The ribosome as a missing link in the evolution of life

Major evolutionary transitions before cells: A journey from molecules to organisms

Basing on logical assumptions and necessary steps of complexification along biological evolution, we propose here an evolutionary path from molecules to cells presenting four ages and three major transitions. At the first age, the basic biomolecules were formed and become abundant. The first transition happened with the event of a chemical symbiosis between nucleic acids and peptides worlds, which marked the emergence of both life and the process of organic encoding. FUCA, the first living process, was composed of self-replicating RNAs linked to amino acids and capable to catalyze their binding. The second transition, from the age of FUCA to the age of progenotes, involved the duplication and recombination of proto-genomes, leading to specialization in protein production and the exploration of protein to metabolite interactions in the prebiotic soup. Enzymes and metabolic pathways were incorporated into biology from protobiotic reactions that occurred without chemical catalysts, step by step. Then, the fourth age brought origin of organisms and lineages, occurring when specific proteins capable to stackle together facilitated the formation of peptidic capsids. LUCA was constituted as a progenote capable to operate the basic metabolic functions of a cell, but still unable to interact with lipid molecules. We present evidence that the evolution of lipid interaction pathways occurred at least twice, with the development of bacterial-like and archaeal-like membranes. Also, data in literature suggest at least two paths for the emergence of DNA biosynthesis, allowing the stabilization of early life strategies in viruses, archaeas and bacterias. Two billion years later, the eukaryotes arouse, and after 1,5 billion years of evolution, they finally learn how to evolve multicellularity via tissue specialization.

Archaeological approaches to RNA virus evolution

Studies with RNA enzymes (ribozymes) and protein enzymes have identified certain structural elements that are present in some cellular mRNAs and viral RNAs. These elements do not share a primary structure and, thus, are not phylogenetically related. However, they have common (secondary/tertiary) structural folds that, according to some lines of evidence, may have an ancient and common origin. The term 'mRNA archaeology' has been coined to refer to the search for such structural/functional relics that may be informative of early evolutionary developments in the cellular and viral worlds and have lasted to the present day. Such identified RNA elements may have developed as biological signals with structural and functional relevance (as if they were buried objects with archaeological value), and coexist with the standard linear information of nucleic acid molecules that is translated into proteins. However, there is a key difference between the methods that extract information from either the primary structure of mRNA or the signals provided by secondary and tertiary structures. The former (sequence comparison and phylogenetic analysis) requires strict continuity of the material vehicle of information during evolution, whereas the archaeological method does not require such continuity. The tools of RNA archaeology (including the use of ribozymes and enzymes to investigate the reactivity of the RNA elements) establish links between the concepts of communication and language theories that have not been incorporated into knowledge of virology, as well as experimental studies on the search for functionally relevant RNA structures.

Ribosomal Protein Cluster Organization in Asgard Archaea

It has been proposed that the superphylum of Asgard Archaea may represent a historical link between the Archaea and Eukarya. Following the discovery of the Archaea, it was soon appreciated that archaeal ribosomes were more similar to those of Eukarya rather than Bacteria. Coupled with other eukaryotic-like features, it has been suggested that the Asgard Archaea may be directly linked to eukaryotes. However, the genomes of Bacteria and non-Asgard Archaea generally organize ribosome-related genes into clusters that likely function as operons. In contrast, eukaryotes typically do not employ an operon strategy. To gain further insight into conservation of the r-protein genes, the genome order of conserved ribosomal protein (r-protein) coding genes was identified in 17 Asgard genomes (thirteen complete genomes and four genomes with less than 20 contigs) and compared with those found previously in non-Asgard archaeal and bacterial genomes. A universal core of two clusters of 14 and 4 cooccurring r-proteins, respectively, was identified in both the Asgard and non-Asgard Archaea. The equivalent genes in the E. coli version of the cluster are found in the S10 and spc operons. The large cluster of 14 r-protein genes (uS19-uL22-uS3-uL29-uS17 from the S10 operon and uL14-uL24-uL5-uS14-uS8-uL6-uL18-uS5-uL30-uL15 from the spc operon) occurs as a complete set in the genomes of thirteen Asgard genomes (five Lokiarchaeotes, three Heimdallarchaeotes, one Odinarchaeote, and four Thorarchaeotes). Four less conserved clusters with partial bacterial equivalents were found in the Asgard. These were the L30e (str operon in Bacteria) cluster, the L18e (alpha operon in Bacteria) cluster, the S24e-S27ae-rpoE1 cluster, and the L31e, L12..L1 cluster. Finally, a new cluster referred to as L7ae was identified. In many cases, r-protein gene clusters/operons are less conserved in their organization in the Asgard group than in other Archaea. If this is generally true for nonribosomal gene clusters, the results may have implications for the history of genome organization. In particular, there may have been an early transition to or from the operon approach to genome organization. Other nonribosomal cellular features may support different relationships. For this reason, it may be important to consider ribosome features separately.

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.

Mosaic translation hypothesis: chimeric polypeptides produced via multiple ribosomal frameshifting as a basis for adaptability

How many different proteins can be produced from a single spliced transcript? Genome annotation projects overlook the coding potential of reading frames other than that of the reference open reading frames (refORFs). Recently, alternative open reading frames (altORFs) and their translational products, alternative proteins, have been shown to carry out important functions in various organisms. AltORFs overlapping refORFs or other altORFs in a different reading frame may be involved in one fundamental mechanism so far overlooked. A few years ago, it was proposed that altORFs may act as building blocks for chimeric (mosaic) polypeptides, which are produced via multiple ribosomal frameshifting events from a single mature transcript. We adopt terminology from that earlier discussion and call this mechanism mosaic translation. This way of extracting and combining genetic information may significantly increase proteome diversity. Thus, we hypothesize that this mechanism may have contributed to the flexibility and adaptability of organisms to a variety of environmental conditions. Specialized ribosomes acting as sensors probably played a central role in this process. Importantly, mosaic translation may be the main source of protein diversity in genomes that lack alternative splicing. The idea of mosaic translation is a testable hypothesis, although its direct demonstration is challenging. Should mosaic translation occur, we would currently highly underestimate the complexity of translation mechanisms and thus the proteome.

Modeling the ribosome as a bipartite graph

Developing mathematical representations of biological systems that can allow predictions is a challenging and important research goal. It is demonstrated here how the ribosome, the nano-machine responsible for synthesizing all proteins necessary for cellular life, can be represented as a bipartite network. Ten ribosomal structures from Bacteria and six from Eukarya are explored. Ribosomal networks are found to exhibit unique properties despite variations in the nodes and edges of the different graphs. The ribosome is shown to exhibit very large topological redundancies, demonstrating mathematical resiliency. These results can potentially explain how it can function consistently despite changes in composition and connectivity. Furthermore, this representation can be used to analyze ribosome function within the large machinery of network theory, where the degrees of freedom are the possible interactions, and can be used to provide new insights for translation regulation and therapeutics.

A Short Tale of the Origin of Proteins and Ribosome Evolution

Proteins are the workhorses of the cell and have been key players throughout the evolution of all organisms, from the origin of life to the present era. How might life have originated from the prebiotic chemistry of early Earth? This is one of the most intriguing unsolved questions in biology. Currently, however, it is generally accepted that amino acids, the building blocks of proteins, were abiotically available on primitive Earth, which would have made the formation of early peptides in a similar fashion possible. Peptides are likely to have coevolved with ancestral forms of RNA. The ribosome is the most evident product of this coevolution process, a sophisticated nanomachine that performs the synthesis of proteins codified in genomes. In this general review, we explore the evolution of proteins from their peptide origins to their folding and regulation based on the example of superoxide dismutase (SOD1), a key enzyme in oxygen metabolism on modern Earth.

Prebiotic chemical refugia: multifaceted scenario for the formation of biomolecules in primitive Earth

The origin of life was a cosmic event happened on primitive Earth. A critical problem to better understand the origins of life in Earth is the search for chemical scenarios on which the basic building blocks of biological molecules could be produced. Classic works in pre-biotic chemistry frequently considered early Earth as an homogeneous atmosphere constituted by chemical elements such as methane (CH₄), ammonia (NH₃), water (H₂O), hydrogen (H₂) and hydrogen sulfide (H₂S). Under that scenario, Stanley Miller was capable to produce amino acids and solved the question about the abiotic origin of proteins. Conversely, the origin of nucleic acids has tricked scientists for decades once nucleotides are complex, though necessary molecules to allow the existence of life. Here we review possible chemical scenarios that allowed not only the formation of nucleotides but also other significant biomolecules. We aim to provide a theoretical solution for the origin of biomolecules at specific sites named "Prebiotic Chemical Refugia." Prebiotic chemical refugium should therefore be understood as a geographic site in prebiotic Earth on which certain chemical elements were accumulated in higher proportion than expected, facilitating the production of basic building blocks for biomolecules. This higher proportion should not be understood as static, but dynamic; once the physicochemical conditions of our planet changed periodically. These different concentration of elements, together with geochemical and astronomical changes along days, synodic months and years provided somewhat periodic changes in temperature, pressure, electromagnetic fields, and conditions of humidity, among other features. Recent and classic works suggesting most likely prebiotic refugia on which the main building blocks for biological molecules might be accumulated are reviewed and discussed.

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).

Genome Evolution from Random Ligation of RNAs of Autocatalytic Sets

The evolutionary origin of the genome remains elusive. Here, I hypothesize that its first iteration, the protogenome, was a multi-ribozyme RNA. It evolved, likely within liposomes (the protocells) forming in dry-wet cycling environments, through the random fusion of ribozymes by a ligase and was amplified by a polymerase. The protogenome thereby linked, in one molecule, the information required to seed the protometabolism (a combination of RNA-based autocatalytic sets) in newly forming protocells. If this combination of autocatalytic sets was evolutionarily advantageous, the protogenome would have amplified in a population of multiplying protocells. It likely was a quasispecies with redundant information, e.g., multiple copies of one ribozyme. As such, new functionalities could evolve, including a genetic code. Once one or more components of the protometabolism were templated by the protogenome (e.g., when a ribozyme was replaced by a protein enzyme), and/or addiction modules evolved, the protometabolism became dependent on the protogenome. Along with increasing fidelity of the RNA polymerase, the protogenome could grow, e.g., by incorporating additional ribozyme domains. Finally, the protogenome could have evolved into a DNA genome with increased stability and storage capacity. I will provide suggestions for experiments to test some aspects of this hypothesis, such as evaluating the ability of ribozyme RNA polymerases to generate random ligation products and testing the catalytic activity of linked ribozyme domains.

Towards the Idea of Molecular Brains

How can single cells without nervous systems perform complex behaviours such as habituation, associative learning and decision making, which are considered the hallmark of animals with a brain? Are there molecular systems that underlie cognitive properties equivalent to those of the brain? This review follows the development of the idea of molecular brains from Darwin’s “root brain hypothesis”, through bacterial chemotaxis, to the recent discovery of neuron-like r-protein networks in the ribosome. By combining a structural biology view with a Bayesian brain approach, this review explores the evolutionary labyrinth of information processing systems across scales. Ribosomal protein networks open a window into what were probably the earliest signalling systems to emerge before the radiation of the three kingdoms. While ribosomal networks are characterised by long-lasting interactions between their protein nodes, cell signalling networks are essentially based on transient interactions. As a corollary, while signals propagated in persistent networks may be ephemeral, networks whose interactions are transient constrain signals diffusing into the cytoplasm to be durable in time, such as post-translational modifications of proteins or second messenger synthesis. The duration and nature of the signals, in turn, implies different mechanisms for the integration of multiple signals and decision making. Evolution then reinvented networks with persistent interactions with the development of nervous systems in metazoans. Ribosomal protein networks and simple nervous systems display architectural and functional analogies whose comparison could suggest scale invariance in information processing. At the molecular level, the significant complexification of eukaryotic ribosomal protein networks is associated with a burst in the acquisition of new conserved aromatic amino acids. Knowing that aromatic residues play a critical role in allosteric receptors and channels, this observation suggests a general role of π systems and their interactions with charged amino acids in multiple signal integration and information processing. We think that these findings may provide the molecular basis for designing future computers with organic processors.

The human tRNA-guanine transglycosylase displays promiscuous nucleobase preference but strict tRNA specificity

Base-modification can occur throughout a transfer RNA molecule; however, elaboration is particularly prevalent at position 34 of the anticodon loop (the wobble position), where it functions to influence protein translation. Previously, we demonstrated that the queuosine modification at position 34 can be substituted with an artificial analogue via the queuine tRNA ribosyltransferase enzyme to induce disease recovery in an animal model of multiple sclerosis. Here, we demonstrate that the human enzyme can recognize a very broad range of artificial 7-deazaguanine derivatives for transfer RNA incorporation. By contrast, the enzyme displays strict specificity for transfer RNA species decoding the dual synonymous NAU/C codons, determined using a novel enzyme-RNA capture-release method. Our data highlight the broad scope and therapeutic potential of exploiting the queuosine incorporation pathway to intentionally engineer chemical diversity into the transfer RNA anticodon.

Protein Homochirality May Be Derived from Primitive Peptide Synthesis by RNA

Homochirality is a feature of life, but its origin is still disputed. Recent theories indicate that the origin of homochirality coincided with that of the RNA world, but proteins have not yet been incorporated into the story. Ribosome is considered a living fossil that survived the RNA world and records the oldest interaction between RNA and proteins. Inspired by several ribosome-related findings, we propose a hypothesis as follows: the substrate chirality preference of some primitive peptide synthesis ribozymes can mediate the chirality transmission from RNA to protein. In return, the chiral preference of protective peptide-RNA interaction can bring these ribozymes an evolutionary advantage and facilitate the expansion of enantiomeric excess in peptides. Monte Carlo simulation results show that this system's chemistry model is plausible. This model can be further tested through investigation of the chirality preference for the interactions between d/l-ribose-composed rRNA homologs and l/d-amino acid-composed peptides.

Social Networking of Quasi-Species Consortia drive Virolution via Persistence

The emergence of cooperative quasi-species consortia (QS-C) thinking from the more accepted quasispecies equations of Manfred Eigen, provides a conceptual foundation from which concerted action of RNA agents can now be understood. As group membership becomes a basic criteria for the emergence of living systems, we also start to understand why the history and context of social RNA networks become crucial for survival and function. History and context of social RNA networks also lead to the emergence of a natural genetic code. Indeed, this QS-C thinking can also provide us with a transition point between the chemical world of RNA replicators and the living world of RNA agents that actively differentiate self from non-self and generate group identity with membership roles. Importantly the social force of a consortia to solve complex, multilevel problems also depend on using opposing and minority functions. The consortial action of social networks of RNA stem-loops subsequently lead to the evolution of cellular organisms representing a tree of life.

Origin of the 16S Ribosomal Molecule from Ancestor tRNAs

We tested the hypothesis that concatemers of ancestral tRNAs gave rise to the 16S ribosomal RNA. We built an ancestral sequence of proto-tRNAs that showed a significant identity of 51.69% and a percentage of structural identity of 0.941 with the 3' upper domain of 16S ribosomal molecule. We also propose a hypothesis in which the small ribosomal subunit emerged by proto-tRNA fusion and worked as a point to bind RNAs in an open structure configuration. In this context, the two ribosomal subunits initially worked independently, and that the subunit junction, with consequent primitive ribosome formation, was mediated by interactions with tRNA molecules during the primordial genetic code formation.

Is it possible that cells have had more than one origin?

Cells occupy a prominent place in the history of life in Earth. The central role of cellular organization can be understood by the fact that "cellular life" is often used as a synonym for life itself. Thus, most characteristics used to define cell overlap with those ones used to define life. However, innovative scenarios for the origin of life are bringing alternative views to describe how cells may have evolved from the open biological systems named progenotes. Here, using a logical and conceptual analysis, we re-evaluate the characteristics used to infer a single origin for cells. We argue that some evidences used to support cell monophyly, such as the presence of elements from the translation mechanism together with the universality of the genetic code, actually indicate a unique origin for all "biological systems", a term used to define not only cells, but also viruses and progenotes. Besides, we present evidence that at least two biochemical pathways as important as (i) DNA replication and (ii) lipid biosynthesis are not homologous between Bacteria and Archaea. The identities observed between the proteins involved in those pathways along representatives of these two ancestral domains of life are too low to indicate common genic ancestry. Altogether these facts can be seen as an indication that cellular organization has possibly evolved two or more times and that LUCA (the Last Universal Common Ancestor) may not have existed as a cellular entity. Thus, we aim to consider the possibility that different strategies acquired by biological systems to exist, such as viral, bacterial and archaeal were most likely originated independently from the evolution of different progenote populations.

The coenzyme/protein pair and the molecular evolution of life

Covering: up to 2020What was first? Coenzymes or proteins? These questions are archetypal examples of causal circularity in living systems. Classically, this "chicken-and-egg" problem was discussed for the macromolecules RNA, DNA and proteins. This report focuses on coenzymes and cofactors and discusses the coenzyme/protein pair as another example of causal circularity in life. Reflections on the origin of life and hypotheses on possible prebiotic worlds led to the current notion that RNA was the first macromolecule, long before functional proteins and hence DNA. So these causal circularities of living systems were solved by a time travel into the past. To tackle the "chicken-and-egg" problem of the protein-coenzyme pair, this report addresses this problem by looking for clues (a) in the first hypothetical biotic life forms such as protoviroids and the last unified common ancestor (LUCA) and (b) in considerations and evidence of the possible prebiotic production of amino acids and coenzymes before life arose. According to these considerations, coenzymes and cofactors can be regarded as very old molecular players in the origin and evolution of life, and at least some of them developed independently of α-amino acids, which here are evolutionarily synonymous with proteins. Discussions on "chicken-and-egg" problems open further doors to the understanding of evolution.

Origin of Life: The Point of No Return

Origin of life research is one of the greatest scientific frontiers of mankind. Many hypotheses have been proposed to explain how life began. Although different hypotheses emphasize different initial phenomena, all of them agree around one important concept: at some point, along with the chain of events toward life, Darwinian evolution emerged. There is no consensus, however, how this occurred. Frequently, the mechanism leading to Darwinian evolution is not addressed and it is assumed that this problem could be solved later, with experimental proof of the hypothesis. Here, the author first defines the minimum components required for Darwinian evolution and then from this standpoint, analyzes some of the hypotheses for the origin of life. Distinctive features of Darwinian evolution and life rooted in the interaction between information and its corresponding structure/function are then reviewed. Due to the obligatory dependency of the information and structure subject to Darwinian evolution, these components must be locked in their origin. One of the most distinctive characteristics of Darwinian evolution in comparison with all other processes is the establishment of a fundamentally new level of matter capable of evolving and adapting. Therefore, the initiation of Darwinian evolution is the "point of no return" after which life begins. In summary: a definition and a mechanism for Darwinian evolution are provided together with a critical analysis of some of the hypotheses for the origin of life.

Origin of the 16S Ribosomal Molecule from Ancestor tRNAs

We tested the hypothesis that concatemers of ancestral tRNAs gave rise to the 16S ribosomal RNA. We built an ancestral sequence of proto-tRNAs that showed a significant identity of 51.69% and a percentage of structural identity of 0.941 with the 16S ribosomal molecule. We also propose a hypothesis for the emergence of translation.

José M. The Ancient History of Peptidyl Transferase Center Formation as Told by Conservation and Information Analyses

The peptidyl transferase center (PTC) is the catalytic center of the ribosome and forms part of the 23S ribosomal RNA. The PTC has been recognized as the earliest ribosomal part and its origins embodied the First Universal Common Ancestor (FUCA). The PTC is frequently assumed to be highly conserved along all living beings. In this work, we posed the following questions: (i) How many 100% conserved bases can be found in the PTC? (ii) Is it possible to identify clusters of informationally linked nucleotides along its sequence? (iii) Can we propose how the PTC was formed? (iv) How does sequence conservation reflect on the secondary and tertiary structures of the PTC? Aiming to answer these questions, all available complete sequences of 23S ribosomal RNA from Bacteria and Archaea deposited on GenBank database were downloaded. Using a sequence bait of 179 bp from the PTC of Thermus termophilus, we performed an optimum pairwise alignment to retrieve the PTC region from 1424 filtered 23S rRNA sequences. These PTC sequences were multiply aligned, and the conserved regions were assigned and observed along the primary, secondary, and tertiary structures. The PTC structure was observed to be more highly conserved close to the adenine located at the catalytical site. Clusters of interrelated, co-evolving nucleotides reinforce previous assumptions that the PTC was formed by the concatenation of proto-tRNAs and important residues responsible for its assembly were identified. The observed sequence variation does not seem to significantly affect the 3D structure of the PTC ribozyme.

Root of the Tree: The Significance, Evolution, and Origins of the Ribosome

The ribosome is an ancient molecular fossil that provides a telescope to the origins of life. Made from RNA and protein, the ribosome translates mRNA to coded protein in all living systems. Universality, economy, centrality and antiquity are ingrained in translation. The translation machinery dominates the set of genes that are shared as orthologues across the tree of life. The lineage of the translation system defines the universal tree of life. The function of a ribosome is to build ribosomes; to accomplish this task, ribosomes make ribosomal proteins, polymerases, enzymes, and signaling proteins. Every coded protein ever produced by life on Earth has passed through the exit tunnel, which is the birth canal of biology. During the root phase of the tree of life, before the last common ancestor of life (LUCA), exit tunnel evolution is dominant and unremitting. Protein folding coevolved with evolution of the exit tunnel. The ribosome shows that protein folding initiated with intrinsic disorder, supported through a short, primitive exit tunnel. Folding progressed to thermodynamically stable β-structures and then to kinetically trapped α-structures. The latter were enabled by a long, mature exit tunnel that partially offset the general thermodynamic tendency of all polypeptides to form β-sheets. RNA chaperoned the evolution of protein folding from the very beginning. The universal common core of the ribosome, with a mass of nearly 2 million Daltons, was finalized by LUCA. The ribosome entered stasis after LUCA and remained in that state for billions of years. Bacterial ribosomes never left stasis. Archaeal ribosomes have remained near stasis, except for the superphylum Asgard, which has accreted rRNA post LUCA. Eukaryotic ribosomes in some lineages appear to be logarithmically accreting rRNA over the last billion years. Ribosomal expansion in Asgard and Eukarya has been incremental and iterative, without substantial remodeling of pre-existing basal structures. The ribosome preserves information on its history.

Comparisons between small ribosomal RNA and theoretical minimal RNA ring secondary structures confirm phylogenetic and structural accretion histories

Ribosomal RNAs are complex structures that presumably evolved by tRNA accretions. Statistical properties of tRNA secondary structures correlate with genetic code integration orders of their cognate amino acids. Ribosomal RNA secondary structures resemble those of tRNAs with recent cognates. Hence, rRNAs presumably evolved from ancestral tRNAs. Here, analyses compare secondary structure subcomponents of small ribosomal RNA subunits with secondary structures of theoretical minimal RNA rings, presumed proto-tRNAs. Two independent methods determined different accretion orders of rRNA structural subelements: (a) classical comparative homology and phylogenetic reconstruction, and (b) a structural hypothesis assuming an inverted onion ring growth where the three-dimensional ribosome's core is most ancient and peripheral elements most recent. Comparisons between (a) and (b) accretions orders with RNA ring secondary structure scales show that recent rRNA subelements are: 1. more like RNA rings with recent cognates, indicating ongoing coevolution between tRNA and rRNA secondary structures; 2. less similar to theoretical minimal RNA rings with ancient cognates. Our method fits (a) and (b) in all examined organisms, more with (a) than (b). Results stress the need to integrate independent methods. Theoretical minimal RNA rings are potential evolutionary references for any sequence-based evolutionary analyses, independent of the focal data from that study.

Transfer RNA: The molecular demiurge in the origin of biological systems

Herein, we review recent works on the role that the tRNA molecule played in the early origins of biological systems. tRNAs gave origin to the first genes (mRNA), the peptidyl transferase center (PTC), the 16S ribosomal molecule, proto-tRNAs were at the core of a proto-translation system, and the anticodon and operational codes appeared in tRNAs molecules. Metabolic pathways emerged from evolutionary pressures of the decoding systems. The transitions from the RNA world to the ribonucleoprotein world to modern biological systems were driven by two kinds of tRNAs transitions, to wit, tRNAs leading to both mRNA and rRNA.

Deamination gradients within codons after 1<->2 position swap predict amino acid hydrophobicity and parallel β-sheet conformational preference

Deaminations C->T and A->G are frequent mutations producing nucleotide content gradients across genomes proportional to singlestrandedness during replication/transcription. Hence, within single codons, deamination risks increase from first to third codon positions, while second codon positions are functionally most crucial. Here genetic codes are analyzed assuming that after anticodons protected codons from deaminations, first and second codon positions swapped (N2N1N3->N1N2N3), with lowest deamination risks for N2 in presumed primitive N2N1N3 codons. N2N1N3, not standard N1N2N3, codon structure minimizes deaminations inversely proportionally to cognate amino acid hydrophobicity and parallel betasheet conformational preference. For N1N2N3, deamination minimization increases with genetic code integration order of cognate amino acids: during the presumed N2N1N3->N1N2N3 codon structure transition, protein synthesis combined direct codon-amino acid interactions for late amino acids and tRNA-based translation for early amino acids. Hence N2N1N3 codons would correspond to tRNA-free translation by spontaneous codon-amino acid affinities, and tRNA-mediated translation presumably caused N2N1N3->N1N2N3 swaps. Results show that rational, not arbitrary rules link codon and amino acid structures. Some analyses detect mitochondrial RNAs and peptides in public data corresponding to systematic position swaps, suggesting occasional swapping polymerase activity.

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.

The primordial tRNA acceptor stem code from theoretical minimal RNA ring clusters

BACKGROUND

Theoretical minimal RNA rings code by design over the shortest length once for each of the 20 amino acids, a start and a stop codon, and form stem-loop hairpins. This defines at most 25 RNA rings of 22 nucleotides. As a group, RNA rings mimick numerous prebiotic and early life biomolecular properties: tRNAs, deamination gradients and replication origins, emergence of codon preferences for the natural circular code, and contents of early protein coding genes. These properties result from the RNA ring's in silico design, based mainly on coding nonredundancy among overlapping translation frames, as the genetic code's codon-amino acid assignments determine. RNA rings resemble ancestral tRNAs, defining RNA ring anticodons and corresponding cognate amino acids. Surprisingly, all examined RNA ring properties coevolve with genetic code integration ranks of RNA ring cognates, as if RNA rings mimick prebiotic and early life evolution.

METHODS

Distances between RNA rings were calculated using different evolutionary models. Associations between these distances and genetic code evolutionary hypotheses detect evolutionary models best describing RNA ring diversification.

RESULTS

Here pseudo-phylogenetic analyses of RNA rings produce clusters corresponding to the primordial code in tRNA acceptor stems, more so when substitution matrices from neutrally evolving pseudogenes are used rather than from functional protein coding genes reflecting selection for conserving amino acid properties.

CONCLUSIONS

Results indicate RNA rings with recent cognates evolved from those with early cognates. Hence RNA rings, as designed by the genetic code's structure, simulate tRNA stem evolution and prebiotic history along neutral chemistry-driven mutation regimes.

Ribosome Evolution and Structural Capacitance

In addition to the canonical loss-of-function mutations, mutations in proteins may additionally result in gain-of-function through the binary activation of cryptic "structural capacitance elements." Our previous bioinformatic analysis allowed us to propose a new mechanism of protein evolution - structural capacitance - that arises via the generation of new elements of microstructure upon mutations that cause a disorder-to-order (D→O) transition in previously disordered regions of proteins. Here we propose that the D→O transition is a necessary follow-on from expected early codon-anticodon and tRNA acceptor stem-amino acid usage, via the accumulation of structural capacitance elements - reservoirs of disorder in proteins. We develop this argument further to posit that structural capacitance is an inherent consequence of the evolution of the genetic code.

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.

Residue conservation elucidates the evolution of r-proteins in ribosomal assembly and function

Ribosomes are the translational machineries having two unequal subunits, small subunit (SSU) and large subunit (LSU) across all the domains of life. Origin and evolution of ribosome are encoded in its structure, and the core of the ribosome is highly conserved. Here, we have used Shannon entropy to analyze the evolution of ribosomal proteins (r-proteins) across the three domains of life. Moreover, we have analyzed the residue conservation at protein-protein (PP) and protein-RNA (PR) interfaces in SSU and LSU. Furthermore, we have studied the evolution of early, intermediate and late binding r-proteins. We show that the r-proteins of Thermus thermophilus are better conserved during the evolution. Furthermore, we find the late binders are better conserved than the early and the intermediate binders. The residues at the interior of the r-proteins are the most conserved followed by those at the interface and the solvent accessible surface. Additionally, we show that the residues at the PP interfaces are better conserved than those at the PR interfaces. However, between PR and PP interfaces, the multi-interface residues at the former are better conserved than those at the latter ones. Our findings may provide insights into the evolution of r-proteins in ribosomal assembly and function.

Viruses as a survival strategy in the armory of life

Viruses have generally been thought of as infectious agents. New data on mimivirus, however, suggests a reinterpretation of this thought. Earth's biosphere seems to contain many more viruses than previously thought and they are relevant in the maintenance of ecosystems and biodiversity. Viruses are not considered to be alive because they are not free-living entities and do not have cellular units. Current hypotheses indicate that some viruses may have been the result of genomic reduction of cellular life forms. However, new studies relating to the origins of biological systems suggest that viruses could also have originated during the transition from First to the Last Universal Common Ancestor (from FUCA to LUCA). Within this setting, life has been established as chemical informational system and could be interpreted as a macrocode of multiple layers. The first entity to acquire these features was the First Universal Common Ancestor (FUCA) that evolved to an intermediate ancestral that could be named T-LUCA (Transitional-LUCA) and be equated to Woese's concept of progenotes. T-LUCA may have remained as undifferentiated subsystems with viruses-like structures. The net result is that both cellular life forms and viruses shared protein synthesis apparatuses. In short, virus is a strategy of life reached by two paths: T-LUCAs like entities and the reduction of cellular life forms.

Emergence of Hierarchical Modularity in Evolving Networks Uncovered by Phylogenomic Analysis

Networks describe how parts associate with each other to form integrated systems which often have modular and hierarchical structure. In biology, network growth involves two processes, one that unifies and the other that diversifies. Here, we propose a biphasic (bow-tie) theory of module emergence. In the first phase, parts are at first weakly linked and associate variously. As they diversify, they compete with each other and are often selected for performance. The emerging interactions constrain their structure and associations. This causes parts to self-organize into modules with tight linkage. In the second phase, variants of the modules diversify and become new parts for a new generative cycle of higher level organization. The paradigm predicts the rise of hierarchical modularity in evolving networks at different timescales and complexity levels. Remarkably, phylogenomic analyses uncover this emergence in the rewiring of metabolomic and transcriptome-informed metabolic networks, the nanosecond dynamics of proteins, and evolving networks of metabolism, elementary functionomes, and protein domain organization.

Origin of the 16S Ribosomal Molecule from Ancestor tRNAs

We tested the hypothesis that concatemers of ancestral tRNAs gave rise to the 16S ribosomal RNA. We built an ancestral sequence of proto-tRNAs that showed a significant identity of 51.69% and a percentage of structural identity of 0.941 with the 16S ribosomal molecule. We also propose a hypothesis for the emergence of translation.

Nervous-Like Circuits in the Ribosome Facts, Hypotheses and Perspectives

In the past few decades, studies on translation have converged towards the metaphor of a “ribosome nanomachine”; they also revealed intriguing ribosome properties challenging this view. Many studies have shown that to perform an accurate protein synthesis in a fluctuating cellular environment, ribosomes sense, transfer information and even make decisions. This complex “behaviour” that goes far beyond the skills of a simple mechanical machine has suggested that the ribosomal protein networks could play a role equivalent to nervous circuits at a molecular scale to enable information transfer and processing during translation. We analyse here the significance of this analogy and establish a preliminary link between two fields: ribosome structure-function studies and the analysis of information processing systems. This cross-disciplinary analysis opens new perspectives about the mechanisms of information transfer and processing in ribosomes and may provide new conceptual frameworks for the understanding of the behaviours of unicellular organisms.

Evolution of the genetic code based on conservative changes of codons, amino acids, and aminoacyl tRNA synthetases

The genetic code, as arranged in the standard tabular form, displays a non-random structure relating to the characteristics of the amino acids. An alternative arrangement can be made by organizing the code according to aminoacyl-tRNA synthetases (aaRSs), codons, and reverse complement codons, which illuminates a coevolutionary process that led to the contemporary genetic code. As amino acids were added to the genetic code, they were recognized by aaRSs that interact with stereochemically similar amino acids. Single nucleotide changes in the codons and anticodons were favored over more extensive changes, such that there was a logical stepwise progression in the evolution of the genetic code. The model presented traces the evolution of the genetic code accounting for these steps. Amino acid frequencies in ancient proteins and the preponderance of GNN codons in mRNAs for ancient proteins indicate that the genetic code began with alanine, aspartate, glutamate, glycine, and valine, with alanine being in the highest proportions. In addition to being consistent in terms of conservative changes in codon nucleotides, the model also is consistent with respect to aaRS classes, aaRS attachment to the tRNA, amino acid stereochemistry, and to a large extent with amino acid physicochemistry, and biochemical pathways.

The Ribosome as a Missing Link in Prebiotic Evolution III: Over-Representation of tRNA- and rRNA-Like Sequences and Plieofunctionality of Ribosome-Related Molecules Argues for the Evolution of Primitive Genomes from Ribosomal RNA Modules

We propose that ribosomal RNA (rRNA) formed the basis of the first cellular genomes, and provide evidence from a review of relevant literature and proteonomic tests. We have proposed previously that the ribosome may represent the vestige of the first self-replicating entity in which rRNAs also functioned as genes that were transcribed into functional messenger RNAs (mRNAs) encoding ribosomal proteins. rRNAs also encoded polymerases to replicate itself and a full complement of the transfer RNAs (tRNAs) required to translate its genes. We explore here a further prediction of our “ribosome-first” theory: the ribosomal genome provided the basis for the first cellular genomes. Modern genomes should therefore contain an unexpectedly large percentage of tRNA- and rRNA-like modules derived from both sense and antisense reading frames, and these should encode non-ribosomal proteins, as well as ribosomal ones with key cell functions. Ribosomal proteins should also have been co-opted by cellular evolution to play extra-ribosomal functions. We review existing literature supporting these predictions. We provide additional, new data demonstrating that rRNA-like sequences occur at significantly higher frequencies than predicted on the basis of mRNA duplications or randomized RNA sequences. These data support our “ribosome-first” theory of cellular 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.

Integrated evolution of ribosomal RNAs, introns, and intron nurseries

The initial components of ribosomes first appeared more than 3.8 billion years ago during a time when many types of RNAs were evolving. While modern ribosomes are complex molecular machines consisting of rRNAs and proteins, they were assembled during early evolution by the association and joining of small functional RNA units. Introns may have provided the means to ligate many of these pieces together. All four classes of introns (group I, group II, spliceosomal, and archaeal) are present in many rRNA gene loci over a broad phylogenetic range. A survey of rRNA intron sequences across the three major life domains suggests that some of the classes of introns may have diverged from one another within rRNA gene loci. Analyses of rRNA sequences revealed self-splicing group I and group II introns are present in ancestral regions of the SSU (small subunit) and LSU (large subunit), whereas spliceosomal and archaeal introns appeared in sections of the rRNA that evolved later. Most classes of introns increased in number for approximately 1 billion years. However, their frequencies are low in the most recently evolved regions added to the SSU and LSU rRNAs. Furthermore, many of the introns appear to have been in the same locations for billions of years, suggesting an ancient origin for these sequences. In this Perspectives paper, I reviewed and analyzed rRNA intron sequences, locations, structural characteristics, and splicing mechanisms; and suggest that rRNA gene loci may have served as evolutionary nurseries for intron formation and diversification.

Possible Emergence of Sequence Specific RNA Aminoacylation via Peptide Intermediary to Initiate Darwinian Evolution and Code Through Origin of Life

One of the most intriguing questions in biological science is how life originated on Earth. A large number of hypotheses have been proposed to explain it, each putting an emphasis on different events leading to functional translation and self-sustained system. Here, we propose a set of interactions that could have taken place in the prebiotic environment. According to our hypothesis, hybridization-induced proximity of short aminoacylated RNAs led to the synthesis of peptides of random sequence. We postulate that among these emerged a type of peptide(s) capable of stimulating the interaction between specific RNAs and specific amino acids, which we call "bridge peptide" (BP). We conclude that translation should have emerged at the same time when the standard genetic code begun to evolve due to the stabilizing effect on RNA-peptide complexes with the help of BPs. Ribosomes, ribozymes, and the enzyme-directed RNA replication could co-evolve within the same period, as logical outcome of RNA-peptide world without the need of RNA only self-sustained step.

Evolution of macromolecular structure: a 'double tale' of biological accretion and diversification

The evolution of structure in biology is driven by accretion and diversification. Accretion brings together disparate parts to form bigger wholes. Diversification provides opportunities for growth and innovation. Here, we review patterns and processes that are responsible for a 'double tale' of accretion and diversification at various levels of complexity, from proteins and nucleic acids to high-rise building structures in cities. Parts are at first weakly linked and associate variously. As they diversify, they compete with each other and are selected for performance. The emerging interactions constrain their structure and associations. This causes parts to self-organise into modules with tight linkage. In a second phase, variants of the modules evolve and become new parts for a new generative cycle of higher-level organisation. Evolutionary genomics and network biology support the 'double tale' of structural module creation and validate an evolutionary principle of maximum abundance that drives the gain and loss of modules.

In the Beginning was a Mutualism - On the Origin of Translation

The origin of translation is critical for understanding the evolution of life, including the origins of life. The canonical genetic code is one of the most dominant aspects of life on this planet, while the origin of heredity is one of the key evolutionary transitions in living world. Why the translation apparatus evolved is one of the enduring mysteries of molecular biology. Assuming the hypothesis, that during the emergence of life evolution had to first involve autocatalytic systems which only subsequently acquired the capacity of genetic heredity, we propose and discuss possible mechanisms, basic aspects of the emergence and subsequent molecular evolution of translation and ribosomes, as well as enzymes as we know them today. It is possible, in this sense, to view the ribosome as a digital-to-analogue information converter. The proposed mechanism is based on the abilities and tendencies of short RNA and polypeptides to fold and to catalyse biochemical reactions. The proposed mechanism is in concordance with the hypothesis of a possible chemical co-evolution of RNA and proteins in the origin of the genetic code or even more generally at the early evolution of life on Earth. The possible abundance and availability of monomers at prebiotic conditions are considered in the mechanism. The hypothesis that early polypeptides were folding on the RNA scaffold is also considered and mutualism in molecular evolutionary development of RNA and peptides is favoured.

Stem-Loop RNA Hairpins in Giant Viruses: Invading rRNA-Like Repeats and a Template Free RNA

We examine the hypothesis that de novo template-free RNAs still form spontaneously, as they did at the origins of life, invade modern genomes, contribute new genetic material. Previously, analyses of RNA secondary structures suggested that some RNAs resembling ancestral (t)RNAs formed recently de novo, other parasitic sequences cluster with rRNAs. Here positive control analyses of additional RNA secondary structures confirm ancestral and de novo statuses of RNA grouped according to secondary structure. Viroids with branched stems resemble de novo RNAs, rod-shaped viroids resemble rRNA secondary structures, independently of GC contents. 5' UTR leading regions of West Nile and Dengue flavivirid viruses resemble de novo and rRNA structures, respectively. An RNA homologous with Megavirus, Dengue and West Nile genomes, copperhead snake microsatellites and levant cotton repeats, not templated by Mimivirus' genome, persists throughout Mimivirus' infection. Its secondary structure clusters with candidate de novo RNAs. The saltatory phyletic distribution and secondary structure of Mimivirus' peculiar RNA suggest occasional template-free polymerization of this sequence, rather than noncanonical transcriptions (swinger polymerization, posttranscriptional editing).

Insuperable problems of the genetic code initially emerging in an RNA world

Differential equations for error-prone information transfer (template replication, transcription or translation) are developed in order to consider, within the theory of autocatalysis, the advent of coded protein synthesis. Variations of these equations furnish a basis for comparing the plausibility of contrasting scenarios for the emergence of specific tRNA aminoacylation, ultimately by enzymes, and the relationship of this process with the origin of the universal system of molecular biological information processing embodied in the Central Dogma. The hypothetical RNA World does not furnish an adequate basis for explaining how this system came into being, but principles of self-organisation that transcend Darwinian natural selection furnish an unexpectedly robust basis for a rapid, concerted transition to genetic coding from a peptide·RNA world.