Revisiting the operational RNA code for amino acids: Ensemble attributes and their implications.

It has been suggested that tRNA acceptor stems specify an operational RNA code for amino acids. In the last 20 years several attributes of the putative code have been elucidated for a small number of model organisms. To gain insight about the ensemble attributes of the code, we analyzed 4925 tRNA sequences from 102 bacterial and 21 archaeal species. Here, we used a classification and regression tree (CART) methodology, and we found that the degrees of degeneracy or specificity of the RNA codes in both Archaea and Bacteria differ from those of the genetic code. We found instances of taxon-specific alternative codes, i.e., identical acceptor stem determinants encrypting different amino acids in different species, as well as instances of ambiguity, i.e., identical acceptor stem determinants encrypting two or more amino acids in the same species. When partitioning the data by class of synthetase, the degree of code ambiguity was significantly reduced. In cryptographic terms, a plausible interpretation of this result is that the class distinction in synthetases is an essential part of the decryption rules for resolving the subset of RNA code ambiguities enciphered by identical acceptor stem determinants of tRNAs acylated by enzymes belonging to the two classes. In evolutionary terms, our findings lend support to the notion that in the pre-DNA world, interactions between tRNA acceptor stems and synthetases formed the basis for the distinction between the two classes; hence, ambiguities in the ancient RNA code were pivotal for the fixation of these enzymes in the genomes of ancestral prokaryotes.