Two types of aminoacyl-tRNA 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 et al., 1990; Nagel and Doolittle, 1991). Yet, this independence is a puzzle inconsistent with the common origin of transfer RNAs, the coevolutionary theory of the genetic code (Wong, 1975, 1981) and other associated data and ideas. We present here the results of antiparallel 'class I versus class II' comparisons of aaRSs within their signature sequences. The two main HIGH- and KMSKS-containing motifs of class I appeared to be complementary to the class II motifs 2 and 1, respectively. The above sequence complementarity along with the mirror-image between crystal structures of complexes formed by the opposite aaRSs and their cognate tRNAs (Ruff et al., 1991), and the generally mirror ('head-to-tail') mapping of the basic functional sites in the sequences of aaRSs from the opposite two classes led us to conclude that these two synthetases emerged synchronously as complementary strands of the same primordial nucleic acid. This conclusion, combined with the hypothesis of tRNA concerted origin (Rodin et al., 1993a,b), may explain many intriguing features of aaRSs and favor the elucidation of the origin of the genetic code.