Relationships among isoacceptor tRNAs seems to support the coevolution theory of the origin of the genetic code

A new method for looking at relationships between nucleotide sequences has been used to analyze divergence both within and between the families of iso accepting tRNA sets. A dendrogram of the relationships between 21 tRNA sets with different amino acid specificities is presented as the result of the analysis. Methionine initiator tRNAs are included as a separate set. The de ndrogram has been interpreted with respect to the final stage of the evolut ionary pathway with the development of highly specific tRNAs from ambiguous molecular adaptors. The location of the sets on the dendrogram was therefo re analyzed in relation to hypotheses on the origin of the genetic code: th e coevolution theory, the physicochemical hypothesis, and the hypothesis of ambiguity reduction of the genetic code. Pairs of 16 sets of isoacceptor t RNAs, whose amino acids are in biosynthetic relationships, occupied contigu ous positions on the dendrogram, thus supporting the coevolution theory of the genetic code.