Dr. Ikehara describes the origin of tRNA as follows.

“Previous hypotheses regarding tRNA have been too strongly influenced by the structure of modern tRNA. Consequently these influence have made it difficult to solve the origin of tRNA. On the contrary, I got a novel hypothesis, called anticodon stem-loop (AntiC-SL). This would explain the origin of tRNA through base sequence analysis of 5’stem sequences of anticodon stem-loop of P.seudomonas aeruginosa tRNA.”

Let Us Explain Anticodon stem-loop (AntiC-SL) tRNA Hypothesis !

The hypothesis suggests that modern tRNA originated from a small but stable hairpin-loop RNA composed of 17 nucleotides, which were produced through random processes. According to the hypothesis, it can be reasonably assumed that modern tRNAs originated from one nonspecific AntiC-SL, which can carry one of GADV-amino acids selected randomly. Furthermore, it can be hypothesized without any large discrepancy that four specific GADV-AntiC-SL tRNAs were formed from one to four nonspecific AntiC-SL tRNAs.

The evolutionary process from one AntiC-SL tRNA to L-form to modern tRNA could be deduced without any large contradiction.

Source: Towards Revealing the Origin of Life, P.120, Fig. 6.10

Source: Towards Revealing the Origin of Life, P.122, Fig. 6.11

＊A small and stable hairpin-loop RNA could be produced by (1) random joining of mononucleotides shown with four colored small circles, (2) degradation of unstable RNA with a smaller number of base pairs than others, followed by complementary strand synthesis, and (3) error-prone transcription by immature [GADV]-protein, which was produced by random joining of [GADV]-amino acids. After steps (2) and (3) were repeated, a sufficiently stable and the small hairpin-loop RNA could be finally obtained. The hairpin-loop RNA became the first AntiC-SL tRNA. As can be seen in the figure, the first primeval AntiC-SL tRNA gene was created with the first AntiC-SL tRNA in parallel.

Dr. Ikehara made the following important observations regarding the origin of tRNA.

“Why could the first AntiC-SL tRNA carry GNC anticodon in the loop? The reason is not because the AntiC-SL tRNA was formed to carry the anticodon in the loop but because GNC anticodon was fortunately contained in the anticodon loop of AntiC-SL tRNA. That is, anticodon, GNC, was fortunately contained in a small and stable hairpin-loop RNA, which was accidentally generated and selected during repeated RNA synthesis and its degradation cycles. In addition, the generation of the AntiC-SL tRNA could also fortunately lead to formation of the first (GNC)n gene.”

＊(GNC)n mean GAC, GUC, GGC and GCC. N means four bases: adenine, A; uracil or thymine, U/T; guanine, G; or cytosine, C.

In addition, Dr. Ikehara expressed the following view.

“One more important question is why the four nucleobases(A,U,G,C) or four nucleotides are used as components of genetic information carriers. Although I do not know the exact reason at the present time, I can state that at least,- one reason would be because the four AntiC-SL RNAs, with which two sets of triplet GNC/CNG base pairs can be formed, could be fortunately produced in the absence of gene. Consequently, life could emerge on this planet owing to the use of the AntiC-SL tRNAs, which were composed of four nucleotides. The use of the AntiC-SL tRNAs would lead to the use of RNA/DNA as genetic information carriers.”

Reference: Ikehara K(2021) Towards Revealing the Origin of Life. Springer Nature