Enzyme systems, evolutionary modification

More than 250 amino acids have been isolated from living systems, but only 20 appear to be encoded by the triplet nucleotide sequence in mRNA. Posttranslation modification accounts for the panoply of amino acids found in contemporary proteins. Hence, the limited number of amino acids encoded by the genome must be a consequence of evolutionary selection. Rohlfing and Saunders (1978) have proposed that it is the specific tripartite interaction among activating enzymes, tRNAs, and amino acid which explains why the amino acid code is limited to the 20 commonly found amino acids. 

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