EVOLUTION OF THE AMINOACYL-TRANSFER-RNA SYNTHETASES AND THE ORIGIN OFTHE GENETIC-CODE

The aminoacyl-tRNA synthetases exist as two enzyme families which were apparently generated by divergent evolution from two primordial synth etases. The two classes of enzymes exhibit intriguing familial relatio nships, in that they are distributed nonrandomly within the codon-amin o acid matrix of the genetic code. For example, all XCX codons code fo r amino acids handled by class II synthetases, and all but one of the XUX codons code for amino acids handled by class I synthetases. One in terpretation of these patterns is that the synthetases coevolved with the genetic code. The more likely explanation, however, is that the sy nthetases evolved in the context of an already-established genetic cod e-a code which developed earlier in an RNA world. The rules which gove rned the development of the genetic code, and led to certain patterns in the coding catalog between codons and amino acids, would also have governed the subsequent evolution of the synthetases in the context of a fixed code, leading to patterns in synthetase distribution such as those observed. These rules are (1) conservative evolution of amino ac id and adapter binding sites and (2) minimization of the disruptive ef fects on protein structure caused by codon meaning changes.