Genetic Amino Acid Coding 



443 



in the second column of Figure 34-1 ) are 

 consistent with the triplet code letters, base 

 sequence studies in vitro, and 87 of 93 

 known single amino acid substitutions as- 

 sumed to have resulted from single base 

 changes. The disagreements with respect to 

 amino acid replacements are relatively few, 

 and for the most part are probably due to 

 an incomplete knowledge of all triplet code 

 letters and to the inclusion of cases in which 

 two or three base changes occurred succes- 

 sively or simultaneously in replacing one 

 amino acid by another. 



The ability of synthetic polyribotides with- 

 out U to result in amino acid incorporation 

 can also be studied using agents (trichlor- 

 acetic acid, for example) that precipitate 

 proteins otherwise soluble in the in vitro sys- 



tem. When homopolyribotides of A, C, or 

 G and mixed polymers with these bases are 

 synthesized and tested, a large number of 

 new triplet code letters without ITs are 

 found. For example, poly A makes poly- 

 lysine; poly C makes polyproline. Guanine- 

 rich polynucleotides do not work well, prob- 

 ably because of the secondary structure due 

 to guanine-guanine interactions. Based on 

 the nucleotide sequences given to the U- 

 containing codons and using sequences which 

 will not duplicate those given to the codons 

 of other amino acids, the base sequences in 

 these new triplets without U's are assigned 

 and listed in the third column of Figure 

 34-1. 



In studying the incorporation of amino 

 acids into protein in vitro, one must use very 



'Sequence proposed by T. H. Jukes. 



^Sequences given are fitted to those in footnote 1 , or ovoid duplicc 

 tion of a sequence for another amino acid. 



figure 34-1. Tentative in vitro messenger RNA codons for amino acids. (After 

 Wahba, A. J., et al., 1963; see reference at end of chapter. See also M. R. Bernfield and 

 M. W. Nirenberg, Science, 147:479-484, 1965.) 



