4 PRIMORDIAL MODES OF TRANSFER-RNA-SYNTHETASE RECOGNITION, DETERMINEDBY THE (G,C) OPERATIONAL CODE

In distinction to single-stranded anticodons built of G, C, A, and U b ases, their presumable double-stranded precursors at the first three p ositions of the acceptor stem are composed almost invariably of G-C an d C-G base pairs. Thus, the ''second'' operational RNA code responsibl e for correct aminoacylation seems to be a (G,C) code preceding the cl assic genetic code, Although historically rooted, the two codes were d estined to diverge quite early. However, closer inspection revealed th at two complementary catalytic domains of class I and class II aminoac yl-tRNA synthetases (aaRSs) multiplied by two, also complementary, G2- C71 and C2-G71 targets in tRNA accepters, yield four (2 x 2) different modes of recognition, It appears therefore that the core four-column organization of the genetic code, associated with the most conservativ e central base of anticodons and codons, was in essence predetermined by these four recognition modes of the (G,C) operational code, The gen eral conclusion follows that the genetic code per se looks like a ''fr ozen accident'' but only beyond the ''2 x 2 = 4'' scope, The four prim ordial modes of tRNA-aaRS recognition are amenable to direct experimen tal verification.