The cell biology and genome of Stentor pyriformis, a giant cell that embeds symbiotic algae in a microtubule meshwork

Endosymbiotic events in which an endosymbiont is retained within a cell that remains capable of phagocytosis, a situation known as mixotrophy, provide potentially important clues about the eukaryotic evolution. Here we describe the cell biology and genome of the giant mixotrophic ciliate Stentor pyriformis. We show that S. pyriformis contains Chlorella variabilis as an endosymbiont that retains the ability to live outside the host. Within the host, the Chlorella cells surrounded by microtubule "baskets" near the cell surface. Photosynthetic efficiency of the Chlorella is reduced inside the Stentor cell compared with outside the host, due to increased nonphotochemical quenching. S. pyriformis displays positive phototaxis via directed swimming that requires the presence of the Chlorella, implying a potential flow of information from the symbiont to direct the orientation and swimming of the host cell. We sequenced the S. pyriformis genome and found that it employs a standard genetic code, similar to other Stentor species but different from most other ciliates. We propose that S. pyriformis will serve as a useful model system for studying endosymbiosis, with unique advantages in terms of size and regenerative ability as well as distinct cellular and genomic features compared with other mixotrophic ciliate models.

Origin of the RNA World in Cold Hadean Geothermal Fields Enriched in Zinc and Potassium: Abiogenesis as a Positive Fallout from the Moon-Forming Impact?

The ubiquitous, evolutionarily oldest RNAs and proteins exclusively use rather rare zinc as transition metal cofactor and potassium as alkali metal cofactor, which implies their abundance in the habitats of the first organisms. Intriguingly, lunar rocks contain a hundred times less zinc and ten times less potassium than the Earth's crust; the Moon is also depleted in other moderately volatile elements (MVEs). Current theories of impact formation of the Moon attribute this depletion to the MVEs still being in a gaseous state when the hot post-impact disk contracted and separated from the nascent Moon. The MVEs then fell out onto juvenile Earth's protocrust; zinc, as the most volatile metal, precipitated last, just after potassium. According to our calculations, the top layer of the protocrust must have contained up to 1019 kg of metallic zinc, a powerful reductant. The venting of hot geothermal fluids through this MVE-fallout layer, rich in metallic zinc and radioactive potassium, both capable of reducing carbon dioxide and dinitrogen, must have yielded a plethora of organic molecules released with the geothermal vapor. In the pools of vapor condensate, the RNA-like molecules may have emerged through a pre-Darwinian selection for low-volatile, associative, mineral-affine, radiation-resistant, nitrogen-rich, and polymerizable molecules.

Chemical Evolution of Life on Earth

Background/Objectives: The origin of genes and genetics is the story of the coevolution of translation systems and the genetic code. Remarkably, the history of the origin of life on Earth was inscribed and preserved in the sequences of tRNAs. Methods: Sequence logos demonstrate the patterning of pre-life tRNA sequences. Results: The pre-life type I and type II tRNA sequences are known to the last nucleotide with only a few ambiguities. Type I and type II tRNAs evolved from ligation of three 31 nt minihelices of highly patterned and known sequence followed by closely related 9 nt internal deletion(s) within ligated acceptor stems. The D loop 17 nt core was a truncated UAGCC repeat. The anticodon and T 17 nt stem-loop-stems are homologous sequences with 5 nt stems and 7 nt U-turn loops that were selected in pre-life to resist ribozyme nucleases and to present a 3 nt anticodon with a single wobble position. The 7 nt T loop in tRNA was selected to interact with the D loop at the "elbow". The 5'-acceptor stem was based on a 7 nt truncated GCG repeat. The 3'-acceptor stem was based on a complementary 7 nt CGC repeat. In pre-life, ACCA-Gly was a primitive adapter molecule ligated to many RNAs, including tRNAs, to synthesize polyglycine. Conclusions: Analysis of sequence logos of tRNAs from an ancient Archaeon substantiates how the pre-life to life transition occurred on Earth. Polyglycine is posited to have aggregated complex molecular assemblies, including minihelices, tRNAs, cooperating molecules, and protocells, leading to the first life on Earth.

Suppression of amber stop codons impairs pathogenicity in Salmonella

Translation terminates at UAG (amber), UGA (opal), and UAA (ochre) stop codons. In nature, readthrough of stop codons can be substantially enhanced by suppressor tRNAs. Stop-codon suppression also provides powerful tools in synthetic biology and disease treatment. How stop-codon suppression affects bacterial pathogenesis is poorly understood. Here, we show that suppression of UAG codons, but not UGA or UAA codons, attenuates expression of Salmonella Pathogenicity Island 1 (SPI-1) genes, which are required for virulence. Consistently, amber suppression abolishes Salmonella infection of macrophages. Systematic genetic and biochemical analyses further show that amber suppression decreases the activity, but not the level, of the master SPI-1 regulator HilD. Our work thus demonstrates an unexpected selectivity of stop codons in regulating Salmonella virulence.

WITHDRAWN: Structural Enzymology, Phylogenetics, Differentiation, and Symbolic Reflexivity at the Dawn of Biology

This manuscript was posted without the final consent of all authors. The authors have therefore withdrawn it. The authors do not wish this work to be cited as reference for the project. If you have any questions, please contact the corresponding author, carter@med.unc.edu .

The Origin(s) of LUCA: Computer Simulation of a New Theory

Carl Woese's thesis of cellular evolution emphasized that the last universal common/cellular ancestor (LUCA) must have evolved by drawing from "global inventions". Yet, existing theories regarding the origin(s) of LUCA have mostly centered upon scenarios that LUCA had evolved mostly independently. In an earlier paper, we advanced a new theory regarding the origin(s) of LUCA that extends Woese's original insights. Our theory centers upon the possibility that different vesicles and protocells can merge with and acquire each other as a form of variation, selection, and retention, driven by wet-and-dry cycles and other similar cyclical processes. In this paper, we use computer simulation to show that under a variety of simulated conditions, LUCA can indeed be produced by our proposed processes. We hope that our study can stimulate laboratory testing of some key hypotheses that vesicles' absorption, acquisition, and merger has indeed been a central force in driving the evolution of LUCA.

The Expanding Universe of Extensions and Tails: Ribosomal Proteins and Histones in RNA and DNA Complex Signaling and Dynamics

This short review bridges two biological fields: ribosomes and nucleosomes-two nucleoprotein assemblies that, along with many viruses, share proteins featuring long filamentous segments at their N- or C-termini. A central hypothesis is that these extensions and tails perform analogous functions in both systems. The evolution of these structures appears closely tied to the emergence of regulatory networks and signaling pathways, facilitating increasingly complex roles for ribosomes and nucleosome alike. This review begins by summarizing the structures and functions of ribosomes and nucleosomes, followed by a detailed comparison highlighting their similarities and differences, particularly in light of recent findings on the roles of ribosomal proteins in signaling and ribosome dynamics. The analysis seeks to uncover whether these systems operate based on shared principles and mechanisms. The nucleosome-ribosome analogy may offer valuable insights into unresolved questions in both fields. For instance, new structural insights from ribosomes might shed light on potential motifs formed by histone tails. From an evolutionary perspective, this study revisits the origins of signaling and regulation in ancient nucleoprotein assemblies, suggesting that tails and extensions may represent remnants of the earliest network systems governing signaling and dynamic control.

Intellectual frameworks to understand complex biochemical systems at the origin of life

Understanding the emergence of complex biochemical systems, such as protein translation, is a great challenge. Although synthetic approaches can provide insight into the potential early stages of life, they do not address the equally important question of why the complex systems of life would have evolved. In particular, the intricacies of the mechanisms governing the transfer of information from nucleic acid sequences to proteins make it difficult to imagine how coded protein synthesis could have emerged from a prebiotic soup. Here we discuss the use of intellectual frameworks in studying the emergence of life. We discuss how one such framework, namely the RNA world theory, has spurred research, and provide an overview of its limitations. We suggest that the emergence of coded protein synthesis could be broken into experimentally tractable problems by treating it as a molecular bricolage-a complex system integrating many different parts, each of which originally evolved for uses unrelated to its modern function-to promote a concrete understanding of its origin.

Flipons and the origin of the genetic code

This paper is focused on the origins of the contemporary genetic code. A novel explanation is proposed for how the mapping of nucleotides in DNA to amino acids in proteins arose that derives from repeat nucleotide sequences able to form alternative nucleic acid structures (ANS), such as the unusual left-handed Z-DNA, triplex, G-quadruplex and I-motif conformations. The scheme identifies sequence-specific contacts that map ANS repeats to dipeptide polymers (DPS). The stereochemistry required naturally evolves into a non-overlapping, triplet code for mapping nucleotides to amino acids. The ANS/DPS complexes form a simple, genetically transmitted, self-templating, autonomously replicating collection of 'tinkers' for Nature to evolve. Tinkers have agency and promote their own synthesis by forming catalytic scaffolds with metals, further enhancing their capabilities. Initial support for the model is provided by computational models built with AlphaFold3. The predictions made are properly falsifiable with the currently available methodology.

The genetic code and its p-adic ultrametric modeling

The genetic code plays a central role in all living organisms and its modeling is important for describing and understanding involved the coding rules. There are many approaches to modeling various aspects of the genetic code. One of the simple and successful mathematical tools for modeling the similarity both between codons and between amino acids, is the ultrametrics and especially the p-adic distance. This article contains an overview of ultrametric (p-adic) modeling of genetic information, and its translation to proteins using the genetic code.

A genomic database furnishes minimal functional glycyl-tRNA synthetases homologous to other, designed class II urzymes

The hypothesis that conserved core catalytic sites could represent ancestral aminoacyl-tRNA synthetases (AARS) drove the design of functional TrpRS, LeuRS, and HisRS 'urzymes'. We describe here new urzymes detected in the genomic record of the arctic fox, Vulpes lagopus. They are homologous to the α-subunit of bacterial heterotetrameric Class II glycyl-tRNA synthetase (GlyRS-B) enzymes. AlphaFold2 predicted that the N-terminal 81 amino acids would adopt a 3D structure nearly identical to our designed HisRS urzyme (HisCA1). We expressed and purified that N-terminal segment and the spliced open reading frame GlyCA1-2. Both exhibit robust single-turnover burst sizes and ATP consumption rates higher than those previously published for HisCA urzymes and comparable to those for LeuAC and TrpAC. GlyCA is more than twice as active in glycine activation by adenosine triphosphate as the full-length GlyRS-B α2 dimer. Michaelis-Menten rate constants for all three substrates reveal significant coupling between Exon2 and both substrates. GlyCA activation favors Class II amino acids that complement those favored by HisCA and LeuAC. Structural features help explain these results. These minimalist GlyRS catalysts are thus homologous to previously described urzymes. Their properties reinforce the notion that urzymes may have the requisite catalytic activities to implement a reduced, ancestral genetic coding alphabet.

Rethinking life and predicting its origin

The definition, origin and recreation of life remain elusive. As others have suggested, only once we put life into reductionist physical terms will we be able to solve those questions. To that end, this work proposes the phenomenon of life to be the product of two dissipative mechanisms. From them, one characterises extant biological life and deduces a testable scenario for its origin. The proposed theory of life allows its replication, reinterprets ecological evolution and creates new constraints on the search for life.

Protein Fold Usages in Ribosomes: Another Glance to the Past

The analysis of protein fold usage, similar to codon usage, offers profound insights into the evolution of biological systems and the origins of modern proteomes. While previous studies have examined fold distribution in modern genomes, our study focuses on the comparative distribution and usage of protein folds in ribosomes across bacteria, archaea, and eukaryotes. We identify the prevalence of certain 'super-ribosome folds,' such as the OB fold in bacteria and the SH3 domain in archaea and eukaryotes. The observed protein fold distribution in the ribosomes announces the future power-law distribution where only a few folds are highly prevalent, and most are rare. Additionally, we highlight the presence of three copies of proto-Rossmann folds in ribosomes across all kingdoms, showing its ancient and fundamental role in ribosomal structure and function. Our study also explores early mechanisms of molecular convergence, where different protein folds bind equivalent ribosomal RNA structures in ribosomes across different kingdoms. This comparative analysis enhances our understanding of ribosomal evolution, particularly the distinct evolutionary paths of the large and small subunits, and underscores the complex interplay between RNA and protein components in the transition from the RNA world to modern cellular life. Transcending the concept of folds also makes it possible to group a large number of ribosomal proteins into five categories of urfolds or metafolds, which could attest to their ancestral character and common origins. This work also demonstrates that the gradual acquisition of extensions by simple but ordered folds constitutes an inexorable evolutionary mechanism. This observation supports the idea that simple but structured ribosomal proteins preceded the development of their disordered extensions.

Impact of the chemical modification of tRNAs anticodon loop on the variability and evolution of codon usage in proteobacteria

Despite the highly conserved nature of the genetic code, the frequency of usage of each codon can vary significantly. The evolution of codon usage is shaped by two main evolutionary forces: mutational bias and selection pressures. These pressures can be driven by environmental factors, but also by the need for efficient translation, which depends heavily on the concentration of transfer RNAs (tRNAs) within the cell. The data presented here supports the proposal that tRNA modifications play a key role in shaping the overall preference of codon usage in proteobacteria. Interestingly, some codons, such as CGA and AGG (encoding arginine), exhibit a surprisingly low level of variation in their frequency of usage, even across genomes with differing GC content. These findings suggest that the evolution of GC content in proteobacterial genomes might be primarily driven by changes in the usage of a specific subset of codons, whose usage is itself influenced by tRNA modifications.

Robust genetic codes enhance protein evolvability

The standard genetic code defines the rules of translation for nearly every life form on Earth. It also determines the amino acid changes accessible via single-nucleotide mutations, thus influencing protein evolvability-the ability of mutation to bring forth adaptive variation in protein function. One of the most striking features of the standard genetic code is its robustness to mutation, yet it remains an open question whether such robustness facilitates or frustrates protein evolvability. To answer this question, we use data from massively parallel sequence-to-function assays to construct and analyze 6 empirical adaptive landscapes under hundreds of thousands of rewired genetic codes, including those of codon compression schemes relevant to protein engineering and synthetic biology. We find that robust genetic codes tend to enhance protein evolvability by rendering smooth adaptive landscapes with few peaks, which are readily accessible from throughout sequence space. However, the standard genetic code is rarely exceptional in this regard, because many alternative codes render smoother landscapes than the standard code. By constructing low-dimensional visualizations of these landscapes, which each comprise more than 16 million mRNA sequences, we show that such alternative codes radically alter the topological features of the network of high-fitness genotypes. Whereas the genetic codes that optimize evolvability depend to some extent on the detailed relationship between amino acid sequence and protein function, we also uncover general design principles for engineering nonstandard genetic codes for enhanced and diminished evolvability, which may facilitate directed protein evolution experiments and the bio-containment of synthetic organisms, respectively.

Theories of the origin of the genetic code: Strong corroboration for the coevolution theory

I analyzed all the theories and models of the origin of the genetic code, and over the years, I have considered the main suggestions that could explain this origin. The conclusion of this analysis is that the coevolution theory of the origin of the genetic code is the theory that best captures the majority of observations concerning the organization of the genetic code. In other words, the biosynthetic relationships between amino acids would have heavily influenced the origin of the organization of the genetic code, as supported by the coevolution theory. Instead, the presence in the genetic code of physicochemical properties of amino acids, which have also been linked to the physicochemical properties of anticodons or codons or bases by stereochemical and physicochemical theories, would simply be the result of natural selection. More explicitly, I maintain that these correlations between codons, anticodons or bases and amino acids are in fact the result not of a real correlation between amino acids and codons, for example, but are only the effect of the intervention of natural selection. Specifically, in the genetic code table we expect, for example, that the most similar codons - that is, those that differ by only one base - will have more similar physicochemical properties. Therefore, the 64 codons of the genetic code table ordered in a certain way would also represent an ordering of some of their physicochemical properties. Now, a study aimed at clarifying which physicochemical property of amino acids has influenced the allocation of amino acids in the genetic code has established that the partition energy of amino acids has played a role decisive in this. Indeed, under some conditions, the genetic code was found to be approximately 98% optimized on its columns. In this same work, it was shown that this was most likely the result of the action of natural selection. If natural selection had truly allocated the amino acids in the genetic code in such a way that similar amino acids also have similar codons - this, not through a mechanism of physicochemical interaction between, for example, codons and amino acids - then it might turn out that even different physicochemical properties of codons (or anticodons or bases) show some correlation with the physicochemical properties of amino acids, simply because the partition energy of amino acids is correlated with other physicochemical properties of amino acids. It is very likely that this would inevitably lead to a correlation between codons (or anticodons or bases) and amino acids. In other words, since the codons (anticodons or bases) are ordered in the genetic code, that is to say, some of their physicochemical properties should also be ordered by a similar order, and given that the amino acids would also appear to have been ordered in the genetic code by selection natural, then it should inevitably turn out that there is a correlation between, for example, the hydrophobicity of anticodons and that of amino acids. Instead, the intervention of natural selection in organizing the genetic code would appear to be highly compatible with the main mechanism of structuring the genetic code as supported by the coevolution theory. This would make the coevolution theory the only plausible explanation for the origin of the genetic code.

Fitness difference between two synonymous mutations of Phytophthora infestans ATP6 gene

BACKGROUND

Sequence variation produced by mutation provides the ultimate source of natural selection for species adaptation. Unlike nonsynonymous mutation, synonymous mutations are generally considered to be selectively neutral but accumulating evidence suggests they also contribute to species adaptation by regulating the flow of genetic information and the development of functional traits. In this study, we analysed sequence characteristics of ATP6, a housekeeping gene from 139 Phytophthora infestans isolates, and compared the fitness components including metabolic rate, temperature sensitivity, aggressiveness, and fungicide tolerance among synonymous mutations.

RESULTS

We found that the housekeeping gene exhibited low genetic variation and was represented by two major synonymous mutants at similar frequency (0.496 and 0.468, respectively). The two synonymous mutants were generated by a single nucleotide substitution but differed significantly in fitness as well as temperature-mediated spatial distribution and expression. The synonymous mutant ending in AT was more common in cold regions and was more expressed at lower experimental temperature than the synonymous mutant ending in GC and vice versa.

CONCLUSION

Our results are consistent with the argument that synonymous mutations can modulate the adaptive evolution of species including pathogens and have important implications for sustainable disease management, especially under climate change.

The tri-flow adaptiveness of codes in major evolutionary transitions

Life codes increase in both number and variety with biological complexity. Although our knowledge of codes is constantly expanding, the evolutionary progression of organic, neural, and cultural codes in response to selection pressure remains poorly understood. Greater clarification of the selective mechanisms is achieved by investigating how major evolutionary transitions reduce spatiotemporal and energetic constraints on transmitting heritable code to offspring. Evolution toward less constrained flows is integral to enduring flow architecture everywhere, in both engineered and natural flow systems. Beginning approximately 4 billion years ago, the most basic level for transmitting genetic material to offspring was initiated by protocell division. Evidence from ribosomes suggests that protocells transmitted comma-free or circular codes, preceding the evolution of standard genetic code. This rudimentary information flow within protocells is likely to have first emerged within the geo-energetic and geospatial constraints of hydrothermal vents. A broad-gauged hypothesis is that major evolutionary transitions overcame such constraints with tri-flow adaptations. The interconnected triple flows incorporated energy-converting, spatiotemporal, and code-based informational dynamics. Such tri-flow adaptations stacked sequence splicing code on top of protein-DNA recognition code in eukaryotes, prefiguring the transition to sexual reproduction. Sex overcame the spatiotemporal-energetic constraints of binary fission with further code stacking. Examples are tubulin code and transcription initiation code in vertebrates. In a later evolutionary transition, language reduced metabolic-spatiotemporal constraints on inheritance by stacking phonetic, phonological, and orthographic codes. In organisms that reproduce sexually, each major evolutionary transition is shown to be a tri-flow adaptation that adds new levels of code-based informational exchange. Evolving biological complexity is also shown to increase the nongenetic transmissibility of code.

The time of appearance of the genetic code

I support the hypothesis that the origin of the genetic code occurred simultaneously with the evolution of cellularity. That is to say, I favour the hypothesis that the origin of the genetic code is a very, very late event in the history of life on Earth. I corroborate this hypothesis with observations favouring the progenote's stage for the Last Universal Common Ancestor (LUCA), for the ancestor of bacteria and that of archaea. Indeed, these progenotic stages would imply that - at that time - the origin of the genetic code was still ongoing simply because this origin would fall within the very definition of progenote. Therefore, if the evolution of cellularity had truly been coeval with the origin of the genetic code - at least in its terminal part - then this would favour theories such as the coevolution theory of the origin of the genetic code because this theory would postulate that this origin must have occurred in extremely complex protocellular conditions and not concerning stereochemical or physicochemical interactions having to do with other stages of the origin of life. In this sense, the coevolution theory would be corroborated while the stereochemical and physicochemical theories would be damaged. Therefore, the origin of the genetic code would be linked to the origin of the cell and not to the origin of life as sometimes asserted. Therefore, I will discuss the late hypothesis of the origin of the genetic code in the context of the theories proposed to explain this origin and more generally of its implications for the early evolution of life.

RNA-Binding Peptides Inspired by the RNA Recognition Motif

β-Hairpin peptides with RNA-binding sequences mimicking the central two β-strands of the RNA recognition motif (RRM) protein domain have been observed to bind in a 2:1 fashion to a series of RNA homooligonucleotides in aqueous solution (PBS buffer, pH 7.40) with binding energies (-27 to -35 kJ mol-1) similar to those of full-size protein RRMs. The peptides display mild selectivities with respect to the binding of the different homooligomers. Binding studies in 500 mM magnesium chloride suggest that the complex formation is not predominantly driven by Coulombic attraction. These peptides represent a starting point for further studies of non-Coulombic binding of RNA by peptides and proteins, which is important in the context of contemporary biology, potential therapeutic applications, and prebiotic peptide-RNA interactions.

Opinion: The Key Steps in the Origin of Life to the Formation of the Eukaryotic Cell

The path from life's origin to the emergence of the eukaryotic cell was long and complex, and as such it is rarely treated in one publication. Here, we offer a sketch of this path, recognizing that there are points of disagreement and that many transitions are still shrouded in mystery. We assume life developed within microchambers of an alkaline hydrothermal vent system. Initial simple reactions were built into more sophisticated reflexively autocatalytic food-generated networks (RAFs), laying the foundation for life's anastomosing metabolism, and eventually for the origin of RNA, which functioned as a genetic repository and as a catalyst (ribozymes). Eventually, protein synthesis developed, leading to life's biology becoming dominated by enzymes and not ribozymes. Subsequent enzymatic innovation included ATP synthase, which generates ATP, fueled by the proton gradient between the alkaline vent flux and the acidic sea. This gradient was later internalized via the evolution of the electron transport chain, a preadaptation for the subsequent emergence of the vent creatures from their microchamber cradles. Differences between bacteria and archaea suggests cellularization evolved at least twice. Later, the bacterial development of oxidative phosphorylation and the archaeal development of proteins to stabilize its DNA laid the foundation for the merger that led to the formation of eukaryotic cells.

Base Pairing Promoted the Self-Organization of Genetic Coding, Catalysis, and Free-Energy Transduction

How Nature discovered genetic coding is a largely ignored question, yet the answer is key to explaining the transition from biochemical building blocks to life. Other, related puzzles also fall inside the aegis enclosing the codes themselves. The peptide bond is unstable with respect to hydrolysis. So, it requires some form of chemical free energy to drive it. Amino acid activation and acyl transfer are also slow and must be catalyzed. All living things must thus also convert free energy and synchronize cellular chemistry. Most importantly, functional proteins occupy only small, isolated regions of sequence space. Nature evolved heritable symbolic data processing to seek out and use those sequences. That system has three parts: a memory of how amino acids behave in solution and inside proteins, a set of code keys to access that memory, and a scoring function. The code keys themselves are the genes for cognate pairs of tRNA and aminoacyl-tRNA synthetases, AARSs. The scoring function is the enzymatic specificity constant, kcat/kM, which measures both catalysis and specificity. The work described here deepens the evidence for and understanding of an unexpected consequence of ancestral bidirectional coding. Secondary structures occur in approximately the same places within antiparallel alignments of their gene products. However, the polar amino acids that define the molecular surface of one are reflected into core-defining non-polar side chains on the other. Proteins translated from base-paired coding strands fold up inside out. Bidirectional genes thus project an inverted structural duality into the proteome. I review how experimental data root the scoring functions responsible for the origins of coding and catalyzed activation of unfavorable chemical reactions in that duality.

Evolution at the Origins of Life?

The role of evolutionary theory at the origin of life is an extensively debated topic. The origin and early development of life is usually separated into a prebiotic phase and a protocellular phase, ultimately leading to the Last Universal Common Ancestor. Most likely, the Last Universal Common Ancestor was subject to Darwinian evolution, but the question remains to what extent Darwinian evolution applies to the prebiotic and protocellular phases. In this review, we reflect on the current status of evolutionary theory in origins of life research by bringing together philosophy of science, evolutionary biology, and empirical research in the origins field. We explore the various ways in which evolutionary theory has been extended beyond biology; we look at how these extensions apply to the prebiotic development of (proto)metabolism; and we investigate how the terminology from evolutionary theory is currently being employed in state-of-the-art origins of life research. In doing so, we identify some of the current obstacles to an evolutionary account of the origins of life, as well as open up new avenues of research.

Evolution of the RNA world: From signals to codes

The accumulated material in evolutionary biology, greatly enhanced by the achievements of modern synthetic biology, allows us to envision certain key hypothetical stages of prebiotic (chemical) evolution. This is often understood as the further evolution in the RNA World towards the RNA-protein World. It is a path towards the emergence of translation and the genetic code (I), signaling pathways with signaling molecules (II), and the appearance of RNA-based components of future gene regulatory networks (III). We believe that these evolutionary paths can be constructively viewed from the perspective of the concept of biological codes (Barbieri, 2003). Crucial evolutionary events in these directions would involve the emergence of RNA-based adaptors. Such adaptors connect two families of functionally and chemically distinct molecules into one functional entity. The emergence of primitive translation processes is undoubtedly the major milestone in the evolutionary path towards modern life. The key aspect here is the appearance of adaptors between amino acids and their cognate triplet codons. The initial steps are believed to involve the emergence of proto-transfer RNAs capable of self-aminoacylation. The second significant evolutionary breakthrough is the development of biochemical regulatory networks based on signaling molecules of the RNA World (ribonucleotides and their derivatives), as well as receptors and effectors (riboswitches) for these messengers. Some authors refer to this as the "lost language of the RNA World." The third evolutionary step is the emergence of signal sequences for ribozymes on the molecules of their RNA targets. This level of regulation in the RNA World is comparable to the gene regulatory networks of modern organisms. We believe that the signal sequences on target molecules have been rediscovered and developed by evolution into the gene regulatory networks of modern cells. In conclusion, the immense diversity of modern biological codes, in some of its key characteristics, can be traced back to the achievements of prebiotic evolution.

Xeno Amino Acids: A Look into Biochemistry as We Do Not Know It

Would another origin of life resemble Earth's biochemical use of amino acids? Here, we review current knowledge at three levels: (1) Could other classes of chemical structure serve as building blocks for biopolymer structure and catalysis? Amino acids now seem both readily available to, and a plausible chemical attractor for, life as we do not know it. Amino acids thus remain important and tractable targets for astrobiological research. (2) If amino acids are used, would we expect the same L-alpha-structural subclass used by life? Despite numerous ideas, it is not clear why life favors L-enantiomers. It seems clearer, however, why life on Earth uses the shortest possible (alpha-) amino acid backbone, and why each carries only one side chain. However, assertions that other backbones are physicochemically impossible have relaxed into arguments that they are disadvantageous. (3) Would we expect a similar set of side chains to those within the genetic code? Many plausible alternatives exist. Furthermore, evidence exists for both evolutionary advantage and physicochemical constraint as explanatory factors for those encoded by life. Overall, as focus shifts from amino acids as a chemical class to specific side chains used by post-LUCA biology, the probable role of physicochemical constraint diminishes relative to that of biological evolution. Exciting opportunities now present themselves for laboratory work and computing to explore how changing the amino acid alphabet alters the universe of protein folds. Near-term milestones include: (a) expanding evidence about amino acids as attractors within chemical evolution; (b) extending characterization of other backbones relative to biological proteins; and (c) merging computing and laboratory explorations of structures and functions unlocked by xeno peptides.

Common evolutionary origins of the bacterial glycyl tRNA synthetase and alanyl tRNA synthetase

Aminoacyl-tRNA synthetases (aaRSs) establish the genetic code. Each aaRS covalently links a given canonical amino acid to a cognate set of tRNA isoacceptors. Glycyl tRNA aminoacylation is unusual in that it is catalyzed by different aaRSs in different lineages of the Tree of Life. We have investigated the phylogenetic distribution and evolutionary history of bacterial glycyl tRNA synthetase (bacGlyRS). This enzyme is found in early diverging bacterial phyla such as Firmicutes, Acidobacteria, and Proteobacteria, but not in archaea or eukarya. We observe relationships between each of six domains of bacGlyRS and six domains of four different RNA-modifying proteins. Component domains of bacGlyRS show common ancestry with (i) the catalytic domain of class II tRNA synthetases; (ii) the HD domain of the bacterial RNase Y; (iii) the body and tail domains of the archaeal CCA-adding enzyme; (iv) the anti-codon binding domain of the arginyl tRNA synthetase; and (v) a previously unrecognized domain that we call ATL (Ancient tRNA latch). The ATL domain has been found thus far only in bacGlyRS and in the universal alanyl tRNA synthetase (uniAlaRS). Further, the catalytic domain of bacGlyRS is more closely related to the catalytic domain of uniAlaRS than to any other aminoacyl tRNA synthetase. The combined results suggest that the ATL and catalytic domains of these two enzymes are ancestral to bacGlyRS and uniAlaRS, which emerged from common protein ancestors by bricolage, stepwise accumulation of protein domains, before the last universal common ancestor of life.

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.

Informatic Capabilities of Translation and Its Implications for the Origins of Life

The ability to encode and convert heritable information into molecular function is a defining feature of life as we know it. The conversion of information into molecular function is performed by the translation process, in which triplets of nucleotides in a nucleic acid polymer (mRNA) encode specific amino acids in a protein polymer that folds into a three-dimensional structure. The folded protein then performs one or more molecular activities, often as one part of a complex and coordinated physiological network. Prebiotic systems, lacking the ability to explicitly translate information between genotype and phenotype, would have depended upon either chemosynthetic pathways to generate its components-constraining its complexity and evolvability- or on the ambivalence of RNA as both carrier of information and of catalytic functions-a possibility which is still supported by a very limited set of catalytic RNAs. Thus, the emergence of translation during early evolutionary history may have allowed life to unmoor from the setting of its origin. The origin of translation machinery also represents an entirely novel and distinct threshold of behavior for which there is no abiotic counterpart-it could be the only known example of computing that emerged naturally at the chemical level. Here we describe translation machinery's decoding system as the basis of cellular translation's information-processing capabilities, and the four operation types that find parallels in computer systems engineering that this biological machinery exhibits.

Alternative Reading Frames are an Underappreciated Source of Protein Sequence Novelty

Protein-coding DNA sequences can be translated into completely different amino acid sequences if the nucleotide triplets used are shifted by a non-triplet amount on the same DNA strand or by translating codons from the opposite strand. Such "alternative reading frames" of protein-coding genes are a major contributor to the evolution of novel protein products. Recent studies demonstrating this include examples across the three domains of cellular life and in viruses. These sequences increase the number of trials potentially available for the evolutionary invention of new genes and also have unusual properties which may facilitate gene origin. There is evidence that the structure of the standard genetic code contributes to the features and gene-likeness of some alternative frame sequences. These findings have important implications across diverse areas of molecular biology, including for genome annotation, structural biology, and evolutionary genomics.

Signal-noise metrics for RNA binding protein identification reveal broad spectrum protein-RNA interaction frequencies and dynamics

Recent efforts towards the comprehensive identification of RNA-bound proteomes have revealed a large, surprisingly diverse family of candidate RNA-binding proteins (RBPs). Quantitative metrics for characterization and validation of protein-RNA interactions and their dynamic interactions have, however, proven analytically challenging and prone to error. Here we report a method termed LEAP-RBP (Liquid-Emulsion-Assisted-Purification of RNA-Bound Protein) for the selective, quantitative recovery of UV-crosslinked RNA-protein complexes. By virtue of its high specificity and yield, LEAP-RBP distinguishes RNA-bound and RNA-free protein levels and reveals common sources of experimental noise in RNA-centric RBP enrichment methods. We introduce strategies for accurate RBP identification and signal-based metrics for quantifying protein-RNA complex enrichment, relative RNA occupancy, and method specificity. In this work, the utility of our approach is validated by comprehensive identification of RBPs whose association with mRNA is modulated in response to global mRNA translation state changes and through in-depth benchmark comparisons with current methodologies.

On the minimal elements of the genetic code and their semiotic functions (degeneracy, complementarity, wobbling)

We address semiotic features of genetic coding, primarily the mechanisms for distinguishing between triplets. It implies that the minimal elements that allow codon recognition and amino acid coding should be identified. Half a century ago, linguist Roman Jakobson and microbiologist François Jacob revealed functional similarities between nucleotides in the genetic code and phonemes in natural language. Developing this analogy, we introduce the concept of a semiotic nucleotide. Unlike "material" nucleotides, its characteristics are limited to the function of differentiation within the processing of genetic information. We demonstrate that, similarly to phonemes, nucleotides are also non-elementary entities and can be represented as a set of two differential features: a) the number of rings and b) the number of hydrogen bonds. This makes it possible to convert semiotic nucleotides into double-byte units of digital information. Proceeding from this assumption, we suggest a new vision of such phenomena as the heterogeneity of the genetic code in terms of coding types, to reveal the code-distinguishing potential of positions within triplets, and represent wobbling as a specific reading regime. All these phenomena relate to the peculiarity of the triplet's third position, where complete or partial neutralization of distinguishing features is possible.

Decoding viruses: An alternative perspective on their history, origins and role in nature

This article provides an alternative perspective on viruses, exploring their origins, ecology, and evolution. Viruses are recognized as the most prevalent biological entities on Earth, permeating nearly all environments and forming the virosphere-a significant biological layer. They play a crucial role in regulating bacterial populations within ecosystems and holobionts, influencing microbial communities and nutrient recycling. Viruses are also key drivers of molecular evolution, actively participating in the maintenance and regulation of ecosystems and cellular organisms. Many eukaryotic genomes contain genomic elements with viral origins, which contribute to organismal equilibrium and fitness. Viruses are involved in the generation of species-specific orphan genes, facilitating adaptation and the development of unique traits in biological lineages. They have been implicated in the formation of vital structures like the eukaryotic nucleus and the mammalian placenta. The presence of virus-specific genes absent in cellular organisms suggests that viruses may pre-date cellular life. Like progenotes, viruses are ribonucleoprotein entities with simpler capsid architectures compared to proteolipidic membranes. This article presents a comprehensive scenario describing major transitions in prebiotic evolution and proposes that viruses emerged prior to the Last Universal Common Ancestor (LUCA) during the progenote era. However, it is important to note that viruses do not form a monophyletic clade, and many viral taxonomic groups originated more recently as reductions of cellular structures. Thus, viral architecture should be seen as an ancient and evolutionarily stable strategy adopted by biological systems. The goal of this article is to reshape perceptions of viruses, highlighting their multifaceted significance in the complex tapestry of life and fostering a deeper understanding of their origins, ecological impact, and evolutionary dynamics.

The Evolution of Life Is a Road Paved with the DNA Quadruplet Symmetry and the Supersymmetry Genetic Code

Symmetries have not been completely determined and explained from the discovery of the DNA structure in 1953 and the genetic code in 1961. We show, during 10 years of investigation and research, our discovery of the Supersymmetry Genetic Code table in the form of 2 × 8 codon boxes, quadruplet DNA symmetries, and the classification of trinucleotides/codons, all built with the same physiochemical double mirror symmetry and Watson-Crick pairing. We also show that single-stranded RNA had the complete code of life in the form of the Supersymmetry Genetic Code table simultaneously with instructions of codons' relationship as to how to develop the DNA molecule on the principle of Watson-Crick pairing. We show that the same symmetries between the genetic code and DNA quadruplet are highly conserved during the whole evolution even between phylogenetically distant organisms. In this way, decreasing disorder and entropy enabled the evolution of living beings up to sophisticated species with cognitive features. Our hypothesis that all twenty amino acids are necessary for the origin of life on the Earth, which entirely changes our view on evolution, confirms the evidence of organic natural amino acids from the extra-terrestrial asteroid Ryugu, which is nearly as old as our solar system.

Mixed anhydrides at the intersection between peptide and RNA autocatalytic sets: evolution of biological coding

We present a scenario for the origin of biological coding, a semiotic relationship between chemical information stored in one location that links to chemical information stored in a separate location. Coding originated from cooperation between two, originally separate, collectively autocatalytic sets (CASs), one for nucleic acids and one for peptides. Upon interaction, a series of RNA folding-directed processes led to their joint cooperativity. The aminoacyl adenylate was the first covalent association made by these two CASs and solidified their interdependence, and is a palimpsest of this era, a relic of the original semiotic relationship between RNA and proteins. Coding was driven by selection pressure to eliminate waste in CASs. Eventually a 1 : 1 relationship between single amino acids and short RNA pieces was established, i.e. the 'genetic code'. The two classes of aaRS enzymes are remnants of the complementary information in two RNA strands, as postulated by Rodin and Ohno. Every stage in the evolution of coding was driven by the downward selection on the components of a system to satisfy the Kantian whole. Coding was engendered because there were two chemically distinct classes of polymers needed for open-ended evolution; systems with only one polymer cannot exhibit this characteristic. Coding is thus synonymous with life as we know it.

Coding From Binding? Molecular Interactions at the Heart of Translation

The mechanism and the evolution of DNA replication and transcription, the key elements of the central dogma of biology, are fundamentally well explained by the physicochemical complementarity between strands of nucleic acids. However, the determinants that have shaped the third part of the dogma-the process of biological translation and the universal genetic code-remain unclear. We review and seek parallels between different proposals that view the evolution of translation through the prism of weak, noncovalent interactions between biological macromolecules. In particular, we focus on a recent proposal that there exists a hitherto unrecognized complementarity at the heart of biology, that between messenger RNA coding regions and the proteins that they encode, especially if the two are unstructured. Reflecting the idea that the genetic code evolved from intrinsic binding propensities between nucleotides and amino acids, this proposal promises to forge a link between the distant past and the present of biological systems.

Origin of life: Drawing the big picture

Trying to provide a broad overview about the origin of life in Earth, the most significant transitions of life before cells are listed and discussed. The current approach emphasizes the symbiotic relationships that emerged with life. We propose a rational, stepwise scenario for the origin of life that starts with the origin of the first biomolecules and steps forward until the origins of the first cells. Along this path, we aim to provide a brief, though comprehensive theoretical model that will consider the following steps: (i) how nucleotides and other biomolecules could be made prebiotically in specific prebiotic refuges; (ii) how the first molecules of RNAs were formed; (iii) how the proto-peptidyl transferase center was built by the concatenation of proto-tRNAs; (iv) how the ribosome and the genetic code could be structured; (v) how progenotes could live and reproduce as "naked" ribonucleoprotein molecules; (vi) how peptides started to bind molecules in the prebiotic soup allowing biochemical pathways to evolve from those bindings; (vii) how genomes got bigger by the symbiotic relationship of progenotes and lateral transference of genetic material; (viii) how the progenote LUCA has been formed by assembling most biochemical routes; (ix) how the first virion capsids probably emerged and evolved; (x) how phospholipid membranes emerged probably twice by the evolution of lipid-binding proteins; (xi) how DNA synthesis have been formed in parallel in Bacteria and Archaea; and, finally, (xii) how DNA-based cells of Bacteria and Archaeabacteria have been constituted. The picture provided is conjectural and present epistemological gaps. Future research will help to advance into the elucidation of gaps and confirmation/refutation of current statements.

Mammalian proteome expansion by stop codon readthrough

Recognition of a stop codon by translation machinery as a sense codon results in translational readthrough instead of termination. This recoding process, termed stop codon readthrough (SCR) or translational readthrough, is found in all domains of life including mammals. The context of the stop codon, local mRNA topology, and molecules that interact with the mRNA region downstream of the stop codon determine SCR. The products of SCR can have localization, stability, and function different from those of the canonical isoforms. In this review, we discuss how recent technological and computational advances have increased our understanding of the SCR process in the mammalian system. Based on the known molecular events that occur during SCR of multiple mRNAs, we propose transient molecular roadblocks on an mRNA downstream of the stop codon as a possible mechanism for the induction of SCR. We argue, with examples, that the insights gained from the natural SCR events can guide us to develop novel strategies for the treatment of diseases caused by premature stop codons. This article is categorized under: Translation > Regulation.

Deciphering the multi-scale, quantitative cis-regulatory code

Uncovering the cis-regulatory code that governs when and how much each gene is transcribed in a given genome and cellular state remains a central goal of biology. Here, we discuss major layers of regulation that influence how transcriptional outputs are encoded by DNA sequence and cellular context. We first discuss how transcription factors bind specific DNA sequences in a dosage-dependent and cooperative manner and then proceed to the cofactors that facilitate transcription factor function and mediate the activity of modular cis-regulatory elements such as enhancers, silencers, and promoters. We then consider the complex and poorly understood interplay of these diverse elements within regulatory landscapes and its relationships with chromatin states and nuclear organization. We propose that a mechanistically informed, quantitative model of transcriptional regulation that integrates these multiple regulatory layers will be the key to ultimately cracking the cis-regulatory code.

A crescendo of competent coding (c3) contains the Standard Genetic Code

The Standard Genetic Code (SGC) can arise by fusion of partial codes evolved in different individuals, perhaps for differing prior tasks. Such code fragments can be unified into an SGC after later evolution of accurate third-position Crick wobble. Late wobble advent fills in the coding table, leaving only later development of translational initiation and termination to reach the SGC in separated domains of life. This code fusion mechanism is computationally implemented here. Late Crick wobble after C3 fusion (c3-lCw) is tested for its ability to evolve the SGC. Compared with previously studied isolated coding tables, or with increasing numbers of parallel, but nonfusing codes, c3-lCw reaches the SGC sooner, is successful in a smaller population, and presents accurate and complete codes more frequently. Notably, a long crescendo of SGC-like codes is exposed for selection of superior translation. c3-lCw also effectively suppresses varied disordered assignments, thus converging on a unified code. Such merged codes closely approach the SGC, making its selection plausible. For example: Under routine conditions, ≈1 of 22 c3-lCw environments evolves codes with ≥20 assignments and ≤3 differences from the SGC, notably including codes identical to the Standard Genetic Code.

Widespread autogenous mRNA-protein interactions detected by CLIP-seq

Autogenous interactions between mRNAs and the proteins they encode are implicated in cellular feedback-loop regulation, but their extent and mechanistic foundation are unclear. It was recently hypothesized that such interactions may be common, reflecting the role of intrinsic nucleobase-amino acid affinities in shaping the genetic code's structure. Here we analyze a comprehensive set of CLIP-seq experiments involving multiple protocols and report on widespread autogenous interactions across different organisms. Specifically, 230 of 341 (67%) studied RNA-binding proteins (RBPs) interact with their own mRNAs, with a heavy enrichment among high-confidence hits and a preference for coding sequence binding. We account for different confounding variables, including physical (overexpression and proximity during translation), methodological (difference in CLIP protocols, peak callers and cell types) and statistical (treatment of null backgrounds). In particular, we demonstrate a high statistical significance of autogenous interactions by sampling null distributions of fixed-margin interaction matrices. Furthermore, we study the dependence of autogenous binding on the presence of RNA-binding motifs and structured domains in RBPs. Finally, we show that intrinsic nucleobase-amino acid affinities favor co-aligned binding between mRNA coding regions and the proteins they encode. Our results suggest a central role for autogenous interactions in RBP regulation and support the possibility of a fundamental connection between coding and binding.

A Cofactor-Based Mechanism for the Origin of the Genetic Code

The origin of the genetic code is probably the central problem of the studies on the origin of life. The key question to answer is the molecular mechanism that allows the association of the amino acids with their triplet codons. We proposed that the codon-anticodon duplex located in the acceptor stem of primitive tRNAs would facilitate the chemical reactions required to synthesize cognate amino acids from simple amino acids (glycine, valine, and aspartic acid) linked to the 3' acceptor end. In our view, various nucleotide-A-derived cofactors (with reactive chemical groups) may be attached to the codon-anticodon duplex, which allows group-transferring reactions from cofactors to simple amino acids, thereby producing the final amino acid. The nucleotide-A-derived cofactors could be incorporated into the RNA duplex (helix) by docking Adenosine (cofactor) into the minor groove via an interaction similar to the A-minor motif, forming a base triple between Adenosine and one complementary base pair of the duplex. Furthermore, we propose that this codon-anticodon duplex could initially catalyze a self-aminoacylation reaction with a simple amino acid. Therefore, the sequence of bases in the codon-anticodon duplex would determine the reactions that occurred during the formation of new amino acids for selective binding of nucleotide-A-derived cofactors.

Emergent properties as by-products of prebiotic evolution of aminoacylation ribozymes

Systems of catalytic RNAs presumably gave rise to important evolutionary innovations, such as the genetic code. Such systems may exhibit particular tolerance to errors (error minimization) as well as coding specificity. While often assumed to result from natural selection, error minimization may instead be an emergent by-product. In an RNA world, a system of self-aminoacylating ribozymes could enforce the mapping of amino acids to anticodons. We measured the activity of thousands of ribozyme mutants on alternative substrates (activated analogs for tryptophan, phenylalanine, leucine, isoleucine, valine, and methionine). Related ribozymes exhibited shared preferences for substrates, indicating that adoption of additional amino acids by existing ribozymes would itself lead to error minimization. Furthermore, ribozyme activity was positively correlated with specificity, indicating that selection for increased activity would also lead to increased specificity. These results demonstrate that by-products of ribozyme evolution could lead to adaptive value in specificity and error tolerance.

The Origin of Genetic Code and Translation in the Framework of Current Concepts on the Origin of Life

The origin of genetic code and translation system is probably the central and most difficult problem in the investigations on the origin of life and one of the most complex problems in the evolutionary biology in general. There are multiple hypotheses on the emergence and development of existing genetic systems that propose the mechanisms for the origin and early evolution of genetic code, as well as for the emergence of replication and translation. Here, we discuss the most well-known of these hypotheses, although none of them provides a description of the early evolution of genetic systems without gaps and assumptions. The RNA world hypothesis is a currently prevailing scientific idea on the early evolution of biological and pre-biological structures, the main advantage of which is the assumption that RNAs as the first living systems were self-sufficient, i.e., capable of functioning as both catalysts and templates. However, this hypothesis has also significant limitations. In particular, no ribozymes with processive polymerase activity have been yet discovered or synthesized. Taking into account the mutual need of proteins and nucleic acids in each other in the current world, many authors propose the early evolution scenarios based on the co-evolution of these two classes of organic molecules. They postulate that the emergence of translation was necessary for the replication of nucleic acids, in contrast to the RNA world hypothesis, according to which the emergence of translation was preceded by the era of self-replicating RNAs. Although such scenarios are less parsimonious from the evolutionary point of view, since they require simultaneous emergence and evolution of two classes of organic molecules, as well as the emergence of synchronized replication and translation, their major advantage is that they explain the development of processive and much more accurate protein-dependent replication.

Model of Genetic Code Structure Evolution under Various Types of Codon Reading

The standard genetic code (SGC) is a set of rules according to which 64 codons are assigned to 20 canonical amino acids and stop coding signal. As a consequence, the SGC is redundant because there is a greater number of codons than the number of encoded labels. This redundancy implies the existence of codons that encode the same genetic information. The size and organization of such synonymous codon blocks are important characteristics of the SGC structure whose evolution is still unclear. Therefore, we studied possible evolutionary mechanisms of the codon block structure. We conducted computer simulations assuming that coding systems at early stages of the SGC evolution were sets of ambiguous codon assignments with high entropy. We included three types of reading systems characterized by different inaccuracy and pattern of codon recognition. In contrast to the previous study, we allowed for evolution of the reading systems and their competition. The simulations performed under minimization of translational errors and reduction of coding ambiguity produced the coding system resistant to these errors. The reading system similar to that present in the SGC dominated the others very quickly. The survived system was also characterized by low entropy and possessed properties similar to that in the SGC. Our simulation show that the unambiguous SGC could emerged from a code with a lower level of ambiguity and the number of tRNAs increased during the evolution.

RNP-world: The ultimate essence of life is a ribonucleoprotein process

The fundamental essence of life is based on process of interaction between nucleic acids and proteins. In a prebiotic world, amino acids, peptides, ions, and other metabolites acted in protobiotic routes at the same time on which RNAs performed catalysis and self-replication. Nevertheless, it was only when nucleic acids and peptides started to interact together in an organized process that life emerged. First, the ignition was sparked with the formation of a Peptidyl Transferase Center (PTC), possibly by concatenation of proto-tRNAs. This molecule that would become the catalytic site of ribosomes started a process of self-organization that gave origin to a protoorganism named FUCA, a ribonucleic ribosomal-like apparatus capable to polymerize amino acids. In that sense, we review hypotheses about the origin and early evolution of the genetic code. Next, populations of open biological systems named progenotes were capable of accumulating and exchanging genetic material, producing the first genomes. Progenotes then evolved in two paths: some presented their own ribosomes and others used available ribosomes in the medium to translate their encoded information. At some point, two different types of organisms emerged from populations of progenotes: the ribosome-encoding organisms (cells) and the capsid-encoding organisms (viruses).

Theories, laws, and models in evo-devo

Evolutionary developmental biology (evo-devo) is the study of the evolution of developmental mechanisms. Here, I review some of the theories, models, and laws in evo-devo, past and present. Nineteenth-century evo-devo was dominated by recapitulation theory and archetypes. It also gave us germ layer theory, the vertebral theory of the skull, floral organs as modified leaves, and the "inverted invertebrate" theory, among others. Newer theories and models include the frameshift theory, the genetic toolkit for development, the ABC model of flower development, the developmental hourglass, the zootype, Urbilateria, and the hox code. Some of these new theories show the influence of archetypes and recapitulation. Interestingly, recent studies support the old "primordial leaf," "inverted invertebrate," and "segmented head" theories. Furthermore, von Baer's first three laws may now need to be rehabilitated, and the hourglass model modified, in view of what Abzhanov has pointed out about the maternal-zygotic transition. There are many supposed "laws" of evo-devo but I argue that these are merely generalizations about trends in particular lineages. I argue that the "body plan" is an archetype, and is often used in such a way that it lacks any scientific meaning. Looking to the future, one challenge for evo-devo will be to develop new theories and models to accommodate the wealth of new data from high-throughput sequencing, including single-cell sequencing. One step in this direction is the use of sophisticated in silico analyses, as in the "transcriptomic hourglass" models.

Average and Standard Deviation of the Error Function for Random Genetic Codes with Standard Stop Codons

The origin of the genetic code has been attributed in part to an accidental assignment of codons to amino acids. Although several lines of evidence indicate the subsequent expansion and improvement of the genetic code, the hypothesis of Francis Crick concerning a frozen accident occurring at the early stage of genetic code evolution is still widely accepted. Considering Crick's hypothesis, mathematical descriptions of hypothetical scenarios involving a huge number of possible coexisting random genetic codes could be very important to explain the origin and evolution of a selected genetic code. This work aims to contribute in this regard, that is, it provides a theoretical framework in which statistical parameters of error functions are calculated. Given a genetic code and an amino acid property, the functional code robustness is estimated by means of a known error function. In this work, using analytical calculations, general expressions for the average and standard deviation of the error function distributions of completely random codes with standard stop codons were obtained. As a possible biological application of these results, any set of amino acids and any pure or mixed amino acid properties can be used in the calculations, such that, in case of having to select a set of amino acids to create a genetic code, possible advantages of natural selection of the genetic codes could be discussed.

Factors in Protobiomonomer Selection for the Origin of the Standard Genetic Code

Natural selection of specific protobiomonomers during abiogenic development of the prototype genetic code is hindered by the diversity of structural, spatial, and rotational isomers that have identical elemental composition and molecular mass (M), but can vary significantly in their physicochemical characteristics, such as the melting temperature T_m, the T_m:M ratio, and the solubility in water, due to different positions of atoms in the molecule. These parameters differ between cis- and trans-isomers of dicarboxylic acids, spatial monosaccharide isomers, and structural isomers of α-, β-, and γ-amino acids. The stable planar heterocyclic molecules of the major nucleobases comprise four (C, H, N, O) or three (C, H, N) elements and contain a single -C=C bond and two nitrogen atoms in each heterocycle involved in C-N and C=N bonds. They exist as isomeric resonance hybrids of single and double bonds and as a mixture of tautomer forms due to the presence of -C=O and/or -NH₂ side groups. They are thermostable, insoluble in water, and exhibit solid-state stability, which is of central importance for DNA molecules as carriers of genetic information. In M-T_m diagrams, proteinogenic amino acids and the corresponding codons are distributed fairly regularly relative to the distinct clusters of purine and pyrimidine bases, reflecting the correspondence between codons and amino acids that was established in different periods of genetic code development. The body of data on the evolution of the genetic code system indicates that the elemental composition and molecular structure of protobiomonomers, and their M, T_m, photostability, and aqueous solubility determined their selection in the emergence of the standard genetic code.