Structural biology. The ribosome is a ribozyme

Proteomics-based characterization of ribosome heterogeneity in adult mouse organs

The translation process, leading to protein synthesis from mRNA, has been long thought to be invariable in all cellular organisms. Increasing evidence shows that it is finely regulated by variable features of the translation machinery. Notably, ribosomes, the functional units of protein synthesis, are suggested to display variations in their composition, depending on the developmental stage, cell type or physio-pathological context, thus hinting a new level of actionable regulation of gene expression. Yet, a comprehensive map of the heterogeneity of ribosome composition in ribosomal proteins (RPs) in different organs and tissues is not available. In this work, we explored tissue-specific ribosome heterogeneity using mass spectrometry-based quantitative proteomic characterization of ribosomal fractions purified from 14 adult mouse organs and tissues. We performed crossed clustering and statistical analyses of RP composition to highlight stable, variable and tissue-specific RPs across organs and tissues. Focusing on specific RPs, we validated their varying abundances using a targeted proteomic approach and western blot analyses, providing further insights into the tissue-specific ribosome RP signature. Finally, we investigated the origin of RP variations in ribosome fraction of the different tissues, by comparing RP relative amounts in our ribosomal proteomic dataset with their corresponding transcript abundances in three independent transcriptomic datasets. Interestingly, we found that, in some tissues, the RP abundance in purified ribosomes does not always correlate with the corresponding RP transcript level, arguing for a translational regulation of RP expression, and/or a regulated incorporation of RPs into ribosomes. Altogether, our data support the notion of a tissue-specific RP signature of ribosomes, which opens avenues to study how specific ribosomal composition provides an additional level of regulation to control gene expression in different tissues and organs.

Direct RNA sequencing of the Escherichia coli epitranscriptome uncovers alterations under heat stress

Modifications of RNA, known as the epitranscriptome, affect gene expression, translation, and splicing in eukaryotes, with implications for developmental processes, cancer, and viral infections. In prokaryotes, regulation at the level of the epitranscriptome is still poorly understood. Here, we used nanopore direct RNA sequencing of Escherichia coli to study RNA modifications and their changes under heat stress. With a single sequencing reaction, we detected most known modification types in ribosomal RNA (rRNA), transfer RNA (tRNA), and messenger RNA (mRNA). RNA sequencing was complemented by a multifaceted approach that included mass spectrometry, deletion mutants, single-nucleotide polymerase chain reaction, and in vitro methylation. Known 5-methylcytidine (m5C) and N6-methyladenosine (m6A) sites in the rRNA were confirmed, but these types of modifications could not be localized in the mRNA. In response to heat stress, levels of m5C, m6A, and N6,N6-dimethyladenosine increased in the 16S rRNA. Sequencing and mass spectrometry data demonstrated a decrease in tRNA modification abundance in the anticodon loop at 45°C. In general, mRNA modifications at 37°C were enriched in the coding regions of genes associated with general metabolism and RNA processing, which shifted to genes involved in cell wall synthesis and membrane transport under heat stress. This study provides new insights into the complexity of post-transcriptional regulation in bacteria.

Prebiotic RNA self-assembling and the origin of life: Mechanistic and molecular modeling rationale for explaining the prebiotic origin and replication of RNA

In recent years, a great interest has been focused on the prebiotic origin of nucleic acids and life on Earth. An attractive idea is that life was initially based on an autocatalytic and autoreplicative RNA (the RNA-world). RNA duplexes are right-handed helical chains with antiparallel orientation, but the rationale for these features is not yet known. An antiparallel (inverted) stacking of purine nucleosides was reported in crystallographic studies. Molecular modeling also supports the inverted orientation of nucleosides. This preferential stacking can also appear when nucleosides are included in a montmorillonite clay matrix. Free-energy values and geometrical parameters show that D-ribose chirality is preferred for the formation of right-handed RNA molecules. Thus, a "zipper" model with antiparallel and auto-intercalated nucleosides linked by phosphate groups can be proposed to form single RNA chains. Unstacking with strand separation and base pairing by H-bonding, results in shortening and inclination of ribose-phosphate chains, leading to right-handed helicity and antiparallel duplexes. Incorporation of complementary precursors on the major groove template by a self-assembly mechanism provides a prebiotic (non-enzymatic) "tetris" replication model by formation of a transient RNA tetrad and tetraplex. Original hairpin motifs appear as simple building units that form typical RNA structures such as hammerheads, cloverleaves and dumbbells. They occur today in the circular viroids and virusoids, as well as in highly branched and complex rRNA molecules.

Effects of lipid membranes on RNA catalytic activity and stability

BACKGOUND INFORMATION

RNA plays crucial roles in cellular organization and metabolism, and modulating its activity is essential for maintaining cellular functions. RNA activity, involving both catalytic (ribozymes) and translation processes, is controlled via myriad mechanisms involving different binding partners such as proteins and smaller polar solutes. We previously reported that lipid membranes can directly interact with the artificial R3C ribozyme changing its activity, however, the effect of lipids on naturally occurring ribozymes remains unknown.

RESULTS

Here, we report that both catalytic activity as well as RNA integrity can be controlled by the presence of different lipid membranes. Gel-phase lipid membranes decreased the activity of hepatitis delta virus ribozyme and increased the activity of a hammerhead ribozyme. The presence of lipid liquid membrane surfaces triggered RNA degradation with greater degradation occurring in the single-stranded regions of RNA.

CONCLUSION

The interplay between RNA activity and stability in the presence of different lipid membranes introduces multiple possibilities, where different combinations of ribozyme and lipid membrane composition could produce different effects on activity.

SIGNIFICANCE

Taken together, these observations support the hypothesis that the activity of both natural and artificial RNAs can be modulated by lipid membranes which, in turn, provides a foundation for the development of novel riboswitch-like molecules, and lipid membrane-based RNA-biosensors.

The Plethora of RNA-Protein Interactions Model a Basis for RNA Therapies

The notion of RNA-based therapeutics has gained wide attractions in both academic and commercial institutions. RNA is a polymer of nucleic acids that has been proven to be impressively versatile, dating to its hypothesized RNA World origins, evidenced by its enzymatic roles in facilitating DNA replication, mRNA decay, and protein synthesis. This is underscored through the activities of riboswitches, spliceosomes, ribosomes, and telomerases. Given its broad range of interactions within the cell, RNA can be targeted by a therapeutic or modified as a pharmacologic scaffold for diseases such as nucleotide repeat disorders, infectious diseases, and cancer. RNA therapeutic techniques that have been researched include, but are not limited to, CRISPR/Cas gene editing, anti-sense oligonucleotides (ASOs), siRNA, small molecule treatments, and RNA aptamers. The knowledge gleaned from studying RNA-centric mechanisms will inevitably improve the design of RNA-based therapeutics. Building on this understanding, we explore the physiological diversity of RNA functions, examine specific dysfunctions, such as splicing errors and viral interactions, and discuss their therapeutic implications.

Transcriptome sequencing and anthocyanin metabolite analysis involved in leaf red color formation of Cinnamomum camphora

Cinnamomum camphora, a key multifunctional tree species, primarily serves in landscaping. Leaf color is crucial for its ornamental appeal, undergoing a transformation to red that enhances the ornamental value of C. camphora. However, the molecular mechanisms underlying this transformation remain largely unexplored. In this study, green leaf (GL), color turning red leaf (RL) and whole red leaf (WRL) were obtained to measure pigment contents, while GL and RL were analyzed for transcriptomic alterations. A decline in chlorophyll content and a rise in anthocyanins were observed during the transition from green to red leaves. Using LC MS/MS, 11 types of anthocyanins showed significant accumulative differences, with cyanidin-3,5-O-diglucoside exhibiting the greatest disparity. Comparative RNA-seq identified 22,948 genes against reference genes, revealing 544 novel genes. Of these, 3,222 genes were up-regulated and 7,391 genes were down-regulated when the FPKM mean value > 1 in at least one group. The ribosome was identified as the most abundant KEGG term, with a substantial number of down-regulated differentially expressed genes (DEGs). The results indicated a downward trend in protein content, with GL exhibiting the highest protein concentration. 22, 4, and 29 DEGs were associated with chlorophyll biosynthesis, chlorophyll degradation, and anthocyanin biosynthesis, respectively. Most DEGs related to chlorophyll biosynthesis were down-regulated. SGR and SGRL, which are associated with chlorophyll degradation, exhibited opposite differential expression, resulting in a significant decrease in chlorophyll content in RL. The significantly up-regulated genes ANS and UFGT are advantageous for anthocyanin biosynthesis, contributing to the red coloration observed. Additionally, differential expression was noted in 40 R2R3-MYBs. Two MYB90 (Ccam01G003512 and Ccam01G003515) homologs of AtMYB113 were also identified showed high levels of up-regulation in RL. These findings suggest a strong correlation between pigment metabolism and transcriptome data, elucidating the mechanism that leads to the red coloration of leaves in C. camphora.

Canada's contributions to RNA research: past, present, and future perspectives

The field of RNA research has provided profound insights into the basic mechanisms modulating the function and adaption of biological systems. RNA has also been at the center stage in the development of transformative biotechnological and medical applications, perhaps most notably was the advent of mRNA vaccines that were critical in helping humanity through the Covid-19 pandemic. Unbeknownst to many, Canada boasts a diverse community of RNA scientists, spanning multiple disciplines and locations, whose cutting-edge research has established a rich track record of contributions across various aspects of RNA science over many decades. Through this position paper, we seek to highlight key contributions made by Canadian investigators to the RNA field, via both thematic and historical viewpoints. We also discuss initiatives underway to organize and enhance the impact of the Canadian RNA research community, particularly focusing on the creation of the not-for-profit organization RNA Canada ARN. Considering the strategic importance of RNA research in biology and medicine, and its considerable potential to help address major challenges facing humanity, sustained support of this sector will be critical to help Canadian scientists play key roles in the ongoing RNA revolution and the many benefits this could bring about to Canada.

A Deep Learning-Driven Sampling Technique to Explore the Phase Space of an RNA Stem-Loop

The folding and unfolding of RNA stem-loops are critical biological processes; however, their computational studies are often hampered by the ruggedness of their folding landscape, necessitating long simulation times at the atomistic scale. Here, we adapted DeepDriveMD (DDMD), an advanced deep learning-driven sampling technique originally developed for protein folding, to address the challenges of RNA stem-loop folding. Although tempering- and order parameter-based techniques are commonly used for similar rare-event problems, the computational costs or the need for a priori knowledge about the system often present a challenge in their effective use. DDMD overcomes these challenges by adaptively learning from an ensemble of running MD simulations using generic contact maps as the raw input. DeepDriveMD enables on-the-fly learning of a low-dimensional latent representation and guides the simulation toward the undersampled regions while optimizing the resources to explore the relevant parts of the phase space. We showed that DDMD estimates the free energy landscape of the RNA stem-loop reasonably well at room temperature. Our simulation framework runs at a constant temperature without external biasing potential, hence preserving the information on transition rates, with a computational cost much lower than that of the simulations performed with external biasing potentials. We also introduced a reweighting strategy for obtaining unbiased free energy surfaces and presented a qualitative analysis of the latent space. This analysis showed that the latent space captures the relevant slow degrees of freedom for the RNA folding problem of interest. Finally, throughout the manuscript, we outlined how different parameters are selected and optimized to adapt DDMD for this system. We believe this compendium of decision-making processes will help new users adapt this technique for the rare-event sampling problems of their interest.

Maize Dek51 encodes a DEAD-box RNA helicase essential for pre-rRNA processing and seed development

Pre-rRNA processing is essential to ribosome biosynthesis. However, the processing mechanism is not fully understood in plants. Here, we report a DEAD-box RNA helicase DEK51 that mediates the 3' end processing of 18S and 5.8S pre-rRNA in maize (Zea mays L.). DEK51 is localized in the nucleolus, and loss of DEK51 arrests maize seed development and blocks the 3' end processing of 18S and 5.8S pre-rRNA. DEK51 interacts with putative key factors in nuclear RNA exosome-mediated pre-rRNA processing, including ZmMTR4, ZmSMO4, ZmRRP44A, and ZmRRP6L2. This suggests that DEK51 facilitates pre-rRNA processing by interacting with the exosome. Loss of ZmMTR4 function arrests seed development and blocks the 3' end processing of 18S and 5.8S pre-rRNA, similar to dek51. DEK51 also interacts with endonucleases ZmUTP24 and ZmRCL1, suggesting that it may also be involved in the cleavage at site A2. These results show the critical role of DEK51 in promoting 3' end processing of pre-rRNA.

Pioneering role of RNA in the early evolution of life

The catalytic, regulatory and structural properties of RNA, combined with their extraordinary ubiquity in cellular processes, are consistent with the proposal that this molecule played a much more conspicuous role in heredity and metabolism during the early stages of biological evolution. This review explores the pivotal role of RNA in the earliest life forms and its relevance in modern biological systems. It examines current models that study the early evolution of life, providing insights into the primordial RNA world and its legacy in contemporary biology.

YjgA plays dual roles in enhancing PTC maturation

Ribosome biogenesis is a highly regulated cellular process that involves the control of numerous assembly factors. The small protein YjgA has been reported to play a role in the late stages of 50S assembly. However, the precise molecular mechanism underlying its function remains unclear. In this study, cryo-electron microscopy (cryo-EM) structures revealed that depletion of YjgA or its N-terminal loop in Escherichia coli both lead to the accumulation of immature 50S particles with structural abnormalities mainly in peptidyl transferase center (PTC) and H68/69 region. CryoDRGN analysis uncovered 8 and 6 distinct conformations of pre50S for ΔyjgA and YjgA-ΔNloop, respectively. These conformations highlighted the role of the N-terminal loop of YjgA in integrating uL16 and stabilizing H89 in PTC, which was further verified by the pull-down assays of YjgA and its mutants with uL16. Together with the function of undocking H68 through the binding of its C-terminal CTLH-like domain to the base of the L1 stalk, YjgA facilitates the maturation of PTC. This study identified critical domains of YjgA contributing to 50S assembly efficiency, providing a comprehensive understanding of the dual roles of YjgA in accelerating ribosome biogenesis and expanding our knowledge of the intricate processes governing cellular protein synthesis.

Massively parallel dissection of RNA in RNA-protein interactions in vivo

Many of the biological functions performed by RNA are mediated by RNA-binding proteins (RBPs), and understanding the molecular basis of these interactions is fundamental to biology. Here, we present massively parallel RNA assay combined with immunoprecipitation (MPRNA-IP) for in vivo high-throughput dissection of RNA-protein interactions and describe statistical models for identifying RNA domains and parsing the structural contributions of RNA. By using custom pools of tens of thousands of RNA sequences containing systematically designed truncations and mutations, MPRNA-IP is able to identify RNA domains, sequences, and secondary structures necessary and sufficient for protein binding in a single experiment. We show that this approach is successful for multiple RNAs of interest, including the long noncoding RNA NORAD, bacteriophage MS2 RNA, and human telomerase RNA, and we use it to interrogate the hitherto unknown sequence or structural RNA-binding preferences of the DNA-looping factor CTCF. By integrating systematic mutation analysis with crosslinking immunoprecipitation, MPRNA-IP provides a novel high-throughput way to elucidate RNA-based mechanisms behind RNA-protein interactions in vivo.

Epigenetics and transgenerational inheritance

Epigenetic modifications can alter the function of genes. The epigenetics changes are caused by environmental effects, which lead to chemical modifications of the DNA or the chromatin. The mechanisms involve the influence of small interfering siRNAs on gene silencing. Epigenetic changes normally last only during the life-time of an individual and are erased in embryos and eggs for a naive progeny. The genomes are reprogrammed and the chemical modifications removed to restart the next generation. However, there are mechanisms that allow the genome to escape from such a clearing effect so that modifications can be transmitted to one or more subsequent generations. In the germline of animal cells small RNAs, including piRNAs, have evolved which guarantee a higher degree of fidelity for transmission of genetic information, guarding especially against the detrimental effect caused by transposon activity. piRNA is essential for transposon silencing for survival of a species and protection of subsequent generations. Inactivation of piRNA results in abundant transposon activity and sperm infertility. The effect in humans has been described but is less distinct. Some stress-induced epigenetic changes are transitory in mice and can be reversed by a change of environment or lifestyle.

Emergent ribozyme behaviors in oxychlorine brines indicate a unique niche for molecular evolution on Mars

Mars is a particularly attractive candidate among known astronomical objects to potentially host life. Results from space exploration missions have provided insights into Martian geochemistry that indicate oxychlorine species, particularly perchlorate, are ubiquitous features of the Martian geochemical landscape. Perchlorate presents potential obstacles for known forms of life due to its toxicity. However, it can also provide potential benefits, such as producing brines by deliquescence, like those thought to exist on present-day Mars. Here we show perchlorate brines support folding and catalysis of functional RNAs, while inactivating representative protein enzymes. Additionally, we show perchlorate and other oxychlorine species enable ribozyme functions, including homeostasis-like regulatory behavior and ribozyme-catalyzed chlorination of organic molecules. We suggest nucleic acids are uniquely well-suited to hypersaline Martian environments. Furthermore, Martian near- or subsurface oxychlorine brines, and brines found in potential lifeforms, could provide a unique niche for biomolecular evolution.

Roles of Nucleic Acids in Protein Folding, Aggregation, and Disease

The role of nucleic acids in protein folding and aggregation is an area of continued research, with relevance to understanding both basic biological processes and disease. In this review, we provide an overview of the trajectory of research on both nucleic acids as chaperones and their roles in several protein misfolding diseases. We highlight key questions that remain on the biophysical and biochemical specifics of how nucleic acids have large effects on multiple proteins' folding and aggregation behavior and how this pertains to multiple protein misfolding diseases.

When Paul Berg meets Donald Crothers: an achiral connection through protein biosynthesis

Outliers in scientific observations are often ignored and mostly remain unreported. However, presenting them is always beneficial since they could reflect the actual anomalies that might open new avenues. Here, we describe two examples of the above that came out of the laboratories of two of the pioneers of nucleic acid research in the area of protein biosynthesis, Paul Berg and Donald Crothers. Their work on the identification of D-aminoacyl-tRNA deacylase (DTD) and 'Discriminator hypothesis', respectively, were hugely ahead of their time and were partly against the general paradigm at that time. In both of the above works, the smallest and the only achiral amino acid turned out to be an outlier as DTD can act weakly on glycine charged tRNAs with a unique discriminator base of 'Uracil'. This peculiar nature of glycine remained an enigma for nearly half a century. With a load of available information on the subject by the turn of the century, our work on 'chiral proofreading' mechanisms during protein biosynthesis serendipitously led us to revisit these findings. Here, we describe how we uncovered an unexpected connection between them that has implications for evolution of different eukaryotic life forms.

RNA therapeutics for infectious diseases

Ribonucleic acids (RNAs), including the messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA), play important roles in living organisms and viruses. In recent years, the RNA-based technologies including the RNAs inhibiting other RNA activities, the RNAs targeting proteins, the RNAs reprograming genetic information, and the RNAs encoding therapeutical proteins, are useful methods to apply in prophylactic and therapeutic vaccines. In this review, we summarize and highlight the current application of the RNA therapeutics, especially on mRNA vaccines which have potential for prevention and treatment against human and animal infectious diseases.

Pairing metagenomics and metaproteomics to characterize ecological niches and metabolic essentiality of gut microbiomes

The genome of a microorganism encodes its potential functions that can be implemented through expressed proteins. It remains elusive how a protein's selective expression depends on its metabolic essentiality to microbial growth or its ability to claim resources as ecological niches. To reveal a protein's metabolic or ecological role, we developed a computational pipeline, which pairs metagenomics and metaproteomics data to quantify each protein's gene-level and protein-level functional redundancy simultaneously. We first illustrated the idea behind the pipeline using simulated data of a consumer-resource model. We then validated it using real data from human and mouse gut microbiome samples. In particular, we analyzed ABC-type transporters and ribosomal proteins, confirming that the metabolic and ecological roles predicted by our pipeline agree well with prior knowledge. Finally, we performed in vitro cultures of a human gut microbiome sample and investigated how oversupplying various sugars involved in ecological niches influences the community structure and protein abundance. The presented results demonstrate the performance of our pipeline in identifying proteins' metabolic and ecological roles, as well as its potential to help us design nutrient interventions to modulate the human microbiome.

Arabidopsis NOTCHLESS plays an important role in root and embryo development

Ribosome biogenesis is a fundamental process in eukaryotic cells. NOTCHLESS (NLE) is involved in 60S ribosome biogenesis in yeast, but its role in Arabidopsis (A. thaliana) remains exclusive. Here, we found that Arabidopsis NLE (AtNLE) is highly conservative in phylogeny, which encoding a WD40-repeat protein. AtNLE is expressed in actively dividing tissues. AtNLE-GFP is localized in the nucleus. AtNLE physically interacts with the MIDAS domain of AtMDN1, a protein involved in the biogenesis of the 60S ribosomal subunit in Arabidopsis. The underexpressing mutant nle-2 shows short roots and reduced cell number in the root meristem. In addition, the null mutant nle-1 is embryo lethal, and defective embryos are arrested at the early globular stage. This work suggests that AtNLE interacts with AtMDN1, and AtNLE functions in root and embryo development.

White and green rust chimneys accumulate RNA in a ferruginous chemical garden

Mechanisms of nucleic acid accumulation were likely critical to life's emergence in the ferruginous oceans of the early Earth. How exactly prebiotic geological settings accumulated nucleic acids from dilute aqueous solutions, is poorly understood. As a possible solution to this concentration problem, we simulated the conditions of prebiotic low-temperature alkaline hydrothermal vents in co-precipitation experiments to investigate the potential of ferruginous chemical gardens to accumulate nucleic acids via sorption. The injection of an alkaline solution into an artificial ferruginous solution under anoxic conditions (O2  < 0.01% of present atmospheric levels) and at ambient temperatures, caused the precipitation of amakinite ("white rust"), which quickly converted to chloride-containing fougerite ("green rust"). RNA was only extractable from the ferruginous solution in the presence of a phosphate buffer, suggesting RNA in solution was bound to Fe2+ ions. During chimney formation, this iron-bound RNA rapidly accumulated in the white and green rust chimney structure from the surrounding ferruginous solution at the fastest rates in the initial white rust phase and correspondingly slower rates in the following green rust phase. This represents a new mechanism for nucleic acid accumulation in the ferruginous oceans of the early Earth, in addition to wet-dry cycles and may have helped to concentrate RNA in a dilute prebiotic ocean.

Ancestry of RNA/RNA interaction regions within segmented ribosomes

The ribosome is the universally conserved ribozyme that translates DNA coded instructions into proteins with the assistance of other RNA molecules, including transfer and messenger RNAs. Of particular interest is the segmentation phenomena, which is found in trypanosomatids and other protists. In these organisms, the large subunit ribosomal RNA is assembled from multiple smaller RNAs. This phenomenon posits several challenges to the folding and stabilization of such ribosomes to retain functionality and efficiency. In earlier studies, RNA/protein interactions were suggested to fully compensate for the fragmentation. Recently, several conserved RNA/RNA interaction regions were described in the cryo-EM structures of segmented ribosomes from trypanosomatids. These regions also seemed to aid in the folding and stabilization of such ribosomes, even before the ribosomal proteins start their association. In the present study, the existence of conserved RNA/RNA interaction regions shared between trypanosomatid and Euglena gracilis segmented ribosomes was confirmed, despite differences in segmentation patterns. Analysis of the crystallographic structures of unsegmented ribosomes from other Eukaryotes, Bacteria, and Archaea allowed us to estimate the relative age of highly conserved RNA/RNA interaction regions. These results strongly suggest that common interaction regions likely date far back into the ribosomes of the last common ancestor. Results also revealed that single hydrogen bonds are overwhelmingly facilitated by the 2'OH, a distinctive RNA feature. This supports the notion that RNA predates DNA and places some constraints on alternative nucleic acids proposals.

Unleashing the potential of peptides in agriculture and beyond

Peptides display a broad range of regulatory functions. Ormancey et al. recently identified an important new mechanism - complementary peptides (cPEPs) - that provide a versatile means to control cell functions. We draw a parallel between RNA and peptide biology, and discuss new routes of investigation and industrial applications opened by this work.

Atomistic simulations of the Escherichia coli ribosome provide selection criteria for translationally active substrates

As genetic code expansion advances beyond L-α-amino acids to backbone modifications and new polymerization chemistries, delineating what substrates the ribosome can accommodate remains a challenge. The Escherichia coli ribosome tolerates non-L-α-amino acids in vitro, but few structural insights that explain how are available, and the boundary conditions for efficient bond formation are so far unknown. Here we determine a high-resolution cryogenic electron microscopy structure of the E. coli ribosome containing α-amino acid monomers and use metadynamics simulations to define energy surface minima and understand incorporation efficiencies. Reactive monomers across diverse structural classes favour a conformational space where the aminoacyl-tRNA nucleophile is <4 Å from the peptidyl-tRNA carbonyl with a Bürgi-Dunitz angle of 76-115°. Monomers with free energy minima that fall outside this conformational space do not react efficiently. This insight should accelerate the in vivo and in vitro ribosomal synthesis of sequence-defined, non-peptide heterooligomers.

Overexpressing Ribosomal Protein L16D Affects Leaf Development but Confers Pathogen Resistance in Arabidopsis

In plant cells, multiple paralogs from ribosomal protein (RP) families are always synchronously expressed, which is likely contributing to ribosome heterogeneity or functional specialization. However, previous studies have shown that most RP mutants share common phenotypes. Consequently, it is difficult to distinguish whether the phenotypes of the mutants have resulted from the loss of specific genes or a global ribosome deficiency. Here, to investigate the role of a specific RP gene, we employed a gene overexpression strategy. We found that Arabidopsis lines overexpressing RPL16D (L16D-OEs) display short and curled rosette leaves. Microscopic observations reveal that both the cell size and cell arrangement are affected in L16D-OEs. The severity of the defect is positively correlated with RPL16D dosage. By combining transcriptomic and proteomic profiling, we found that overexpressing RPL16D decreases the expression of genes involved in plant growth, but increases the expression of genes involved in immune response. Overall, our results suggest that RPL16D is involved in the balance between plant growth and immune response.

Biological Catalysis and Information Storage Have Relied on N-Glycosyl Derivatives of β-D-Ribofuranose since the Origins of Life

Most naturally occurring nucleotides and nucleosides are N-glycosyl derivatives of β-d-ribose. These N-ribosides are involved in most metabolic processes that occur in cells. They are essential components of nucleic acids, forming the basis for genetic information storage and flow. Moreover, these compounds are involved in numerous catalytic processes, including chemical energy production and storage, in which they serve as cofactors or coribozymes. From a chemical point of view, the overall structure of nucleotides and nucleosides is very similar and simple. However, their unique chemical and structural features render these compounds versatile building blocks that are crucial for life processes in all known organisms. Notably, the universal function of these compounds in encoding genetic information and cellular catalysis strongly suggests their essential role in the origins of life. In this review, we summarize major issues related to the role of N-ribosides in biological systems, especially in the context of the origin of life and its further evolution, through the RNA-based World(s), toward the life we observe today. We also discuss possible reasons why life has arisen from derivatives of β-d-ribofuranose instead of compounds based on other sugar moieties.

Nucleotides Entrapped in Liposome Nanovesicles as Tools for Therapeutic and Diagnostic Use in Biomedical Applications

The use of nucleotides for biomedical applications is an old desire in the scientific community. As we will present here, there are references published over the past 40 years with this intended use. The main problem is that, as unstable molecules, nucleotides require some additional protection to extend their shelf life in the biological environment. Among the different nucleotide carriers, the nano-sized liposomes proved to be an effective strategic tool to overcome all these drawbacks related to the nucleotide high instability. Moreover, due to their low immunogenicity and easy preparation, the liposomes were selected as the main strategy for delivery of the mRNA developed for COVID-19 immunization. For sure this is the most important and relevant example of nucleotide application for human biomedical conditions. In addition, the use of mRNA vaccines for COVID-19 has increased interest in the application of this type of technology to other health conditions. For this review article, we will present some of these examples, especially focused on the use of liposomes to protect and deliver nucleotides for cancer therapy, immunostimulatory activities, enzymatic diagnostic applications, some examples for veterinarian use, and the treatment of neglected tropical disease.

How to build a protoribosome: structural insights from the first protoribosome constructs that have proven to be catalytically active

The modern ribosome catalyzes all coded protein synthesis in extant organisms. It is likely that its core structure is a direct descendant from the ribosome present in the last common ancestor (LCA). Hence, its earliest origins likely predate the LCA and therefore date further back in time. Of special interest is the pseudosymmetrical region (SymR) that lies deep within the large subunit (LSU) where the peptidyl transfer reaction takes place. It was previously proposed that two RNA oligomers, representing the P- and A-regions of extant ribosomes dimerized to create a pore-like structure, which hosted the necessary properties that facilitate peptide bond formation. However, recent experimental studies show that this may not be the case. Instead, several RNA constructs derived exclusively from the P-region were shown to form a homodimer capable of peptide bond synthesis. Of special interest will be the origin issues because the homodimer would have allowed a pre-LCA ribosome that was significantly smaller than previously proposed. For the A-region, the immediate issue will likely be its origin and whether it enhances ribosome performance. Here, we reanalyze the RNA/RNA interaction regions that most likely lead to SymR formation in light of these recent findings. Further, it has been suggested that the ability of these RNA constructs to dimerize and enhance peptide bond formation is sequence-dependent. We have analyzed the implications of sequence variations as parts of functional and nonfunctional constructs.

Ribosome biogenesis factors-from names to functions

The assembly of ribosomal subunits is a highly orchestrated process that involves a huge cohort of accessory factors. Most eukaryotic ribosome biogenesis factors were first identified by genetic screens and proteomic approaches of pre-ribosomal particles in Saccharomyces cerevisiae. Later, research on human ribosome synthesis not only demonstrated that the requirement for many of these factors is conserved in evolution, but also revealed the involvement of additional players, reflecting a more complex assembly pathway in mammalian cells. Yet, it remained a challenge for the field to assign a function to many of the identified factors and to reveal their molecular mode of action. Over the past decade, structural, biochemical, and cellular studies have largely filled this gap in knowledge and led to a detailed understanding of the molecular role that many of the players have during the stepwise process of ribosome maturation. Such detailed knowledge of the function of ribosome biogenesis factors will be key to further understand and better treat diseases linked to disturbed ribosome assembly, including ribosomopathies, as well as different types of cancer.

The discovery and development of RNA-based therapies for treatment of HIV-1 infection

INTRODUCTION

Long-term control of HIV-1 infection can potentially be achieved using autologous stem cell transplants with gene-modified cells. Non-coding RNAs represent a diverse class of therapeutic agents including ribozymes, RNA aptamers and decoys, small interfering RNAs, short hairpin RNAs, and U1 interference RNAs that can be designed to inhibit HIV-1 replication. They have been engineered for delivery as drugs to complement current HIV-1 therapies and as gene therapies for a potential HIV-1 functional cure.

AREAS COVERED

This review surveys the past three decades of development of these RNA technologies with a focus on their efficacy and safety for treating HIV-1 infections. We describe the mechanisms of each RNA-based agent, targets they have been developed against, efforts to enhance their stability and efficacy, and we evaluate their performance in past and ongoing preclinical and clinical trials.

EXPERT OPINION

RNA-based technologies are among the top candidates for gene therapies where they can be stably expressed for long-term suppression of HIV-1. Advances in both gene and drug delivery strategies and improvements to non-coding RNA stability and antiviral properties will cooperatively drive forward progress in improving drug therapy and engineering HIV-1 resistant cells.

Ribocentre: a database of ribozymes

Ribozymes are excellent systems in which to study 'sequence - structure - function' relationships in RNA molecules. Understanding these relationships may greatly help structural modeling and design of functional RNA structures and some functional structural modules could be repurposed in molecular design. At present, there is no comprehensive database summarising all the natural ribozyme families. We have therefore created Ribocentre, a database that collects together sequence, structure and mechanistic data on 21 ribozyme families. This includes available information on timelines, sequence families, secondary and tertiary structures, catalytic mechanisms, applications of the ribozymes together with key publications. The database is publicly available at https://www.ribocentre.org.

The Origin of Translation: Bridging the Nucleotides and Peptides

Extant biology uses RNA to record genetic information and proteins to execute biochemical functions. Nucleotides are translated into amino acids via transfer RNA in the central dogma. tRNA is essential in translation as it connects the codon and the cognate amino acid. To reveal how the translation emerged in the prebiotic context, we start with the structure and dissection of tRNA, followed by the theory and hypothesis of tRNA and amino acid recognition. Last, we review how amino acids assemble on the tRNA and further form peptides. Understanding the origin of life will also promote our knowledge of artificial living systems.

The volume changes of unfolding of dsDNA

High hydrostatic pressure can have profound effects on the stability of biomacromolecules. The magnitude and direction (stabilizing or destabilizing) of this effect is defined by the volume changes in the system, ΔV. Positive volume changes will stabilize the starting native state, whereas negative volume changes will lead to the stabilization of the final unfolded state. For the DNA double helix, experimental data suggested that when the thermostability of dsDNA is below 50°C, increase in hydrostatic pressure will lead to destabilization; i.e., helix-to-coil transition has negative ΔV. In contrast, the dsDNA sequences with the thermostability above 50°C showed positive ΔV values and were stabilized by hydrostatic pressure. In order to get insight into this switch in the response of dsDNA to hydrostatic pressure as a function of temperature, first we further validated this trend using experimental measurements of ΔV for 10 different dsDNA sequences using pressure perturbation calorimetry. We also developed a computational protocol to calculate the expected volume changes of dsDNA unfolding, which was benchmarked against the experimental set of 50 ΔV values that included, in addition to our data, the values from the literature. Computation predicts well the experimental values of ΔV. Such agreement between computation and experiment lends credibility to the computation protocol and provides molecular level rational for the observed temperature dependence of ΔV that can be traced to the hydration. Difference in the ΔV value for A/T versus G/C basepairs is also discussed.

Computationally-guided design and selection of high performing ribosomal active site mutants

Understanding how modifications to the ribosome affect function has implications for studying ribosome biogenesis, building minimal cells, and repurposing ribosomes for synthetic biology. However, efforts to design sequence-modified ribosomes have been limited because point mutations in the ribosomal RNA (rRNA), especially in the catalytic active site (peptidyl transferase center; PTC), are often functionally detrimental. Moreover, methods for directed evolution of rRNA are constrained by practical considerations (e.g. library size). Here, to address these limitations, we developed a computational rRNA design approach for screening guided libraries of mutant ribosomes. Our method includes in silico library design and selection using a Rosetta stepwise Monte Carlo method (SWM), library construction and in vitro testing of combined ribosomal assembly and translation activity, and functional characterization in vivo. As a model, we apply our method to making modified ribosomes with mutant PTCs. We engineer ribosomes with as many as 30 mutations in their PTCs, highlighting previously unidentified epistatic interactions, and show that SWM helps identify sequences with beneficial phenotypes as compared to random library sequences. We further demonstrate that some variants improve cell growth in vivo, relative to wild type ribosomes. We anticipate that SWM design and selection may serve as a powerful tool for rRNA engineering.

MXene-Based Nucleic Acid Biosensors for Agricultural and Food Systems

MXene is a two-dimensional (2D) nanomaterial that exhibits several superior properties suitable for fabricating biosensors. Likewise, the nucleic acid (NA) in oligomerization forms possesses highly specific biorecognition ability and other features amenable to biosensing. Hence the combined use of MXene and NA is becoming increasingly common in biosensor design and development. In this review, MXene- and NA-based biosensors are discussed in terms of their sensing mechanisms and fabrication details. MXenes are introduced from their definition and synthesis process to their characterization followed by their use in NA-mediated biosensor fabrication. The emphasis is placed on the detection of various targets relevant to agricultural and food systems, including microbial pathogens, chemical toxicants, heavy metals, organic pollutants, etc. Finally, current challenges and future perspectives are presented with an eye toward the development of advanced biosensors with improved detection performance.

A Tale of Water Molecules in the Ribosomal Peptidyl Transferase Reaction

The peptidyl transferase center (PTC) in the large subunit of the ribosome plays a critical role in protein synthesis by catalyzing the formation of peptide bonds with an astounding speed of about 15 to 20 peptide bonds per second. The ribosome coordinates the nucleophilic attack and deprotonation in the rate-limiting step at the PTC. However, the details of peptide bond formation within the ribosome, particularly the precise role of the two water molecules in the PTC, remain unclear. Here, we propose a novel stepwise "proton shuttle" mechanism which corroborates all the reported experimental measurements so far. In this mechanism, a water molecule close to A76 of peptidyl-tRNA 2'- and 3'-O stabilizes the transition state. The other one adjacent to the carbonyl oxygen of peptidyl-tRNA actively participates in the proton shuttle, playing the catalytic role of ribosome-catalyzed peptide bond formation.

Extracellular N6 -isopentenyladenosine (i6A) addition induces cotranscriptional i6A incorporation into ribosomal RNAs

N⁶ -isopentenyladenosine (i⁶A), a modified adenosine monomer, is known to induce cell death upon its addition to the culture medium. However, the molecular fate of extracellularly added i⁶A has yet to be identified. Here we show that i⁶A addition to cell culture medium results in i⁶A incorporation into cellular RNA in several cell lines, including the 5-fluorouracil (5-FU)-resistant human oral squamous cell carcinoma cell line FR2-SAS and its parental 5-FU-sensitive cell line SAS. i⁶A was predominantly incorporated into 18S and 28S rRNAs, and i⁶A incorporation into total RNA was mostly suppressed by treating these cell lines with an RNA polymerase I (Pol I) inhibitor. i⁶A was incorporated into RNA even upon inactivation of TRIT1, the only cellular i⁶A-modifying enzyme. These results indicate that upon cellular uptake of i⁶A, it is anabolized to be used for Pol I transcription. Interestingly, at lower i⁶A concentrations, the cytotoxic effect of i⁶A was substantially more pronounced in FR2-SAS cells than in SAS cells. Moreover, in FR2-SAS cells, i⁶A treatment decreased the rate of cellular protein synthesis and increased intracellular protein aggregation, and these effects were more pronounced than in SAS cells. Our work provides insights into the molecular fate of extracellularly applied i⁶A in the context of intracellular nucleic acid anabolism and suggests investigation of i⁶A as a candidate for a chemotherapy agent against 5-FU-resistant cancer cells.

Evolutionary Origin and Functional Diversification of Aminotransferases

Aminotransferases (ATs) are pyridoxal 5′-phosphate–dependent enzymes that catalyze the transamination reactions between amino acid donor and keto acid acceptor substrates. Modern AT enzymes constitute ∼2% of all classified enzymatic activities, play central roles in nitrogen metabolism, and generate multitude of primary and secondary metabolites. ATs likely diverged into four distinct AT classes before the appearance of the last universal common ancestor and further expanded to a large and diverse enzyme family. Although the AT family underwent an extensive functional specialization, many AT enzymes retained considerable substrate promiscuity and multifunctionality because of their inherent mechanistic, structural, and functional constraints. This review summarizes the evolutionary history, diverse metabolic roles, reaction mechanisms, and structure–function relationships of the AT family enzymes, with a special emphasis on their substrate promiscuity and multifunctionality. Comprehensive characterization of AT substrate specificity is still needed to reveal their true metabolic functions in interconnecting various branches of the nitrogen metabolic network in different organisms.

Chiral proofreading during protein biosynthesis and its evolutionary implications

Homochirality of biomacromolecules is a prerequisite for their proper functioning and hence essential for all life forms. This underscores the role of cellular chiral checkpoints in enforcing homochirality during protein biosynthesis. D-aminoacyl-tRNA deacylase (DTD) is an enzyme that performs 'Chirality-based proofreading' to remove D-amino acids mistakenly attached to tRNAs, thus recycling them for further rounds of translation. Paradoxically, owing to its L-chiral rejection mode of action, DTD can remove glycine as well, which is an achiral amino acid. However, this activity is modulated by discriminator base (N73) in tRNA, a unique element that protects the cognate Gly-tRNA^Gly . Here, we review our recent work showing various aspects of DTD and tRNA^Gly co-evolution and its key role in maintaining proper translation surveillance in both bacteria and eukaryotes. Moreover, we also discuss two major optimization events on DTD and tRNA that resolved compatibility issues among the archaeal and the bacterial translation apparatuses. Importantly, such optimizations are necessary for the emergence of mitochondria and successful eukaryogenesis.

Ciliate Research. From Myth to Trendsetting Science

This special issue of the Journal of Eukaryotic Microbiology (JEM) summarizes achievements obtained by generations of researchers with ciliates in widely different disciplines. In fact, ciliates range among the first cells seen under the microscope centuries ago. Their beauty made them an object of scientia amabilis and their manifold reactions made them attractive for college experiments and finally challenged causal analyses at the cellular level. Some of this work was honored by a Nobel Prize. Some observations yielded a baseline for additional novel discoveries, occasionally facilitated by specific properties of some ciliates. This also offers some advantage in the exploration of closely related parasites (malaria). Articles contributed here by colleagues from all over the world encompass a broad spectrum of ciliate life, from genetics to evolution, from molecular cell biology to ecology, from intercellular signaling to epigenetics etc. This introductory chapter, largely based on my personal perception, aims at integrating work presented in this special issue of JEM into a broader historical context up to current research.

Peptide Bond Formation between Aminoacyl-Minihelices by a Scaffold Derived from the Peptidyl Transferase Center

The peptidyl transferase center (PTC) in the ribosome is composed of two symmetrically arranged tRNA-like units that contribute to peptide bond formation. We prepared units of the PTC components with putative tRNA-like structure and attempted to obtain peptide bond formation between aminoacyl-minihelices (primordial tRNAs, the structures composed of a coaxial stack of the acceptor stem on the T-stem of tRNA). One of the components of the PTC, P1c2^UGGU (74-mer), formed a dimer and a peptide bond was formed between two aminoacyl-minihelices tethered by the dimeric P1c2^UGGU. Peptide synthesis depended on both the existence of the dimeric P1c2^UGGU and the sequence complementarity between the ACCA-3′ sequence of the minihelix. Thus, the tRNA-like structures derived from the PTC could have originated as a scaffold of aminoacyl-minihelices for peptide bond formation through an interaction of the CCA sequence of minihelices. Moreover, with the same origin, some would have evolved to constitute the present PTC of the ribosome, and others to function as present tRNAs.

RNA World Modeling: A Comparison of Two Complementary Approaches

Simple Summary

Despite years of dedicated research, scientists are still not sure what the first ”living” cell would have looked like. One of the most well-known hypotheses is the RNA world hypothesis, which assumes that, in the beginning, life relied on RNA molecules instead of DNA as information carriers and primitive enzymes. The population of such RNAs is made up of self-replicating molecules (replicases) that could make copies of themselves and parasite molecules that could only be copied by replicases. In this study, we further investigated the interplay between these hypothetical prebiotic RNA species, since it plays a crucial role in generating diversity and complexity in prebiotic molecular evolution. We compared two approaches that are commonly used to investigate such simple prebiotic systems, representing different modeling and observation scales—namely, microscopic and macroscopic. In both cases, we were able to obtain consistent results.

Abstract

The origin of life remains one of the major scientific questions in modern biology. Among many hypotheses aiming to explain how life on Earth started, RNA world is probably the most extensively studied. It assumes that, in the very beginning, RNA molecules served as both enzymes and as genetic information carriers. However, even if this is true, there are many questions that still need to be answered—for example, whether the population of such molecules could achieve stability and retain genetic information for many generations, which is necessary in order for evolution to start. In this paper, we try to answer this question based on the parasite–replicase model (RP model), which divides RNA molecules into enzymes (RNA replicases) capable of catalyzing replication and parasites that do not possess replicase activity but can be replicated by RNA replicases. We describe the aforementioned system using partial differential equations and, based on the analysis of the simulation, surmise general rules governing its evolution. We also compare this approach with one where the RP system is modeled and implemented using a multi-agent modeling technique. We show that approaching the description and analysis of the RP system from different perspectives (microscopic represented by MAS and macroscopic depicted by PDE) provides consistent results. Therefore, applying MAS does not lead to erroneous results and allows us to study more complex situations where many cases are concerned, which would not be possible through the PDE model.

Toward Convergence in Folding Simulations of RNA Tetraloops: Comparison of Enhanced Sampling Techniques and Effects of Force Field Modifications

Atomistic molecular dynamics simulations represent an established technique for investigation of RNA structural dynamics. Despite continuous development, contemporary RNA simulations still suffer from suboptimal accuracy of empirical potentials (force fields, ffs) and sampling limitations. Development of efficient enhanced sampling techniques is important for two reasons. First, they allow us to overcome the sampling limitations, and second, they can be used to quantify ff imbalances provided they reach a sufficient convergence. Here, we study two RNA tetraloops (TLs), namely the GAGA and UUCG motifs. We perform extensive folding simulations and calculate folding free energies (ΔG_fold^°) with the aim to compare different enhanced sampling techniques and to test several modifications of the nonbonded terms extending the AMBER OL3 RNA ff. We demonstrate that replica-exchange solute tempering (REST2) simulations with 12-16 replicas do not show any sign of convergence even when extended to a timescale of 120 μs per replica. However, the combination of REST2 with well-tempered metadynamics (ST-MetaD) achieves good convergence on a timescale of 5-10 μs per replica, improving the sampling efficiency by at least 2 orders of magnitude. Effects of ff modifications on ΔG_fold^° energies were initially explored by the reweighting approach and then validated by new simulations. We tested several manually prepared variants of the gHBfix potential which improve stability of the native state of both TLs by ∼2 kcal/mol. This is sufficient to conveniently stabilize the folded GAGA TL while the UUCG TL still remains under-stabilized. Appropriate adjustment of van der Waals parameters for C-H···O5' base-phosphate interaction may further stabilize the native states of both TLs by ∼0.6 kcal/mol.

Recent Advances on DNAzyme-Based Sensing

DNAzymes are functional nucleic acid with catalytic activity. Owing to the high sensitivity, excellent programmability, and flexible obtainment through in vitro selection, RNA-cleaving DNAzymes have attracted increasing interest in developing DNAzyme-based sensors. In this review, we summarize the recent advances on DNAzyme-based sensing applications. We initially conclude two general strategies to expand the library of DNAzymes, in vitro selection to discover new DNAzymes towards different targets of interest and chemical modifications to endue the existing DNAzymes with new function or properties. We then discuss the recent applications of DNAzyme-based sensors for the detection of a variety of important biomolecules both in vitro and in vivo . Finally, perspectives on the challenges and future directions in the development of DNAzyme-based sensors are provided.

Rolling circle RNA synthesis catalysed by RNA

RNA-catalyzed RNA replication is widely considered a key step in the emergence of life’s first genetic system. However, RNA replication can be impeded by the extraordinary stability of duplex RNA products, which must be dissociated for re-initiation of the next replication cycle. Here, we have explored rolling circle synthesis (RCS) as a potential solution to this strand separation problem. We observe sustained RCS by a triplet polymerase ribozyme beyond full-length circle synthesis with strand displacement yielding concatemeric RNA products. Furthermore, we show RCS of a circular Hammerhead ribozyme capable of self-cleavage and re-circularization. Thus, all steps of a viroid-like RNA replication pathway can be catalyzed by RNA alone. Finally, we explore potential RCS mechanisms by molecular dynamics simulations, which indicate a progressive build-up of conformational strain upon RCS with destabilization of nascent strand 5′- and 3′-ends. Our results have implications for the emergence of RNA replication and for understanding the potential of RNA to support complex genetic processes.

Many organisms today rely on a trio of molecules for their survival: DNA, to store their genetic information; proteins, to conduct the biological processes required for growth or replication; and RNA, to mainly act as an intermediary between DNA and proteins. Yet, how these inanimate molecules first came together to form a living system remains unclear.

Circumstantial evidence suggests that the first lifeforms relied to a much greater exrtent on RNA to conduct all necessary biological processes. There is no trace of this ‘RNA world’ today, but molecular ‘fossils’ may exist in current biology. Viroids, for example, are agents which can infect and replicate inside plant cells. They are formed of nothing but a circular strand of RNA that serves not only as genetic storage but also as ribozymes (RNA-based enzymes). Viroids need proteins from the host plant to replicate, but scientists have been able to engineer ribozymes that can copy complex RNA strands. This suggests that viroid-like replication could be achieved using only RNA.

Kristoffersen et al. put this idea to the test and showed that it is possible to use RNA enzymatic activity alone to carry out all the steps of a viroid-like copying mechanism. This process included copying a viroid-like RNA circle with RNA, followed by trimming the copy to the right size and reforming the circle. These two latter steps could be carried out by a ribozyme that could itself be encoded on the RNA circle. A computer simulation indicated that RNA synthesis on the circle caused increasing tension that could ease some of the barriers to replication.

These results increase our understanding of how RNA copying by RNA could be possible. This may lead to developing molecular models of a primordial RNA-based replication, which could be used to investigate early genetic systems and may have potential applications in synthetic biology.

Bacterial Response to Oxidative Stress and RNA Oxidation

Bacteria have to cope with oxidative stress caused by distinct Reactive Oxygen Species (ROS), derived not only from normal aerobic metabolism but also from oxidants present in their environments. The major ROS include superoxide O₂⁻, hydrogen peroxide H₂O₂ and radical hydroxide HO^•. To protect cells under oxidative stress, bacteria induce the expression of several genes, namely the SoxRS, OxyR and PerR regulons. Cells are able to tolerate a certain number of free radicals, but high levels of ROS result in the oxidation of several biomolecules. Strikingly, RNA is particularly susceptible to this common chemical damage. Oxidation of RNA causes the formation of strand breaks, elimination of bases or insertion of mutagenic lesions in the nucleobases. The most common modification is 8-hydroxyguanosine (8-oxo-G), an oxidized form of guanosine. The structure and function of virtually all RNA species (mRNA, rRNA, tRNA, sRNA) can be affected by RNA oxidation, leading to translational defects with harmful consequences for cell survival. However, bacteria have evolved RNA quality control pathways to eliminate oxidized RNA, involving RNA-binding proteins like the members of the MutT/Nudix family and the ribonuclease PNPase. Here we summarize the current knowledge on the bacterial stress response to RNA oxidation, namely we present the different ROS responsible for this chemical damage and describe the main strategies employed by bacteria to fight oxidative stress and control RNA damage.

Chemical Modifications of Ribosomal RNA

Cellular RNAs in all three kingdoms of life are modified with diverse chemical modifications. These chemical modifications expand the topological repertoire of RNAs, and fine-tune their functions. Ribosomal RNA in yeast contains more than 100 chemically modified residues in the functionally crucial and evolutionary conserved regions. The chemical modifications in the rRNA are of three types-methylation of the ribose sugars at the C2-positionAbstract (Nm), isomerization of uridines to pseudouridines (Ψ), and base modifications such as (methylation (mN), acetylation (acN), and aminocarboxypropylation (acpN)). The modifications profile of the yeast rRNA has been recently completed, providing an excellent platform to analyze the function of these modifications in RNA metabolism and in cellular physiology. Remarkably, majority of the rRNA modifications and the enzymatic machineries discovered in yeast are highly conserved in eukaryotes including humans. Mutations in factors involved in rRNA modification are linked to several rare severe human diseases (e.g., X-linked Dyskeratosis congenita, the Bowen-Conradi syndrome and the William-Beuren disease). In this chapter, we summarize all rRNA modifications and the corresponding enzymatic machineries of the budding yeast.

A Computational Study of RNA Tetraloop Thermodynamics, Including Misfolded States

An important characteristic of RNA folding is the adoption of alternative configurations of similar stability, often referred to as misfolded configurations. These configurations are considered to compete with correctly folded configurations, although their rigorous thermodynamic and structural characterization remains elusive. Tetraloop motifs found in large ribozymes are ideal systems for an atomistically detailed computational quantification of folding free energy landscapes and the structural characterization of their constituent free energy basins, including nonnative states. In this work, we studied a group of closely related 10-mer tetraloops using a combined parallel tempering and metadynamics technique that allows a reliable sampling of the free energy landscapes, requiring only knowledge that the stem folds into a canonical A-RNA configuration. We isolated and analyzed unfolded, folded, and misfolded populations that correspond to different free energy basins. We identified a distinct misfolded state that has a stability very close to that of the correctly folded state. This misfolded state contains a predominant population that shares the same structural features across all tetraloops studied here and lacks the noncanonical A-G base pair in its loop portion. Further analysis performed with biased trajectories showed that although this competitive misfolded state is not an essential intermediate, it is visited in most of the transitions from unfolded to correctly folded states. Moreover, the tetraloops can transition from this misfolded state to the correctly folded state without requiring extensive unfolding.

Net Charges of the Ribosomal Proteins of the S10 and spc Clusters of Halophiles Are Inversely Related to the Degree of Halotolerance

Net positive charge(s) on ribosomal proteins (r-proteins) have been reported to influence the assembly and folding of ribosomes. A high percentage of r-proteins from extremely halophilic archaea are known to be acidic or even negatively charged. Those proteins that remain positively charged are typically far less positively charged. Here, the analysis is extended to non-archaeal halophilic bacteria, eukaryotes, and halotolerant archaea. The net charges (pH 7.4) of the r-proteins that comprise the S10-spc operon/cluster from individual microbial and eukaryotic genomes were estimated and intercompared. It was observed that, as a general rule, the net charges of individual proteins remained mostly basic as the salt tolerance of the bacterial strains increased from 5 to 15%. The most striking exceptions were the extremely halophilic bacterial strains, Salinibacter ruber SD01, Acetohalobium arabaticum DSM 5501 and Selenihalanaerobacter shriftii ATCC BAA-73, which are reported to require a minimum of 18% to 21% salt for their growth. All three strains have higher numbers of acidic S10-spc cluster r-proteins than what is seen in the moderate halophiles or the halotolerant strains. Of the individual proteins, only uL2 never became acidic. uS14 and uL16 also seldom became acidic. The net negative charges on several of the S10-spc cluster r-proteins are a feature generally shared by all extremely halophilic archaea and bacteria. The S10-spc cluster r-proteins of halophilic fungi and algae (eukaryotes) were exceptions: these were positively charged despite the halophilicity of the organisms. IMPORTANCE The net charges (at pH 7.4) of the ribosomal proteins (r-proteins) that comprise the S10-spc cluster show an inverse relationship with the halophilicity/halotolerance levels in both bacteria and archaea. In non-halophilic bacteria, the S10-spc cluster r-proteins are generally basic (positively charged), while the rest of the proteomes in these strains are generally acidic. On the other hand, the whole proteomes of the extremely halophilic strains are overall negatively charged, including the S10-spc cluster r-proteins. Given that the distribution of charged residues in the ribosome exit tunnel influences cotranslational folding, the contrasting charges observed in the S10-spc cluster r-proteins have potential implications for the rate of passage of these proteins through the ribosomal exit tunnel. Furthermore, the universal protein uL2, which lies in the oldest part of the ribosome, is always positively charged irrespective of the strain/organism it belongs to. This has implications for its role in the prebiotic context.

Hitting Times of Some Critical Events in RNA Origins of Life

Can a replicase be found in the vast sequence space by random drift? We partially answer this question through a proof-of-concept study of the times of occurrence (hitting times) of some critical events in the origins of life for low-dimensional RNA sequences using a mathematical model and stochastic simulation studies from Python software. We parameterize fitness and similarity landscapes for polymerases and study a replicating population of sequences (randomly) participating in template-directed polymerization. Under the ansatz of localization where sequence proximity correlates with spatial proximity of sequences, we find that, for a replicating population of sequences, the hitting and establishment of a high-fidelity replicator depends critically on the polymerase fitness and sequence (spatial) similarity landscapes and on sequence dimension. Probability of hitting is dominated by landscape curvature, whereas hitting time is dominated by sequence dimension. Surface chemistries, compartmentalization, and decay increase hitting times. Compartmentalization by vesicles reveals a trade-off between vesicle formation rate and replicative mass, suggesting that compartmentalization is necessary to ensure sufficient concentration of precursors. Metabolism is thought to be necessary to replication by supplying precursors of nucleobase synthesis. We suggest that the dynamics of the search for a high-fidelity replicase evolved mostly during the final period and, upon hitting, would have been followed by genomic adaptation of genes and to compartmentalization and metabolism, effecting degree-of-freedom gains of replication channel control over domain and state to ensure the fidelity and safe operations of the primordial genetic communication system of life.