Figure 1.

Dual complementarity of the second bases in consensus tRNAs and their aaRS synthetase class membership. Shown are class I × class II pairs of amino acids with their corresponding complementary anticodons and second bases in the 5′ acceptor strand of cognate consensus tRNA genes. The pairs are arranged according to the four-column organization of the genetic code (Table 1). As a result, nonrandom four (2 × 2) modes of primordial tRNA–aaRS recognition becomes evident. Indeed, although four hydrophobic amino acids of the first column (Val, Ile, Met, and Leu) and three small amino acids of the second column (Ala, Pro, Ser) have the same G2-C71 base pair, these two groups are recognized by the different synthetases. The same is true for their complementary partners with C2-G71 from the third and fourth columns, respectively. An asterisk marks the “nonsense” anticodon TCA that might be originally assigned to Trp as in the present-day mitochondrial code (18). D, mean values of physico-chemical distances (from ref. 19), calculated for each of the four groups and for the two pairs of complementarily linked groups (bold). The ambiguous “S” symbolizes either G or C. Even if the corresponding four pairs (with tRNA^Cys and tRNA^Trp) are included into the statistical test, they should fit the “2 × 2” matrix of the genetic code at least in two cases. In sum, 14 of 17 pairs show the dual complementarity correlated with this “2 × 2” matrix. If in addition the “wobbling” G-U partnership is assumed between the first and third bases of anticodons (see text), this gives a total of 24 of 28 such pairs, which is evidently a nonrandom value.