This from the site: chemie.unibas.chhabi.htmlOpen ↗ From the logic of the above
arguments it should follow that, indeed, certain amino acids must have been preferentially
esterified with certain RNA sequences, before the existence of any protein enzymes. A tentative clue
to this assumption could be the interesting fact that, contrary to ribosomes, none of the known
aminoacyl tRNA synthetases (ARS, the enzymes that catalyse the aminoacylation of their cognate tRNA)
contain RNA, suggesting that todayís ARSís evolved after the appearance of a primitive translation
apparatus.
TH: This too suggests that the tNA's first attached to the AA's in another way.
This also means that modern tRNA identities, i.e. specific contacts between ARS and cognate tRNAs
responsible for their correct aminoacylation [84], must be different from an early ësecond genetic
codeí which was responsible for the recognition between a nucleotide sequence called ëparacodoní
(or ëprotocodoní or ëchargeoní) and a single cognate amino acid [85]. Remnants of a second genetic
code from early organisms, possibly from the RNA world, have been found in tRNA sequences carrying
some of the simple, presumably primordial amino acids (alanine, glycine, aspartic acid, valine,
leucine, isoleucine tRNA) [86]. A search through 1400 tRNA sequences revealed that, in those
alanine, glycine, etc. tRNAs, the nucleotide positions 3 to 5 (paired with nucleotides 70 to 68)
that are close to the aminoacyl-carrying single-stranded ends (pos. 73 to 76) seem to match
unexpectedly well with their respective anticodon sequence much further away (pos. 34 to 36).
TH: This seems to support my idea of two anticodon loops.
This supports a theory of the origin of the genetic code (only for the first few simple amino acids
[87]) according to which the paracodon must have been in close proximity to the aminoacyl group of
some simple, early adaptor molecule, presumably a short 2í/3í-aminoacylated RNA hairpin [68]. This
proximity might have allowed for a primitive recognition between the helical region of the
paracodon and the aminoacyl residue. Further evolution of these adaptors could have generated the
clover leaf secondary structure of modern tRNA by a genetic duplication of one half of the
primitive hairpin adaptor [88].
Comment? Tom Hendricks