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UUG, and for glycine and tryptophaiu' are UGG. Similarly, code letters 

 have been inferred for the other amino acids, using other copolymers: 

 UA, UC, UAC, UAG, and UCG. It is noteworthy that the code is de- 

 generate in the sense that more than one triplet has been found to code 

 for leucine and for a few other amino acids. Since there are 20 amino 

 acids and 64 triplets, some degeneracy vvas expected. Not all of the 

 possibilities have yet been found. It may require a use of other organisms 

 and perhaps a different method to complete the translation. Until that is 

 accomplished, the universality of the code will remain an open cjuestion. 



The correctness of the code as presently known is supported by the 

 remarkable consistency with which it fits the data on known amino acid 

 substitutions in proteins resulting from mutation. For example, the 

 amino acid composition of the tobacco mosaic virus (TMV) protein has 

 been established, and single amino acid changes have been found by 

 Wittman and by Tsugita and Fraenkel-Conrat in a series of mutant 

 viruses resulting from in vitro deamination by HNO2 (p. 37). In 16 of 

 these mutants, the amino acid changes when coded in terms of the cor- 

 responding triplets, could be accounted for by deamination of a single 

 nucleotide of the triplet. There are also a few instances which do not 

 fit the present code, and may reflect its degeneracy. All of the known 

 amino acid substitutions in mutant hemoglobins have also been examined, 

 and most of the changes here too are consistent with single nucleotide 

 changes in the nucleic acid code. 



Support of the universality of the code comes from in vitro studies of 

 protein synthesis in mixed systems with various components coming from 

 different organisms. Perhaps the most dramatic example is the synthesis 

 of TMV protein by the E. co/i-ribosome-supernatant system to which 

 TMV-RNA was added as "messenger' or template-RNA. This result also 

 represents the first reported instance of the in vitro synthesis of a protein 

 of known properties determined by an added template-RNA. 



Although the amino acid sequence of newly synthesized protein is 

 determined by m-RNA, there is an obligate requirement for s-RNA, and 

 it is responsible for the actual translation step. Although m-RNA carries 

 the code, one may ask. does it "know" the code? To answer this ques- 

 tion, a direct experiment was designed and performed in the laboratories 

 of Lipmann and of Benzer. The cysteine-s-RNA complex vvas isolated 

 and, by reaction with Raney nickel, the cysteine w as converted to alanine, 

 giving rise to a new complex carrying the amino acid alanine but the code 

 for cysteine. This complex was then introduced into an in vitro system for 

 protein synthesis. Alanine was incorporated in the place of cysteine, 

 demonstrating that the specificity of the s-RNA and not that of the amino 

 acid itself or of the m-RNA determined the incorporation. 



