Sprecher
Beschreibung
In extant life, DNA stores genetic information while proteins carry out biochemical functions. The central dogma explains how nucleotides are translated into amino acids through tRNA. tRNA is essential in translation as it connects the codon and the cognate amino acids. However, we are still uncertain about the basis on which codon:amino acid assignments were initially made. Two hypotheses have been in debate for 60 years: stereochemical theory and frozen accident theory. Limited experimental work bearing on this question has left answers mainly in the realm of conjecture.
We reported an enzyme-free RNA aminoacylation reaction. In a tRNA acceptor stem-overhang mimic, overhang sequences do not influence the chemoselectivity of aminoacylation significantly, but the terminal three base pairs of the stem do. Computational simulations have also demonstrated that the amino acids interact with the three base pairs, leading to chemoselectivity and stereoselectivity. The findings support early suggestions of a second genetic code in the tRNA acceptor stem.
The terminal three base pairs that selectively transfer a specific amino acid share no similarity to the codon of the same amino acid, which prompted us to wonder if the earliest assignment was determined by the difference in binding strength between the mRNA triplet codon and the tRNA anticodon loop. The binding kinetics of 64 triplets to anticodon loops were measured using isothermal titration calorimetry (ITC) and biolayer interferometry (BLI). Cytosine-ending codons from four-fold degenerate boxes bind tighter than codons from two-fold degenerate boxes. BLI assay also shows that two anticodon loops can bind to a single-stranded RNA if they are in the four-fold degenerate boxes. The dataset suggests that prebiotic amino acids are first assigned to the anticodons which bind codons tighter.
Combining the data from the tRNA acceptor stem and anticodon loop, the two hypotheses mentioned above can be reconciled. The amino acids interact with the acceptor stem to selectively aminoacylate the CCA end, while the tRNA acceptor stem domain engages with the anticodon domain randomly. Therefore, a better explanation for the Origin of Genetic Code is likely a combination of the two hypotheses.
