Transfer RNAs with complementary anticodons: could they reflect early evolution of discriminative genetic code adaptors?
In accordance with the hypercycle theory of M. Eigen and P. Schuster [(1979) Hypercycle: A Principle of Natural Self-Organization (Springer, New York)], the ancestors of modern tRNAs appear to have emerged via the shortest possible way, both complementary strands of a short symmetrical double helix serving as pre-tRNAs with complementary anticodons. This conclusion is based upon results of comparative sequence analysis of the 17-base-long anticodon loop and stem of tRNAs totaling 896 and especially of 22 pairs of consensus tRNAs with complementary or quasi-complementary anticodons. With regard to the anticodon loop and stem of pairs of consensus tRNAs, complementary distances were considerably less than direct distances--i.e., antiparallel pairing invariably yielded fewer mismatches than direct pairing. Furthermore, the smallest complementary distance was detected when two antiparallel sequences formed irregular G-U bonds in their anticodon triplets. The above implies that pre-tRNAs in peribiotic times were long hairpin structures having 73 bases or more, the middle base of an anticodon being the center of symmetry. Accordingly, each pair of pre-tRNAs with complementary anticodons should have been almost identical with each other except for their three central bases. The above situation appears to have dictated the early establishment of direct links between anticodons and the type of amino acids with which tRNAs are to be charged. This direct link is still maintained between modern aminoacyl-tRNA synthetases and anticodons. Replication of the double helices concertedly generated new codons for the same pair of amino acids. Thus, occurrence of synonymous as well as certain "palindromic" features of the genetic code table might have been determined by this mechanism.