tRNA evolution from the proto-tRNA minihelix world

Information Gradient among Nucleotide Sequences of Essential RNAs from an Evolutionary Perspective

We hypothesize that the first ancestral "protocell" molecular structures, i.e., the first RNAs and peptides that gradually transformed into real cells once the Earth had cooled sufficiently for organic molecules to appear, have left traces in the RNAs and the genes in present cells. We propose a circular RNA that could have been one of these ancestral structures whose vestigial pentameric subsequences would mark the evolution from this key moment when the protocells began to join with living organisms. In particular, we propose that, in present RNAs (ribosomal or messenger), which play an important role in the metabolism of current cells, we look for traces of the proposed primitive structure in the form of pentamers (or longer fragments) that belong to their nucleotide sequence. The result obtained can be summarized in the existence of a gradient of occurrence of such pentamers, with a high frequency for the most vital functions (protein synthesis, nucleic synthesis, cell respiration, etc.). This gradient is also visible between organisms, from the oldest (Archaea) to the most recent (Eukaryotes) in the evolution of species.

Quantitative and qualitative detection of tRNAs, tRNA halves and tRFs in human cancer samples: Molecular grounds for biomarker development and clinical perspectives

Transfer RNAs (tRNAs) are small non-coding RNAs playing a central role during protein synthesis. Besides translation, growing evidence suggests that in many contexts, precursor or mature tRNAs can also be processed into smaller fragments playing many non-canonical regulatory roles in different biological pathways with oncogenic relevance. Depending on the source, these molecules can be classified as tRNA halves (also known as tiRNAs) or tRNA-derived fragments (tRFs), and furtherly divided into 5'-tRNA and 3'-tRNA halves, or tRF-1, tRF-2, tRF-3, tRF-5, and i-tRF, respectively. Unlike DNA and mRNA, high-throughput sequencing of tRNAs is challenging, because of technical limitations of currently developed sequencing methods. In recent years, different sequencing approaches have been proposed allowing the quantification and identification of an increasing number of tRNA fragments with critical functions in distinct physiological and pathophysiological processes. In the present review, we discussed pros and cons of recent advances in different sequencing methods, also introducing the expanding repertoire of bioinformatics tool and resources specifically focused on tRNA research and discussing current issues in the study of these small RNA molecules. Furthermore, we discussed the potential value of tRNA fragments as diagnostic and prognostic biomarkers for different types of cancers.

A Proposal of the Ur-RNAome

It is widely accepted that the earliest RNA molecules were folded into hairpins or mini-helixes. Herein, we depict the 2D and 3D conformations of those earliest RNA molecules with only RNY triplets, which Eigen proposed as the primeval genetic code. We selected 26 species (13 bacteria and 13 archaea). We found that the free energy of RNY hairpins was consistently lower than that of their corresponding shuffled controls. We found traces of the three ribosomal RNAs (16S, 23S, and 5S), tRNAs, 6S RNA, and the RNA moieties of RNase P and the signal recognition particle. Nevertheless, at this stage of evolution there was no genetic code (as seen in the absence of the peptidyl transferase centre and any vestiges of the anti-Shine-Dalgarno sequence). Interestingly, we detected the anticodons of both glycine (GCC) and threonine (GGU) in the hairpins of proto-tRNA.

The 3 31 Nucleotide Minihelix tRNA Evolution Theorem and the Origin of Life

There are no theorems (proven theories) in the biological sciences. We propose that the 3 31 nt minihelix tRNA evolution theorem be universally accepted as one. The 3 31 nt minihelix theorem completely describes the evolution of type I and type II tRNAs from ordered precursors (RNA repeats and inverted repeats). Despite the diversification of tRNAome sequences, statistical tests overwhelmingly support the theorem. Furthermore, the theorem relates the dominant pathway for the origin of life on Earth, specifically, how tRNAomes and the genetic code may have coevolved. Alternate models for tRNA evolution (i.e., 2 minihelix, convergent and accretion models) are falsified. In the context of the pre-life world, tRNA was a molecule that, via mutation, could modify anticodon sequences and teach itself to code. Based on the tRNA sequence, we relate the clearest history to date of the chemical evolution of life. From analysis of tRNA evolution, ribozyme-mediated RNA ligation was a primary driving force in the evolution of complexity during the pre-life-to-life transition. TRNA formed the core for the evolution of living systems on Earth.

Investigation of tRNA-based relatedness within the Planctomycetes-Verrucomicrobia-Chlamydiae (PVC) superphylum: a comparative analysis

The PVC superphylum is a diverse group of prokaryotes that require stringent growth conditions. RNA is a fascinating molecule to find evolutionary relatedness according to the RNA World Hypothesis. We conducted tRNA gene analysis to find evolutionary relationships in the PVC phyla. The analysis of genomic data (P = 9, V = 4, C = 8) revealed that the number of tRNA genes varied from 28 to 90 in Planctomycetes and Chlamydia, respectively. Verrucomicrobia has whole genomes and the longest scaffold (3 + 1), with tRNA genes ranging from 49 to 53 in whole genomes and 4 in the longest scaffold. Most tRNAs in the E. coli genome clustered with homologs, but approximately 43% clustered with tRNAs encoding different amino acids. Planctomyces, Akkermansia, Isosphaera, and Chlamydia were similar to E. coli tRNAs. In a phylum, tRNAs coding for different amino acids clustered at a range of 8 to 10%. Further analysis of these tRNAs showed sequence similarity with Cyanobacteria, Proteobacteria, Viridiplantae, Ascomycota and Basidiomycota (Eukaryota). This indicates the possibility of horizontal gene transfer or, otherwise, a different origin of tRNA in PVC bacteria. Hence, this work proves its importance for determining evolutionary relatedness and potentially identifying bacteria using tRNA. Thus, the analysis of these tRNAs indicates that primitive RNA may have served as the genetic material of LUCA before being replaced by DNA. A quantitative analysis is required to test these possibilities that relate the evolutionary significance of tRNA to the origin of life.

Self-empowerment of life through RNA networks, cells and viruses

Our understanding of the key players in evolution and of the development of all organisms in all domains of life has been aided by current knowledge about RNA stem-loop groups, their proposed interaction motifs in an early RNA world and their regulative roles in all steps and substeps of nearly all cellular processes, such as replication, transcription, translation, repair, immunity and epigenetic marking. Cooperative evolution was enabled by promiscuous interactions between single-stranded regions in the loops of naturally forming stem-loop structures in RNAs. It was also shown that cooperative RNA stem-loops outcompete selfish ones and provide foundational self-constructive groups (ribosome, editosome, spliceosome, etc.). Self-empowerment from abiotic matter to biological behavior does not just occur at the beginning of biological evolution; it is also essential for all levels of socially interacting RNAs, cells and viruses.

Self-empowerment of life through RNA networks, cells and viruses

Our understanding of the key players in evolution and of the development of all organisms in all domains of life has been aided by current knowledge about RNA stem-loop groups, their proposed interaction motifs in an early RNA world and their regulative roles in all steps and substeps of nearly all cellular processes, such as replication, transcription, translation, repair, immunity and epigenetic marking. Cooperative evolution was enabled by promiscuous interactions between single-stranded regions in the loops of naturally forming stem-loop structures in RNAs. It was also shown that cooperative RNA stem-loops outcompete selfish ones and provide foundational self-constructive groups (ribosome, editosome, spliceosome, etc.). Self-empowerment from abiotic matter to biological behavior does not just occur at the beginning of biological evolution; it is also essential for all levels of socially interacting RNAs, cells and viruses.

Primitive Oligomeric RNAs at the Origins of Life on Earth

There are several theories on the origin of life, which differ by choosing the preponderant factor of emergence: main function (autocatalysis versus replication), initial location (black smokers versus ponds) or first molecule (RNA versus DNA). Among the two last ones, the first assumes that an RNA world involving a collaboration of small RNAs with amino-acids pre-existed and the second that DNA-enzyme-lipid complexes existed first. The debate between these classic theories is not closed and the arguments for one or the other of these theories have recently fueled a debate in which the two have a high degree of likelihood. It therefore seems interesting to propose a third intermediate way, based on the existence of an RNA that may have existed before the latter stages postulated by these theories, and therefore may be the missing link towards a common origin of them. To search for a possible ancestral structure, we propose as candidate a small RNA existing in ring or hairpin form in the early stages of life, which could have acted as a "proto-ribosome" by favoring the synthesis of the first peptides. Remnants of this putative candidate RNA exist in molecules nowadays involved in the ribosomal factory, the concentrations of these relics depending on the seniority of these molecules within the translation process.

Structural Computational Analysis of the Natural History of Class I aminoacyl-tRNA Synthetases Suggests their Role in Establishing the Genetic Code

The evolutionary history of Class I aminoacyl-tRNA synthetases (aaRS) through the reconstruction of ancestral sequences is presented. From structural molecular modeling, we sought to understand its relationship with the acceptor arms and the tRNA anticodon loop, how this relationship was established, and the possible implications in determining the genetic code and the translation system. The results of the molecular docking showed that in 7 out 9 aaRS, the acceptor arm and the anticodon loop bond practically in the same region. Domain accretion process in aaRS and repositioning of interactions between tRNAs and aaRS are illustrated. Based on these results, we propose that the operational code and the anticodon code coexisted, competing for the aaRS catalytic region, while consequently contributed to the stabilization of these proteins.

Menzerath-Altmann's Law of Syntax in RNA Accretion History

RNA evolves by adding substructural parts to growing molecules. Molecular accretion history can be dissected with phylogenetic methods that exploit structural and functional evidence. Here, we explore the statistical behaviors of lengths of double-stranded and single-stranded segments of growing tRNA, 5S rRNA, RNase P RNA, and rRNA molecules. The reconstruction of character state changes along branches of phylogenetic trees of molecules and trees of substructures revealed strong pushes towards an economy of scale. In addition, statistically significant negative correlations and strong associations between the average lengths of helical double-stranded stems and their time of origin (age) were identified with the Pearson's correlation and Spearman's rho methods. The ages of substructures were derived directly from published rooted trees of substructures. A similar negative correlation was detected in unpaired segments of rRNA but not for the other molecules studied. These results suggest a principle of diminishing returns in RNA accretion history. We show this principle follows a tendency of substructural parts to decrease their size when molecular systems enlarge that follows the Menzerath-Altmann's law of language in full generality and without interference from the details of molecular growth.

Evolution of the genetic code

Diverse models have been advanced for the evolution of the genetic code. Here, models for tRNA, aminoacyl-tRNA synthetase (aaRS) and genetic code evolution were combined with an understanding of EF-Tu suppression of tRNA 3rd anticodon position wobbling. The result is a highly detailed scheme that describes the placements of all amino acids in the standard genetic code. The model describes evolution of 6-, 4-, 3-, 2- and 1-codon sectors. Innovation in column 3 of the code is explained. Wobbling and code degeneracy are explained. Separate distribution of serine sectors between columns 2 and 4 of the code is described. We conclude that very little chaos contributed to evolution of the genetic code and that the pattern of evolution of aaRS enzymes describes a history of the evolution of the code. A model is proposed to describe the biological selection for the earliest evolution of the code and for protocell evolution.

Universal Codons with Enrichment from GC to AU Nucleotide Composition Reveal a Chronological Assignment from Early to Late Along with LUCA Formation

The emergence of a primitive genetic code should be considered the most essential event during the origin of life. Almost a complete set of codons (as we know them) should have been established relatively early during the evolution of the last universal common ancestor (LUCA) from which all known organisms descended. Many hypotheses have been proposed to explain the driving forces and chronology of the evolution of the genetic code; however, none is commonly accepted. In the current paper, we explore the features of the genetic code that, in our view, reflect the mechanism and the chronological order of the origin of the genetic code. Our hypothesis postulates that the primordial RNA was mostly GC-rich, and this bias was reflected in the order of amino acid codon assignment. If we arrange the codons and their corresponding amino acids from GC-rich to AU-rich, we find that: 1. The amino acids encoded by GC-rich codons (Ala, Gly, Arg, and Pro) are those that contribute the most to the interactions with RNA (if incorporated into short peptides). 2. This order correlates with the addition of novel functions necessary for the evolution from simple to longer folded peptides. 3. The overlay of aminoacyl-tRNA synthetases (aaRS) to the amino acid order produces a distinctive zonal distribution for class I and class II suggesting an interdependent origin. These correlations could be explained by the active role of the bridge peptide (BP), which we proposed earlier in the evolution of the genetic code.

Why Is AUG the Start Codon?: Theoretical Minimal RNA Rings: Maximizing Coded Information Biases 1st Codon for the Universal Initiation Codon AUG

The rational design of theoretical minimal RNA rings predetermines AUG as the universal start codon. This design maximizes coded amino acid diversity over minimal sequence length, defining in silico theoretical minimal RNA rings, candidate ancestral genes. RNA rings code for 21 amino acids and a stop codon after three consecutive translation rounds, and form a degradation-delaying stem-loop hairpin. Twenty-five RNA rings match these constraints, ten start with the universal initiation codon AUG. No first codon bias exists among remaining RNA rings. RNA ring design predetermines AUG as initiation codon. This is the only explanation yet for AUG as start codon. RNA ring design determines additional RNA ring gene- and tRNA-like properties described previously, because it presumably mimics constraints on life's primordial RNAs.

RNA Rings Strengthen Hairpin Accretion Hypotheses for tRNA Evolution: A Reply to Commentaries by Z.F. Burton and M. Di Giulio

Theoretical minimal RNA ring design ensures coding over the shortest length once for each coding signal (start and stop codons, and each amino acid) and their hairpin configuration. These constraints define 25 RNA rings which surprisingly resemble ancestral tRNA loops, suggesting commonalities between RNA ring design and proto-tRNAs. RNA rings share several other properties with tRNAs, suggesting that primordial RNAs were multifunctional peptide coding sequences and structural RNAs. Two hypotheses, respectively, by M. Di Giulio and Z.F. Burton, derived from cloverleaf structural symmetries suggest that two and three, respectively, stem-loop hairpins agglutinated into tRNAs. Their authors commented that their respective structure-based hypotheses reflect better tRNA structure than RNA rings. Unlike these hypotheses, RNA ring design uses no tRNA-derived information, rendering model predictive power comparisons senseless. Some analyses of RNA ring primary and secondary structures stress RNA ring splicing in their predicted anticodon's midst, indicating ancestrality of split tRNAs, as the two-piece model predicts. Advancement of knowledge, rather than of specific hypotheses, gains foremost by examining independent hypotheses for commonalities, and only secondarily for discordances. RNA rings mimick ancestral biomolecules including tRNAs, and their evolution, and constitute an interesting synthetic system for early prebiotic evolution tests/simulations.

Accretion history of large ribosomal subunits deduced from theoretical minimal RNA rings is congruent with histories derived from phylogenetic and structural methods

Accretions of tRNAs presumably formed the large complex ribosomal RNA structures. Similarities of tRNA secondary structures with rRNA secondary structures increase with the integration order of their cognate amino acid in the genetic code, indicating tRNA evolution towards rRNA-like structures. Here analyses rank secondary structure subelements of three large ribosomal RNAs (Prokaryota: Archaea: Thermus thermophilus; Bacteria: Escherichia coli; Eukaryota: Saccharomyces cerevisiae) in relation to their similarities with secondary structures formed by presumed proto-tRNAs, represented by 25 theoretical minimal RNA rings. These ranks are compared to those derived from two independent methods (ranks provide a relative evolutionary age to the rRNA substructure), (a) cladistic phylogenetic analyses and (b) 3D-crystallography where core subelements are presumed ancient and peripheral ones recent. Comparisons of rRNA secondary structure subelements with RNA ring secondary structures show congruence between ranks deduced by this method and both (a) and (b) (more with (a) than (b)), especially for RNA rings with predicted ancient cognate amino acid. Reconstruction of accretion histories of large rRNAs will gain from adequately integrating information from independent methods. Theoretical minimal RNA rings, sequences deterministically designed in silico according to specific coding constraints, might produce adequate scales for prebiotic and early life molecular evolution.

Debating tRNA Origins

The tRNA is a critical component in all modern translation systems as well as an important intermediate in models of early protein coding systems. In the following works, proponents for each of the major hypotheses for tRNA origin and evolution engage in discussion of the merits for each model.

Genome-wide analysis revealed novel molecular features and evolution of Anti-codons in cyanobacterial tRNAs

Transfer RNAs (tRNAs) play important roles to decode the genetic information contained in mRNA in the process of translation. The tRNA molecules possess conserved nucleotides at specific position to regulate the unique function. However, several nucleotides at different position of the tRNA undergo modification to maintain proper stability and function. The major modifications include the presence of pseudouridine (Ψ) residue instead of uridine and the presence of m⁵-methylation sites. We found that, Ψ13 is conserved in D-stem, whereas Ψ38 & Ψ39 were conserved in the anti-codon loop (AL) and anti-codon arm (ACA), respectively. Furthermore, Ψ55 found to be conserved in the Ψ loop. Although, fourteen possible methylation sites can be found in the tRNA, cyanobacterial tRNAs were found to possess conserved G9, m³C₃₂, C₃₆, A₃₇, m⁵C₃₈ and U₅₄ methylation sites. The presence of multiple conserved methylation sites might be responsible for providing necessary stability to the tRNA. The evolutionary study revealed, tRNA^Met and tRNA^Ile were evolved earlier than other tRNA isotypes and their evolution is date back to at least 4000 million years ago. The presence of novel pseudouridination and m⁵-methylation sites in the cyanobacterial tRNAs are of particular interest for basic biology. Further experimental study can delineate their functional significance in protein translation.

The Uroboros Theory of Life's Origin: 22-Nucleotide Theoretical Minimal RNA Rings Reflect Evolution of Genetic Code and tRNA-rRNA Translation Machineries

Theoretical minimal RNA rings attempt to mimick life's primitive RNAs. At most 25 22-nucleotide-long RNA rings code once for each biotic amino acid, a start and a stop codon and form a stem-loop hairpin, resembling consensus tRNAs. We calculated, for each RNA ring's 22 potential splicing positions, similarities of predicted secondary structures with tRNA vs. rRNA secondary structures. Assuming rRNAs partly derived from tRNA accretions, we predict positive associations between relative secondary structure similarities with rRNAs over tRNAs and genetic code integration orders of RNA ring anticodon cognate amino acids. Analyses consider for each secondary structure all nucleotide triplets as potential anticodon. Anticodons for ancient, chemically inert cognate amino acids are most frequent in the 25 RNA rings. For RNA rings with primordial cognate amino acids according to tRNA-homology-derived anticodons, tRNA-homology and coding sequences coincide, these are separate for predicted cognate amino acids that presumably integrated late the genetic code. RNA ring secondary structure similarity with rRNA over tRNA secondary structures associates best with genetic code integration orders of anticodon cognate amino acids when assuming split anticodons (one and two nucleotides at the spliced RNA ring 5' and 3' extremities, respectively), and at predicted anticodon location in the spliced RNA ring's midst. Results confirm RNA ring homologies with tRNAs and CDs, ancestral status of tRNA half genes split at anticodons, the tRNA-rRNA axis of RNA evolution, and that single theoretical minimal RNA rings potentially produce near-complete proto-tRNA sets. Hence genetic code pre-existence determines 25 short circular gene- and tRNA-like RNAs. Accounting for each potential splicing position, each RNA ring potentially translates most amino acids, realistically mimicks evolution of the tRNA-rRNA translation machinery. These RNA rings 'of creation' remind the uroboros' (snake biting its tail) symbolism for creative regeneration.

Origin of the Genetic Code Is Found at the Transition between a Thioester World of Peptides and the Phosphoester World of Polynucleotides

The early metabolism arising in a Thioester world gave rise to amino acids and their simple peptides. The catalytic activity of these early simple peptides became instrumental in the transition from Thioester World to a Phosphate World. This transition involved the appearances of sugar phosphates, nucleotides, and polynucleotides. The coupling of the amino acids and peptides to nucleotides and polynucleotides is the origin for the genetic code. Many of the key steps in this transition are seen in the catalytic cores of the nucleotidyltransferases, the class II tRNA synthetases (aaRSs) and the CCA adding enzyme. These catalytic cores are dominated by simple beta hairpin structures formed in the Thioester World. The code evolved from a proto-tRNA, a tetramer XCCA interacting with a proto-aminoacyl-tRNA synthetase (aaRS) activating Glycine and Proline. The initial expanded code is found in the acceptor arm of the tRNA, the operational code. It is the coevolution of the tRNA with the aaRSs that is at the heart of the origin and evolution of the genetic code. There is also a close relationship between the accretion models of the evolving tRNA and that of the ribosome.

The Origin of tRNA Deduced from Pseudomonas aeruginosa 5' Anticodon-Stem Sequence : Anticodon-stem loop hypothesis

The riddle of the origin of life is unsolved as yet. One of the best ways to solve the riddle would be to find a vestige of the first life from databases of DNA and/or protein of modern organisms. It would be, especially, important to know the origin of tRNA, because it mediates between genetic information and the amino acid sequence of a protein. Here I attempt to find a vestige of the origin and evolution of tRNA from base sequences of Pseudomonas aeruginosa tRNA gene. It was first perceived that 5' anticodon (AntiC) stem sequences of P. aeruginosa tRNA for translation of G-start codon (GNN) are intimately and mutually related. Then, mutual relations among all of the forty-two 5' AntiC stem sequences of P. aeruginosa tRNA were examined. These relationships imply that P. aeruginosa tRNA originated from four anticodon stem-loops (AntiC-SL) translating GNC codons to the corresponding four amino acids, Gly, Ala, Asp and Val (where N is G, C, A, or T). In contrast to the case of AntiC-stem sequence, a mutual relation map could not be drawn with D-, T- and acceptor-stem sequences of P. aeruginosa tRNA. Thus I conclude that the four AntiC-SLs were the first primeval tRNAs.

A tRNA- and Anticodon-Centric View of the Evolution of Aminoacyl-tRNA Synthetases, tRNAomes, and the Genetic Code

Pathways of standard genetic code evolution remain conserved and apparent, particularly upon analysis of aminoacyl-tRNA synthetase (aaRS) lineages. Despite having incompatible active site folds, class I and class II aaRS are homologs by sequence. Specifically, structural class IA aaRS enzymes derive from class IIA aaRS enzymes by in-frame extension of the protein N-terminus and by an alternate fold nucleated by the N-terminal extension. The divergence of aaRS enzymes in the class I and class II clades was analyzed using the Phyre2 protein fold recognition server. The class I aaRS radiated from the class IA enzymes, and the class II aaRS radiated from the class IIA enzymes. The radiations of aaRS enzymes bolster the coevolution theory for evolution of the amino acids, tRNAomes, the genetic code, and aaRS enzymes and support a tRNA anticodon-centric perspective. We posit that second- and third-position tRNA anticodon sequence preference (C>(U~G)>A) powerfully selected the sectoring pathway for the code. GlyRS-IIA appears to have been the primordial aaRS from which all aaRS enzymes evolved, and glycine appears to have been the primordial amino acid around which the genetic code evolved.

What viruses tell us about evolution and immunity: beyond Darwin?

We describe mechanisms of genetic innovation mediated by viruses and related elements that, during evolution, caused major genetic changes beyond what was anticipated by Charles Darwin. Viruses and related elements introduced genetic information and have shaped the genomes and immune systems of all cellular life forms. None of these mechanisms contradict Darwin's theory of evolution but extend it by means of sequence information that has recently become available. Not only do small increments of genetic information contribute to evolution, but also do major events such as infection by viruses or bacteria, which can supply new genetic information to a host by horizontal gene transfer. Thereby, viruses and virus-like elements act as major drivers of evolution.

Ribosome Structure, Function, and Early Evolution

Ribosomes are among the largest and most dynamic molecular motors. The structure and dynamics of translation initiation and elongation are reviewed. Three ribosome motions have been identified for initiation and translocation. A swivel motion between the head/beak and the body of the 30S subunit was observed. A tilting dynamic of the head/beak versus the body of the 30S subunit was detected using simulations. A reversible ratcheting motion was seen between the 30S and the 50S subunits that slide relative to one another. The 30S⁻50S intersubunit contacts regulate translocation. IF2, EF-Tu, and EF-G are homologous G-protein GTPases that cycle on and off the same site on the ribosome. The ribosome, aminoacyl-tRNA synthetase (aaRS) enzymes, transfer ribonucleic acid (tRNA), and messenger ribonucleic acid (mRNA) form the core of information processing in cells and are coevolved. Surprisingly, class I and class II aaRS enzymes, with distinct and incompatible folds, are homologs. Divergence of class I and class II aaRS enzymes and coevolution of the genetic code are described by analysis of ancient archaeal species.

Hypothesis: Spontaneous Advent of the Prebiotic Translation System via the Accumulation of L-Shaped RNA Elements

The feasibility of self-assembly of a translation system from prebiotic random RNA chains is a question that is central to the ability to conceive life emerging by natural processes. The spontaneous materialization of a translation system would have required the autonomous formation of proto-transfer RNA (tRNA) and proto-ribosome molecules that are indispensable for translating an RNA chain into a polypeptide. Currently, the vestiges of a non-coded proto-ribosome, which could have only catalyzed the formation of a peptide bond between random amino acids, is consensually localized in the region encircling the peptidyl transferase center of the ribosomal large subunit. The work presented here suggests, based on high resolution structures of ribosomes complexed with messenger RNA (mRNA) and tRNAs, that three types of L-shaped RNA building blocks derived from the modern ribosome, alongside with an L-shaped proto-tRNA, each composed of about 70-mer, could have randomly occurred in the prebiotic world and combined to form a simple translation system. The model of the initial coded proto-ribosome, which includes the active sites of both ribosomal subunits, together with a bridging element, incorporates less than 6% of the current prokaryotic rRNA, yet it integrates all of the ribosomal components that are vital for synthesizing the earliest coded polypeptides.

Type-II tRNAs and Evolution of Translation Systems and the Genetic Code

Because tRNA is the core biological intellectual property that was necessary to evolve translation systems, tRNAomes, ribosomes, aminoacyl-tRNA synthetases, and the genetic code, the evolution of tRNA is the core story in evolution of life on earth. We have previously described the evolution of type-I tRNAs. Here, we use the same model to describe the evolution of type-II tRNAs, with expanded V loops. The models are strongly supported by inspection of typical tRNA diagrams, measuring lengths of V loop expansions, and analyzing the homology of V loop sequences to tRNA acceptor stems. Models for tRNA evolution provide a pathway for the inanimate-to-animate transition and for the evolution of translation systems, the genetic code, and cellular life.

Aminoacyl-tRNA synthetase evolution and sectoring of the genetic code

The genetic code sectored via tRNA charging errors, and the code progressed toward closure and universality because of evolution of aminoacyl-tRNA synthetase (aaRS) fidelity and translational fidelity mechanisms. Class I and class II aaRS folds are identified as homologs. From sequence alignments, a structurally conserved Zn-binding domain common to class I and class II aaRS was identified. A model for the class I and class II aaRS alternate folding pathways is posited. Five mechanisms toward code closure are highlighted: 1) aaRS proofreading to remove mischarged amino acids from tRNA; 2) accurate aaRS active site specification of amino acid substrates; 3) aaRS-tRNA anticodon recognition; 4) conformational coupling proofreading of the anticodon-codon interaction; and 5) deamination of tRNA wobble adenine to inosine. In tRNA anticodons there is strong wobble sequence preference that results in a broader spectrum of contacts to synonymous mRNA codon wobble bases. Adenine is excluded from the anticodon wobble position of tRNA unless it is modified to inosine. Uracil is generally preferred to cytosine in the tRNA anticodon wobble position. Because of wobble ambiguity when tRNA reads mRNA, the maximal coding capacity of the three nucleotide code read by tRNA is 31 amino acids + stops.

Rooted tRNAomes and evolution of the genetic code

We advocate for a tRNA- rather than an mRNA-centric model for evolution of the genetic code. The mechanism for evolution of cloverleaf tRNA provides a root sequence for radiation of tRNAs and suggests a simplified understanding of code evolution. To analyze code sectoring, rooted tRNAomes were compared for several archaeal and one bacterial species. Rooting of tRNAome trees reveals conserved structures, indicating how the code was shaped during evolution and suggesting a model for evolution of a LUCA tRNAome tree. We propose the polyglycine hypothesis that the initial product of the genetic code may have been short chain polyglycine to stabilize protocells. In order to describe how anticodons were allotted in evolution, the sectoring-degeneracy hypothesis is proposed. Based on sectoring, a simple stepwise model is developed, in which the code sectors from a 1→4→8→∼16 letter code. At initial stages of code evolution, we posit strong positive selection for wobble base ambiguity, supporting convergence to 4-codon sectors and ∼16 letters. In a later stage, ∼5–6 letters, including stops, were added through innovating at the anticodon wobble position. In archaea and bacteria, tRNA wobble adenine is negatively selected, shrinking the maximum size of the primordial genetic code to 48 anticodons. Because 64 codons are recognized in mRNA, tRNA-mRNA coevolution requires tRNA wobble position ambiguity leading to degeneracy of the code.

tRNA structure and evolution and standardization to the three nucleotide genetic code

Cloverleaf tRNA with a 75 nucleotide (nt) core is posited to have evolved from ligation of three 31 nt minihelices followed by symmetric internal deletions of 9 nt within ligated acceptor stems. Statistical tests strongly support the model. Although the tRNA anticodon loop and T loop are homologs, their U-turns have been treated as distinct motifs. An appropriate comparison, however, shows that intercalation of D loop G19 between T loop bases 4 and 5 causes elevation of T loop base 5 and flipping of T loop bases 6 and 7 out of the 7 nt loop. In the anticodon loop, by contrast, loop bases 3–7 stack tightly to form a stiff connection to mRNA. Furthermore, we identify ancient repeat sequences of 3 (GCG), 5 (UAGCC) and 17 nt (∼CCGGGUUCAAAACCCGG) that comprise 75 out of 75 nts of the tRNA cloverleaf core. To present a sufficiently stiff 3-nt anticodon, a 7-nt anticodon loop was necessary with a U-turn between loop positions 2 and 3. Cloverleaf tRNA, therefore, was a radical evolutionary innovation essential for the 3-nt code. Conservation of GCG and UAGCC repeat sequences indicates that cloverleaf tRNA is at the interface between a strange RNA repeat world and the first evolution of molecules that fold to assume biologic functions. We posit that cloverleaf tRNA was the molecular archetype around which translation systems evolved.