The genetic code describes translational assignments between codons and amino acids.
tRNAs and aminoacyl-tRNA synthetases (aaRSs) are those molecules by means of which
these assignments are established. Any aaRS recognizes its tRNAs according to some
of their nucleotides called identity elements (IEs). Let a 1Mut-similarity Sim (1Mut)
be the average similarity between such tRNA genes whose codons differ by one point
mutation. We showed that: (1) a global maximum of Sim (1Mut) is reached at the standard
genetic code 27 times for 4 sets of IEs of tRNA genes of eukaryotic species, while
it is so only 5 times for similarities Sim (C&R) between all tRNA genes whose codons
lie in the same column or row of the code. Therefore, point mutations of anticodons
were tested by nature to recruit tRNAs from one isoaccepting group to another, (2)
because plain similarities Sim (all) between tRNA genes of species within any of the
three domains of life are higher than between tRNA genes of species belonging to different
domains, tRNA genes retained information about early evolution of cells, (3) we searched
the order of tRNAs in which they were most probably assigned to their codons and amino
acids. The beginning Ala, (Val), Pro, Ile, Lys, Arg, Trp, Met, Asp, Cys, (Ser) of
our resulting chronology lies under a plateau on a graph of Sim (1Mut,IE)(univ.ancestors)
plotted over this chronology for a set S(IE) of all IEs of tRNA genes, whose universal
ancestors were separately computed for each codon. This plateau has remained preserved
along the whole line of evolution of the code and is consistent with observations
of Ribas de Pouplana and Schimmel [2001. Aminoacy1-tRNA synthetases: potential markers
of genetic code development. Trends Biochem. Sci. 26, 591-598] that specific pairs
of aaRSs-one from each of their two classes-can be docked simultaneously onto the
acceptor stem of tRNA and hence an interaction existed between their ancestors using
a reduced code, (4) sharpness of a local maximum of Sim (1Mut) at the standard code
is almost 100% along our chronologies.