The evolution of the genetic code revisited.

The evolution of the genetic code in terms of the adoption of new codons has previously been related to the relative thermostability of codon-anticodon interactions such that the most stable interactions have been hypothesised to represent the most ancient coding capacity. This derivation is critically dependent on the accuracy of the experimentally determined stability parameters. A new set of parameters recently determined for B-DNA reveals that the codon-anticodon pairs for the codes in non-plant mitochondria on the one hand and prokaryotic and eukaryotic organisms on the other can be unequivocally divided into two classes - the most stable base steps define a common code specified by the first two bases in a codon while the less stable base steps correlate with divergent usage and the adoption of a 3-letter code. This pattern suggests that the fixation of codons for A, G, P, V, S, T, D/E, R may have preceded the divergence of the non-plant mitochondrial line from other organisms. Other variations in the code correlate with the least stable codon-anticodon pairs.