

 in uptake of label, resulting from the incorporation of only one of the 20 

 amino acids, phenylalanine, into a polyphenylalanine peptide. Thus it 

 was shown that uridylic acid alone coded for phenylalanine. A direct 

 estimate of the coding ratio was not possible because poly U acted cat- 

 alytically. Subsequently, purely genetic studies by a group at Cambridge 

 have provided other evidence that a triplet is sufficient. 



Subsequently, with the use of synthetic polynucleotide copolymers, a 

 set of triplets for all 20 amino acids have been worked out in the labora- 

 tories of Nirenberg and of Ochoa. As an example, with the copolymer 

 poly UG, containing about five times as many uridylic acid residues as 

 guanylic, several amino acids, are incorporated, including phenylalanine, 

 leucine, valine, glycine, and tryptophane. From the relative amount of 

 each of these found in the protein fraction, it is possible to distinguish 

 two classes, relative to phenylalanine: leucine and valine, in which the 

 incorporation is about 1/5 that of phenylalanine, and glycine and trypto- 

 phane in which the incorporation is very much lower. On this evidence 

 it has been proposed that the coding letters for leucine and valine are 



