PHYLOGENETIC ANALYSIS OF THE AMINOACYL-TRANSFER-RNA SYNTHETASES

Numerous aminoacyl-tRNA synthetase sequences have been aligned by comp uter and phylogenetic trees constructed from them for the two classes of these enzymes. Branching orders based on a consensus of these trees have been proposed for the two groups. Although the order of appearan ce can be rationalized to fit many different scenarios having to do wi th the genetic code, the invention of a system for translating nucleic acid sequences into polypeptide chains must have pre dated the existe nce of these proteins. In the past, a variety of schemes has been prop osed for matching amino acids and tRNAs. Most of these have invoked di rect recognition of one by the other, whether or not the anticodon was involved. Often ignored is the possibility of a non-protein (presumab ly RNA) matchmaker for bringing the two into conjunction. If such had been the case, then the contemporary aminoacyl-tRNA synthetases could have entered the system gradually, each specific type replacing its ma tchmaking RNA counterpart in turn. A simple displacement scheme of thi s sort accommodates the existence of two different families of these e nzymes, the second being introduced well before the first had undergon e sufficient genetic duplications to specify the full gamut of amino a cids. Such a scheme is also consistent with similar amino acids often, but not always, being the substrates of enzymes with the most similar amino acid sequences.