Empirical evidence indicates with increasing clearness that ribonucleic acid (RNA) plays a vital role in protein synthesis. It appears rational to assume that the sequence of amino acids characterizing a given protein is uniquely determined by the sequence of nucleotides in the ribonucleic acid molecule.While RNA is a polymer of four different nucleotides, proteins are polymers of 20 different amino acids. Since it is possible to form 20 kinds of triplets from four different elements, this suggests that each of the 20 amino acids is determined by a triplet of nucleotides, taken without regard to order.'The fact that the internucleotide distances are comparable with the distances of amino acid residues in a protein when both are in the extended form makes it plausible that a given amino acid shares its determining nucleotides with neighboring amino acids-. This would necessitate a correlation between neighboring residues in protein sequences, making certain pairs favored and others excluded (1,2). However, studies by Gamow, Rich, and Ycas (2) show that there does not appear to be any such interresidue correlation, and all sequences are apparently possible. Thus it appears more probable that the number of determining nucleotides exceeds by a factor of 3 the number of amino acid residues in the synthesized protein, so that neighboring residues do not share determining nucleotides.It is not possible to test this hypothesis critically against available information on amino acid sequences, since all sequences are permitted. However, the model predicts that the statistical frequency of amino acid residues in proteins will show certain regularities. We have therefore attempted to test whether the distribution of amino acids predicted from the proposed model corresponds with analytically found distributions.If one arranges the amino acids in a protein in order of abundance, repeats this on a collection of proteins,2 and takes the mean values (without regard to identity) of the most abundant, second most abundant, third most abundant, etc., one obtains a curve shown in Figure 1. This will be referred to as a "distribution." In 1011
CHEMISTRY: M. YCAS mercaptalbumin, and bovine serum mercaptalbumin carried out in this laboratory are in complete agreement with the results presented here and may be quantitatively interpreted by the charge-fluctuation concept. These studies will be reported in a separate communication.
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