A Physical Basis for Degeneracy in the Amino Acid Code

Abstract

If two or more nucleotide sequences specify the same amino acid, the code is said to be degenerate for that amino acid. Evidence that such degeneracy exists in E. coli comes from experiments on the stimulation of ribosomal incorporation of amino acids by synthetic polyribonucleotides. For instance, Martin et al. [1] and Speyer et al. [2] found that either poly UC or poly UG stimulates incorporation of leucine, so that at least two different combinations code for the same amino acid. Since the specificity of coding seems to reside in sRNA molecules functioning as adaptors [3,4], such degeneracy might be due to the existence of two sRNA acceptors for leucine, each having different coding properties. Two peaks of leucine-acceptor activity were demonstrated in yeast sRNA by Doctor et al. [5] using countercurrent distribution. Berg and Lagerkvist [6] have shown that E. coli sRNA also contains two leucine acceptors with different acceptor-terminal nucleotide sequences. In this paper, two leucine acceptors in E. coli, separated by countercurrent distribution, are shown to have different coding properties. One responds preferentially to poly UC, the other to poly UG.