2 TYPES OF AMINOACYL-TRANSFER-RNA SYNTHETASES COULD BE ORIGINALLY ENCODED BY COMPLEMENTARY STRANDS OF THE SAME NUCLEIC-ACID

The lack of even a marginal similarity between the two aminoacyl-tRNA synthetase (aaRS) classes suggests their independent origins (Eriani e t al., 1990; Nagel and Doolittle, 1991). Yet, this independence is a p uzzle inconsistent with the common origin of transfer RNAs, the coevol utionary theory of the genetic code (Wong, 1975, 1981) and other assoc iated data and ideas. We present here the results of antiparallel 'cla ss I versus class II' comparisons of aaRSs within their signature sequ ences. The two main HIGH- and KMSKS-containing motifs of class I appea red to be complementary to the class II motifs 2 and I, respectively. The above sequence complementarity along with the mirror-image between crystal structures of complexes formed by the opposite aaRSs and thei r cognate tRNAs (Ruff et al., 1991), and the generally mirror ('head-t o-tail') mapping of the basic functional sites in the sequences of aaR Ss from the opposite two classes led us to conclude that these two syn thetases emerged synchronously as complementary strands of the same pr imordial nucleic acid. This conclusion, combined with the hypothesis o f tRNA concerted origin (Rodin et al., 1993a,b), may explain many intr iguing features of aaRSs and favor the elucidation of the origin of th e genetic code.