Degenerate transfer RNAs from brewer's yeast

Experiments utilizing synthetic polyribonucleotides containing randomly ordered bases as templates in a cell-free protein-synthesizing system from E. coli have demonstrated stimulation of amino acid incorporation by the majority of the 64 triplet codons (Nirenberg et al., 1963; Speyer et al., 1963). Since most amino acids are coded by two or more codons, it appears that there is extensive degeneracy in the genetic code. Recent studies on the trinucleotide-stimulated binding of aminoacyl sRNA to ribosomes (Nirenberg and Leder, 1964; Nirenberg et al., 1965; Söll et al., 1965) have also provided evidence for extensive degeneracy. The question arises as to whether this degeneracy is due to a single species of tRNA able to recognize more than one codon, or more than one species of tRNA, each with different anticodons but specific for the same amino acid.