[U-08] Biogeochemistry of CO world

[U08-06] Steps to the emergence of life viewed from GADV hypothesis

★Invited Papers

*Kenji Ikehara¹ (1.G&L Kyosei Institute)

Keywords:The origin of life, GADV hypothesis

Mystery of the origin of life has not been elucidated still now,in spite of strenuous efforts of many researchers. For the purpose, it is necessary to clarify formation processes of 6 members (gene, tRNA, genetic code, protein, metabolism and cell structure) composing fundamental life system, which is essential for Earth-life to live. However, only random reactions should proceed on the primitive Earth before the emergence of life or in the absence of gene. Therefore, it is necessary for solving the mystery of the emergence of life to make clear how all the 6 members were produced through random reactions. However, it would be impossible to produce (mature) genes and (mature) proteins as used by extant organisms through random polymerization of the respective monomers. On the other hand, I tried to solve one problem about 25 years ago, where entirely new genes, which do not have any meaningful homology with base sequences of any genes used by extant organisms, are created. Consequently, I got an important concept, pseudo-replication or protein 0^th-order structure, suggesting that immature but water-soluble globular [GADV]-proteins could be produced even by random joining of [GADV]-amino acids. One of the protein 0^th-order structures is a specific amino acid composition, in which roughly equal amounts of [GADV]-amino acids are contained [1,2]. That was triggered to propose [GADV]-protein world hypothesis or GADV hypothesis on the origin of life, with which the mystery might be solved [3,4]. The steps to the emergence of life are as follows [5]. (1) [GADV]-amino acids were synthesized with prebiotic means and accumulated on the primitive Earth (chemical evolution). (2) Immature [GADV]-proteins, actually aggregates of [GADV]-peptides, were produced during wetting-drying cycles of [GADV]-amino acids in pools on the primitive Earth (the origin of protein). (3) [GADV]-microspheres were formed through association of [GADV]-proteins (the origin of cell structure). (4) Proto-metabolic system was formed in [GADV]-microspheres using the immature [GADV]-protein catalysts (the origin of metabolism). (5) Primeval anticodon stem-loop (AntiC-SL) tRNA(s) were formed as the smallest but sufficiently stable hairpin loop RNA, which is composed of 17 nucleotides. The first AntiC-SL tRNA was formed during repeated random polymerization of four types of nucleotides, which were produced through the proto-metabolic system, and its degradation (the origin of tRNA). (6) GNC primeval genetic code was established by accidentally freezing between four types of AntiC-SL tRNAs carrying one of GNC codons and four types of [GADV]-amino acids (the origin of genetic code). (7) Single-stranded (ss)-(GNC)_n RNA was formed by random joining of GNC anticodons carried by primeval tRNAs and double-stranded (ds)-(GNC)_n RNA was formed by complementary strand synthesis of the ss-(GNC)_n RNA. Successively, mature (GNC)_n gene was formed through maturation of the immature [GADV]-protein with a weak catalytic activity, which was synthesized by expression of a random (GNC)_n sequence on one strand of ds-(GNC)_n RNA (the origin of gene). (8) Thus, the most primeval fundamental life system was established and the first Earth-life emerged (the origin of life). Those are the steps to the emergence of life viewed from GADV hypothesis on the origin of life. Features of the GADV hypothesis are as follows [5]. (1) All the 6 members composing fundamental life system were formed through the respective random processes. (2) The establishment process of the fundamental life system can be explained by considering that 6 members were formed by piling up one by one in order of protein, cell structure, metabolism, tRNA, genetic code to gene. (3) The formation order of any 6 members cannot be exchanged. (4) The GADV hypothesis is an idea based on “protein/metabolism-early theory”, but not on “gene/selfreplication-early theory”. Please refer the following references for more information.
References 1. Ikehara, K.; Int. J. Mol. Sci., 2009, 10, 1525–1537. 2. Ikehara, K. Orig. Life Evol. Biosph., 2014, 44, 279–281. 3. Ikehara, K. J. Biosci. 2002, 27, 165–186. 4. Ikehara, K.; Chem. Rec., 2005, 107–118. 5. Ikehara, K. Towards Revealing the Origin of life.—Presenting the GADV Hypothesis; Springer Nature, Gewerbestrasse: Cham, Switzerland, 2021.