Programmed translational frameshifting

Targeting RNA with small molecules, from RNA structures to precision medicines: IUPHAR review: 40

RNA plays important roles in regulating both health and disease biology in all kingdoms of life. Notably, RNA can form intricate three-dimensional structures, and their biological functions are dependent on these structures. Targeting the structured regions of RNA with small molecules has gained increasing attention over the past decade, because it provides both chemical probes to study fundamental biology processes and lead medicines for diseases with unmet medical needs. Recent advances in RNA structure prediction and determination and RNA biology have accelerated the rational design and development of RNA-targeted small molecules to modulate disease pathology. However, challenges remain in advancing RNA-targeted small molecules towards clinical applications. This review summarizes strategies to study RNA structures, to identify small molecules recognizing these structures, and to augment the functionality of RNA-binding small molecules. We focus on recent advances in developing RNA-targeted small molecules as potential therapeutics in a variety of diseases, encompassing different modes of actions and targeting strategies. Furthermore, we present the current gaps between early-stage discovery of RNA-binding small molecules and their clinical applications, as well as a roadmap to overcome these challenges in the near future.

Mapping the structural landscape of the yeast Ty3 retrotransposon RNA genome

Long terminal repeat (LTR)-retrotransposons are significant contributors to the evolution and diversity of eukaryotic genomes. Their RNA genomes (gRNA) serve as a template for protein synthesis and reverse transcription to a DNA copy, which can integrate into the host genome. Here, we used the SHAPE-MaP strategy to explore Ty3 retrotransposon gRNA structure in yeast and under cell-free conditions. Our study reveals the structural dynamics of Ty3 gRNA and the well-folded core, formed independently of the cellular environment. Based on the detailed map of Ty3 gRNA structure, we characterized the structural context of cis-acting sequences involved in reverse transcription and frameshifting. We also identified a novel functional sequence as a potential initiator for Ty3 gRNA dimerization. Our data indicate that the dimer is maintained by direct interaction between short palindromic sequences at the 5' ends of the two Ty3 gRNAs, resembling the model characteristic for other retroelements like HIV-1 and Ty1. This work points out a range of cell-dependent and -independent Ty3 gRNA structural changes that provide a solid background for studies on RNA structure-function relationships important for retroelement biology.

The RNA Helicase Ded1 from Yeast Is Associated with the Signal Recognition Particle and Is Regulated by SRP21

The DEAD-box RNA helicase Ded1 is an essential yeast protein involved in translation initiation that belongs to the DDX3 subfamily. The purified Ded1 protein is an ATP-dependent RNA-binding protein and an RNA-dependent ATPase, but it was previously found to lack substrate specificity and enzymatic regulation. Here we demonstrate through yeast genetics, yeast extract pull-down experiments, in situ localization, and in vitro biochemical approaches that Ded1 is associated with, and regulated by, the signal recognition particle (SRP), which is a universally conserved ribonucleoprotein complex required for the co-translational translocation of polypeptides into the endoplasmic reticulum lumen and membrane. Ded1 is physically associated with SRP components in vivo and in vitro. Ded1 is genetically linked with SRP proteins. Finally, the enzymatic activity of Ded1 is inhibited by SRP21 in the presence of SCR1 RNA. We propose a model where Ded1 actively participates in the translocation of proteins during translation. Our results provide a new understanding of the role of Ded1 during translation.

Ribosomal frameshifting at normal codon repeats recodes functional chimeric proteins in human

Ribosomal frameshifting refers to the process that ribosomes slip into +1 or -1 reading frame, thus produce chimeric trans-frame proteins. In viruses and bacteria, programmed ribosomal frameshifting can produce essential trans-frame proteins for viral replication or regulation of other biological processes. In humans, however, functional trans-frame protein derived from ribosomal frameshifting is scarcely documented. Combining multiple assays, we show that short codon repeats could act as cis-acting elements that stimulate ribosomal frameshifting in humans, abbreviated as CRFS hereafter. Using proteomic analyses, we identified many putative CRFS events from 32 normal human tissues supported by trans-frame peptides positioned at codon repeats. Finally, we show a CRFS-derived trans-frame protein (HDAC1-FS) functions by antagonizing the activities of HDAC1, thus affecting cell migration and apoptosis. These data suggest a novel type of translational recoding associated with codon repeats, which may expand the coding capacity of mRNA and diversify the regulation in human.

Mitochondrial Genomes in Perkinsus Decode Conserved Frameshifts in All Genes

Mitochondrial genomes of apicomplexans, dinoflagellates, and chrompodellids that collectively make up the Myzozoa, encode only three proteins (Cytochrome b [COB], Cytochrome c oxidase subunit 1 [COX1], Cytochrome c oxidase subunit 3 [COX3]), contain fragmented ribosomal RNAs, and display extensive recombination, RNA trans-splicing, and RNA-editing. The early-diverging Perkinsozoa is the final major myzozoan lineage whose mitochondrial genomes remained poorly characterized. Previous reports of Perkinsus genes indicated independent acquisition of non-canonical features, namely the occurrence of multiple frameshifts. To determine both ancestral myzozoan and novel perkinsozoan mitochondrial genome features, we sequenced and assembled mitochondrial genomes of four Perkinsus species. These data show a simple ancestral genome with the common reduced coding capacity but disposition for rearrangement. We identified 75 frameshifts across the four species that occur as distinct types and that are highly conserved in gene location. A decoding mechanism apparently employs unused codons at the frameshift sites that advance translation either +1 or +2 frames to the next used codon. The locations of frameshifts are seemingly positioned to regulate protein folding of the nascent protein as it emerges from the ribosome. The cox3 gene is distinct in containing only one frameshift and showing strong selection against residues that are otherwise frequently encoded at the frameshift positions in cox1 and cob. All genes lack cysteine codons implying a reduction to 19 amino acids in these genomes. Furthermore, mitochondrion-encoded rRNA fragment complements are incomplete in Perkinsus spp. but some are found in the nuclear DNA suggesting import into the organelle. Perkinsus demonstrates further remarkable trajectories of organelle genome evolution including pervasive integration of frameshift translation into genome expression.

Noncanonical amino acid mutagenesis in response to recoding signal-enhanced quadruplet codons

While amber suppression is the most common approach to introduce noncanonical amino acids into proteins in live cells, quadruplet codon decoding has potential to enable a greatly expanded genetic code with up to 256 new codons for protein biosynthesis. Since triplet codons are the predominant form of genetic code in nature, quadruplet codon decoding often displays limited efficiency. In this work, we exploited a new approach to significantly improve quadruplet UAGN and AGGN (N = A, U, G, C) codon decoding efficiency by using recoding signals imbedded in mRNA. With representative recoding signals, the expression level of mutant proteins containing UAGN and AGGN codons reached 48% and 98% of that of the wild-type protein, respectively. Furthermore, this strategy mitigates a common concern of reading-through endogenous stop codons with amber suppression-based system. Since synthetic recoding signals are rarely found near the endogenous UAGN and AGGN sequences, a low level of undesirable suppression is expected. Our strategy will greatly enhance the utility of noncanonical amino acid mutagenesis in live-cell studies.

Phenotypic mutations contribute to protein diversity and shape protein evolution

Errors in DNA replication generate genetic mutations, while errors in transcription and translation lead to phenotypic mutations. Phenotypic mutations are orders of magnitude more frequent than genetic ones, yet they are less understood. Here, we review the types of phenotypic mutations, their quantifications, and their role in protein evolution and disease. The diversity generated by phenotypic mutation can facilitate adaptive evolution. Indeed, phenotypic mutations, such as ribosomal frameshift and stop codon readthrough, sometimes serve to regulate protein expression and function. Phenotypic mutations have often been linked to fitness decrease and diseases. Thus, understanding the protein heterogeneity and phenotypic diversity caused by phenotypic mutations will advance our understanding of protein evolution and have implications on human health and diseases.

CGG repeats trigger translational frameshifts that generate aggregation-prone chimeric proteins

CGG repeat expansions in the FMR1 5’UTR cause the neurodegenerative disease Fragile X-associated tremor/ataxia syndrome (FXTAS). These repeats form stable RNA secondary structures that support aberrant translation in the absence of an AUG start codon (RAN translation), producing aggregate-prone peptides that accumulate within intranuclear neuronal inclusions and contribute to neurotoxicity. Here, we show that the most abundant RAN translation product, FMRpolyG, is markedly less toxic when generated from a construct with a non-repetitive alternating codon sequence in place of the CGG repeat. While exploring the mechanism of this differential toxicity, we observed a +1 translational frameshift within the CGG repeat from the arginine to glycine reading frame. Frameshifts occurred within the first few translated repeats and were triggered predominantly by RNA sequence and structural features. Short chimeric R/G peptides form aggregates distinct from those formed by either pure arginine or glycine, and these chimeras induce toxicity in cultured rodent neurons. Together, this work suggests that CGG repeats support translational frameshifting and that chimeric RAN translated peptides may contribute to CGG repeat-associated toxicity in FXTAS and related disorders.

Hyper-swivel head domain motions are required for complete mRNA-tRNA translocation and ribosome resetting

Translocation of messenger RNA (mRNA) and transfer RNA (tRNA) substrates through the ribosome during protein synthesis, an exemplar of directional molecular movement in biology, entails a complex interplay of conformational, compositional, and chemical changes. The molecular determinants of early translocation steps have been investigated rigorously. However, the elements enabling the ribosome to complete translocation and reset for subsequent protein synthesis reactions remain poorly understood. Here, we have combined molecular simulations with single-molecule fluorescence resonance energy transfer imaging to gain insights into the rate-limiting events of the translocation mechanism. We find that diffusive motions of the ribosomal small subunit head domain to hyper-swivelled positions, governed by universally conserved rRNA, can maneuver the mRNA and tRNAs to their fully translocated positions. Subsequent engagement of peptidyl-tRNA and disengagement of deacyl-tRNA from mRNA, within their respective small subunit binding sites, facilitate the ribosome resetting mechanism after translocation has occurred to enable protein synthesis to resume.

Abracadabra, One Becomes Two: The Importance of Context in Viral -1 Programmed Ribosomal Frameshifting

The constrained nature of viral genomes has allowed a translational sleight of hand known as −1 Programmed Ribosomal Frameshifting (−1 PRF) to flourish. Numerous studies have sought to tease apart the mechanisms and implications of −1PRF utilizing a few techniques. The dual-luciferase assay and ribosomal profiling have driven the PRF field to make great advances; however, the use of these assays means that the full impact of the genomic and cellular context on −1 PRF is often lost. Here, we discuss how the Minimal Frameshifting Element (MFE) and its constraints can hide contextual effects on −1 PRF. We review how sequence elements proximal to the traditionally defined MFE, such as the coronavirus attenuator sequence, can affect the observed rates of −1 PRF. Further, the MFE-based approach fully obscured −1 PRF in Barley yellow dwarf virus and would render the exploration of −1 PRF difficult in Porcine reproductive and respiratory syndrome virus, Encephalomyocarditis virus, Theiler’s murine encephalomyelitis virus, and Sindbis virus. Finally, we examine how the cellular context of tRNA abundance, miRNAs, and immune response elements can affect −1 PRF. The use of MFE was instrumental in establishing the basic foundations of PRF; however, it has become clear that the contextual impact on −1 PRF is no longer the exception so much as it is the rule and argues for new approaches to study −1PRF that embrace context. We therefore urge our field to expand the strategies and methods used to explore −1 PRF.

Premature translation termination mediated non-ER stress induced ATF6 activation by a ligand-dependent ribosomal frameshifting circuit

The −1 programmed ribosomal frameshifting (−1 PRF) has been explored as a gene regulatory circuit for synthetic biology applications. The −1 PRF usually uses an RNA pseudoknot structure as the frameshifting stimulator. Finding a ligand-responsive pseudoknot with efficient −1 PRF activity is time consuming and is becoming a bottleneck for its development. Inserting a guanine to guanine (GG)–mismatch pair in the 5′-stem of a small frameshifting pseudoknot could attenuate −1 PRF activity by reducing stem stability. Thus, a ligand-responsive frameshifting pseudoknot can be built using GG-mismatch–targeting small molecules to restore stem stability. Here, a pseudoknot requiring stem–loop tertiary interactions for potent frameshifting activity was used as the engineering template. This considerably amplified the effect of mismatch destabilization, and led to creation of a mammalian −1 PRF riboswitch module capable of mediating premature translation termination as a synthetic regulatory mode. Application of the synthetic circuit allowed ligand-dependent ATF6N mimic formation for the activation of protein folding–related genes involved in the unfolded protein response without an ER-stress inducing agent. With the availability of mismatch-targeting molecules, the tailored module thus paves the way for various mismatch plug-ins to streamline highly efficient orthogonal ligand-dependent −1 PRF stimulator development in the synthetic biology toolbox.

Crystal structure of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) frameshifting pseudoknot

SARS-CoV-2 produces two long viral protein precursors from one open reading frame using a highly conserved RNA pseudoknot that enhances programmed -1 ribosomal frameshifting. The 1.3 Å-resolution X-ray structure of the pseudoknot reveals three coaxially stacked helices buttressed by idiosyncratic base triples from loop residues. This structure represents a frameshift-stimulating state that must be deformed by the ribosome and exhibits base-triple-adjacent pockets that could be targeted by future small-molecule therapeutics.

Canary in a coal mine: collided ribosomes as sensors of cellular conditions

The recent discovery that collision of ribosomes triggers quality control and stress responses in eukaryotes has shifted the perspective of the field. Collided eukaryotic ribosomes adopt a unique structure, acting as a ubiquitin signaling platform for various response factors. While several of the signals that determine which downstream pathways are activated have been uncovered, we are only beginning to learn how the specificity for the activation of each process is achieved during collisions. This review will summarize those findings and how ribosome-associated factors act as molecular sentinels, linking aberrations in translation to the overall cellular state. Insights into how cells respond to ribosome collision events will provide greater understanding of the role of the ribosome in the maintenance of cellular homeostasis.

Quantitative Control for Stoichiometric Protein Synthesis

Bacterial protein synthesis rates have evolved to maintain preferred stoichiometries at striking precision, from the components of protein complexes to constituents of entire pathways. Setting relative protein production rates to be well within a factor of two requires concerted tuning of transcription, RNA turnover, and translation, allowing many potential regulatory strategies to achieve the preferred output. The last decade has seen a greatly expanded capacity for precise interrogation of each step of the central dogma genome-wide. Here, we summarize how these technologies have shaped the current understanding of diverse bacterial regulatory architectures underpinning stoichiometric protein synthesis. We focus on the emerging expanded view of bacterial operons, which encode diverse primary and secondary mRNA structures for tuning protein stoichiometry. Emphasis is placed on how quantitative tuning is achieved. We discuss the challenges and open questions in the application of quantitative, genome-wide methodologies to the problem of precise protein production. Expected final online publication date for the Annual Review of Microbiology, Volume 75 is October 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Non-essential ribosomal proteins in bacteria and archaea identified using COGs

Ribosomal proteins (RPs) are highly conserved across the bacterial and archaeal domains. Although many RPs are essential for survival, genome analysis demonstrates the absence of some RP genes in many bacterial and archaeal genomes. Furthermore, global transposon mutagenesis and/or targeted deletion showed that elimination of some RP genes had only a moderate effect on the bacterial growth rate. Here, we systematically analyze the evolutionary conservation of RPs in prokaryotes by compiling the list of the ribosomal genes that are missing from one or more genomes in the recently updated version of the Clusters of Orthologous Genes (COG) database. Some of these absences occurred because the respective genes carried frameshifts, presumably, resulting from sequencing errors, while others were overlooked and not translated during genome annotation. Apart from these annotation errors, we identified multiple genuine losses of RP genes in a variety of bacteria and archaea. Some of these losses are clade-specific, whereas others occur in symbionts and parasites with dramatically reduced genomes. The lists of computationally and experimentally defined non-essential ribosomal genes show a substantial overlap, revealing a common trend in prokaryote ribosome evolution that could be linked to the architecture and assembly of the ribosomes. Thus, RPs that are located at the surface of the ribosome and/or are incorporated at a late stage of ribosome assembly are more likely to be non-essential and to be lost during microbial evolution, particularly, in the course of genome compaction.IMPORTANCEIn many prokaryote genomes, one or more ribosomal protein (RP) genes are missing. Analysis of 1,309 prokaryote genomes included in the COG database shows that only about half of the RPs are universally conserved in bacteria and archaea. In contrast, up to 16 other RPs are missing in some genomes, primarily, tiny (<1 Mb) genomes of host-associated bacteria and archaea. Ten universal and nine archaea-specific ribosomal proteins show clear patterns of lineage-specific gene loss. Most of the RPs that are frequently lost from bacterial genomes are located on the ribosome periphery and are non-essential in Escherichia coli and Bacillus subtilis These results reveal general trends and common constraints in the architecture and evolution of ribosomes in prokaryotes.

New Insights into the Functions of Nucleic Acids Controlled by Cellular Microenvironments

The right-handed double-helical B-form structure (B-form duplex) has been widely recognized as the canonical structure of nucleic acids since it was first proposed by James Watson and Francis Crick in 1953. This B-form duplex model has a monochronic and static structure and codes genetic information within a sequence. Interestingly, DNA and RNA can form various non-canonical structures, such as hairpin loops, left-handed helices, triplexes, tetraplexes of G-quadruplex and i-motif, and branched junctions, in addition to the canonical structure. The formation of non-canonical structures depends not only on sequence but also on the surrounding environment. Importantly, these non-canonical structures may exhibit a wide variety of biological roles by changing their structures and stabilities in response to the surrounding environments, which undergo vast changes at specific locations and at specific times in cells. Here, we review recent progress regarding the interesting behaviors and functions of nucleic acids controlled by molecularly crowded cellular conditions. New insights gained from recent studies suggest that nucleic acids not only code genetic information in sequences but also have unknown functions regarding their structures and stabilities through drastic structural changes in cellular environments.

Transcript Regulation of the Recoded Archaeal α-l-Fucosidase In Vivo

Genetic decoding is flexible, due to programmed deviation of the ribosomes from standard translational rules, globally termed "recoding". In Archaea, recoding has been unequivocally determined only for termination codon readthrough events that regulate the incorporation of the unusual amino acids selenocysteine and pyrrolysine, and for -1 programmed frameshifting that allow the expression of a fully functional α-l-fucosidase in the crenarchaeon Saccharolobus solfataricus, in which several functional interrupted genes have been identified. Increasing evidence suggests that the flexibility of the genetic code decoding could provide an evolutionary advantage in extreme conditions, therefore, the identification and study of interrupted genes in extremophilic Archaea could be important from an astrobiological point of view, providing new information on the origin and evolution of the genetic code and on the limits of life on Earth. In order to shed some light on the mechanism of programmed -1 frameshifting in Archaea, here we report, for the first time, on the analysis of the transcription of this recoded archaeal α-l-fucosidase and of its full-length mutant in different growth conditions in vivo. We found that only the wild type mRNA significantly increased in S. solfataricus after cold shock and in cells grown in minimal medium containing hydrolyzed xyloglucan as carbon source. Our results indicated that the increased level of fucA mRNA cannot be explained by transcript up-regulation alone. A different mechanism related to translation efficiency is discussed.

Mutations in domain IV of elongation factor EF-G confer -1 frameshifting

A recent crystal structure of a ribosome complex undergoing partial translocation in the absence of elongation factor EF-G showed disruption of codon-anticodon pairing and slippage of the reading frame by -1, directly implicating EF-G in preservation of the translational reading frame. Among mutations identified in a random screen for dominant-lethal mutations of EF-G were a cluster of six that map to the tip of domain IV, which has been shown to contact the codon-anticodon duplex in trapped translocation intermediates. In vitro synthesis of a full-length protein using these mutant EF-Gs revealed dramatically increased -1 frameshifting, providing new evidence for a role for domain IV of EF-G in maintaining the reading frame. These mutations also caused decreased rates of mRNA translocation and rotational movement of the head and body domains of the 30S ribosomal subunit during translocation. Our results are in general agreement with recent findings from Rodnina and coworkers based on in vitro translation of an oligopeptide using EF-Gs containing mutations at two positions in domain IV, who found an inverse correlation between the degree of frameshifting and rates of translocation. Four of our six mutations are substitutions at positions that interact with the translocating tRNA, in each case contacting the RNA backbone of the anticodon loop. We suggest that EF-G helps to preserve the translational reading frame by preventing uncoupled movement of the tRNA through these contacts; a further possibility is that these interactions may stabilize a conformation of the anticodon that favors base-pairing with its codon.

Regulators of Viral Frameshifting: More Than RNA Influences Translation Events

Programmed ribosomal frameshifting (PRF) is a conserved translational recoding mechanism found in all branches of life and viruses. In bacteria, archaea, and eukaryotes PRF is used to downregulate protein production by inducing a premature termination of translation, which triggers messenger RNA (mRNA) decay. In viruses, PRF is used to drive the production of a new protein while downregulating the production of another protein, thus maintaining a stoichiometry optimal for productive infection. Traditionally, PRF motifs have been defined by the characteristics of two cis elements: a slippery heptanucleotide sequence followed by an RNA pseudoknot or stem-loop within the mRNA. Recently, additional cis and new trans elements have been identified that regulate PRF in both host and viral translation. These additional factors suggest PRF is an evolutionarily conserved process whose function and regulation we are just beginning to understand.

Molecular Dynamics Simulations Suggest a Non-Doublet Decoding Model of -1 Frameshifting by tRNASer3

In-frame decoding in the ribosome occurs through canonical or wobble Watson-Crick pairing of three mRNA codon bases (a triplet) with a triplet of anticodon bases in tRNA. Departures from the triplet-triplet interaction can result in frameshifting, meaning downstream mRNA codons are then read in a different register. There are many mechanisms to induce frameshifting, and most are insufficiently understood. One previously proposed mechanism is doublet decoding, in which only codon bases 1 and 2 are read by anticodon bases 34 and 35, which would lead to -1 frameshifting. In E. coli, tRNA^Ser3_GCU can induce -1 frameshifting at alanine (GCA) codons. The logic of the doublet decoding model is that the Ala codon's GC could pair with the tRNA^Ser3's GC, leaving the third anticodon residue U36 making no interactions with mRNA. Under that model, a U36C mutation would still induce -1 frameshifting, but experiments refute this. We perform all-atom simulations of wild-type tRNA^Ser3, as well as a U36C mutant. Our simulations revealed a hydrogen bond between U36 of the anticodon and G1 of the codon. The U36C mutant cannot make this interaction, as it lacks the hydrogen-bond-donating H3. The simulation thus suggests a novel, non-doublet decoding mechanism for -1 frameshifting by tRNA^Ser3 at Ala codons.

A tRNA-mimic Strategy to Explore the Role of G34 of tRNAGly in Translation and Codon Frameshifting

Decoding of the 61 sense codons of the genetic code requires a variable number of tRNAs that establish codon-anticodon interactions. Thanks to the wobble base pairing at the third codon position, less than 61 different tRNA isoacceptors are needed to decode the whole set of codons. On the tRNA, a subtle distribution of nucleoside modifications shapes the anticodon loop structure and participates to accurate decoding and reading frame maintenance. Interestingly, although the 61 anticodons should exist in tRNAs, a strict absence of some tRNAs decoders is found in several codon families. For instance, in Eukaryotes, G34-containing tRNAs translating 3-, 4- and 6-codon boxes are absent. This includes tRNA specific for Ala, Arg, Ile, Leu, Pro, Ser, Thr, and Val. tRNA^Gly is the only exception for which in the three kingdoms, a G34-containing tRNA exists to decode C3 and U3-ending codons. To understand why G34-tRNA^Gly exists, we analysed at the genome wide level the codon distribution in codon +1 relative to the four GGN Gly codons. When considering codon GGU, a bias was found towards an unusual high usage of codons starting with a G whatever the amino acid at +1 codon. It is expected that GGU codons are decoded by G34-containing tRNA^Gly, decoding also GGC codons. Translation studies revealed that the presence of a G at the first position of the downstream codon reduces the +1 frameshift by stabilizing the G34•U3 wobble interaction. This result partially explains why G34-containing tRNA^Gly exists in Eukaryotes whereas all the other G34-containing tRNAs for multiple codon boxes are absent.

Localized Context-Dependent Effects of the "Ambush" Hypothesis: More Off-Frame Stop Codons Downstream of Shifty Codons

The ambush hypothesis speculates that off-frame stop codons increase translational efficiency after ribosomal frameshifts by stopping early frameshifted translation. Some evidences fit this hypothesis: (1) synonymous codon usages increase with their potential contribution to off-frame stops; (2) the genetic code assigns frequent amino acids to codon families contributing to off-frame stops; (3) positive biases for off-frame stops (AT rich) occur despite adverse nucleotide (GC) biases; and (4) mitochondrial off-frame stop codon densities increase with ribosomal structural instability, potential proxy of frameshift frequencies. In this study, analyses of vertebrate mitogenes and tRNA synthetase genes from all superkingdoms and viruses test a new prediction of the ambush hypothesis: sequences immediately downstream of frameshift-inducing homopolymer codons (AAA, CCC, GGG, and TTT) are off-frame stop rich. Codons immediately downstream of homopolymer codons form more than average off-frame stops, biases are stronger than for corresponding upstream distances and for any other group of synonymous codons. Sequences downstream of that high-density region are off-frame stop depleted. This decrease suggests that off-frame stops, combined with suppressor tRNAs regulate translation of overlapping coding sequences. Results show the predictive power of the ambush hypothesis, from macroevolutionary (genetic code structure) to detailed gene sequence anatomy.

Single-Frame, Multiple-Frame and Framing Motifs in Genes

We study the distribution of new classes of motifs in genes, a research field that has not been investigated to date. A single-frame motif SF has no trinucleotide in reading frame (frame 0) that occurs in a shifted frame (frame 1 or 2), e.g., the dicodon AAACAA is SF as the trinucleotides AAA and CAA do not occur in a shifted frame. A motif which is not single-frame SF is multiple-frame MF. Several classes of MF motifs are defined and analysed. The distributions of single-frame SF motifs (associated with an unambiguous trinucleotide decoding in the two 5′–3′ and 3′–5′ directions) and 5′ unambiguous motifs 5′U (associated with an unambiguous trinucleotide decoding in the 5′–3′ direction only) are analysed without and with constraints. The constraints studied are: initiation and stop codons, periodic codons {AAA,CCC,GGG,TTT}, antiparallel complementarity and parallel complementarity. Taken together, these results suggest that the complementarity property involved in the antiparallel (DNA double helix, RNA stem) and parallel sequences could also be fundamental for coding genes with an unambiguous trinucleotide decoding in the two 5′–3′ and 3′–5′ directions or the 5′–3′ direction only. Furthermore, the single-frame motifs SF with a property of trinucleotide decoding and the framing motifs F (also called circular code motifs; first introduced by Michel (2012)) with a property of reading frame decoding may have been involved in the early life genes to build the modern genetic code and the extant genes. They could have been involved in the stage without anticodon-amino acid interactions or in the Implicated Site Nucleotides (ISN) of RNA interacting with the amino acids. Finally, the SF and MF dipeptides associated with the SF and MF dicodons, respectively, are studied and their importance for biology and the origin of life discussed.

mRNA-Mediated Duplexes Play Dual Roles in the Regulation of Bidirectional Ribosomal Frameshifting

In contrast to -1 programmed ribosomal frameshifting (PRF) stimulation by an RNA pseudoknot downstream of frameshifting sites, a refolding upstream RNA hairpin juxtaposing the frameshifting sites attenuates -1 PRF in human cells and stimulates +1 frameshifting in yeast. This eukaryotic functional mimicry of the internal Shine-Dalgarno (SD) sequence-mediated duplex was confirmed directly in the 70S translation system, indicating that both frameshifting regulation activities of upstream hairpin are conserved between 70S and 80S ribosomes. Unexpectedly, a downstream pseudoknot also possessed two opposing hungry codon-mediated frameshifting regulation activities: attenuation of +1 frameshifting and stimulation of a non-canonical -1 frameshifting within the +1 frameshift-prone CUUUGA frameshifting site in the absence of release factor 2 (RF2) in vitro. However, the -1 frameshifting activity of the downstream pseudoknot is not coupled with its +1 frameshifting attenuation ability. Similarly, the +1 frameshifting activity of the upstream hairpin is not required for its -1 frameshifting attenuation function Thus, each of the mRNA duplexes flanking the two ends of a ribosomal mRNA-binding channel possesses two functions in bi-directional ribosomal frameshifting regulation: frameshifting stimulation and counteracting the frameshifting activity of each other.

Refining the Ambush Hypothesis: Evidence That GC- and AT-Rich Bacteria Employ Different Frameshift Defence Strategies

Stop codons are frequently selected for beyond their regular termination function for error control. The “ambush hypothesis” proposes out-of-frame stop codons (OSCs) terminating frameshifted translations are selected for. Although early indirect evidence was partially supportive, recent evidence suggests OSC frequencies are not exceptional when considering underlying nucleotide content. However, prior null tests fail to control amino acid/codon usages or possible local mutational biases. We therefore return to the issue using bacterial genomes, considering several tests defining and testing against a null. We employ simulation approaches preserving amino acid order but shuffling synonymous codons or preserving codons while shuffling amino acid order. Additionally, we compare codon usage in amino acid pairs, where one codon can but the next, otherwise identical codon, cannot encode an OSC. OSC frequencies exceed expectations typically in AT-rich genomes, the +1 frame and for TGA/TAA but not TAG. With this complex evidence, simply rejecting or accepting the ambush hypothesis is not warranted. We propose a refined post hoc model, whereby AT-rich genomes have more accidental frameshifts, handled by RF2–RF3 complexes (associated with TGA/TAA) and are mostly +1 (or −2) slips. Supporting this, excesses positively correlate with in silico predicted frameshift probabilities. Thus, we propose a more viable framework, whereby genomes broadly adopt one of the two strategies to combat frameshifts: preventing frameshifting (GC-rich) or permitting frameshifts but minimizing impacts when most are caught early (AT-rich). Our refined framework holds promise yet some features, such as the bias of out-of-frame sense codons, remain unexplained.

Mechanism of tRNA-mediated +1 ribosomal frameshifting

Accurate translation of the genetic code is critical to ensure expression of proteins with correct amino acid sequences. Certain tRNAs can cause a shift out of frame (i.e., frameshifting) due to imbalances in tRNA concentrations, lack of tRNA modifications or insertions or deletions in tRNAs (called frameshift suppressors). Here, we determined the structural basis for how frameshift-suppressor tRNA^SufA6 (a derivative of tRNA^Pro) reprograms the mRNA frame to translate a 4-nt codon when bound to the bacterial ribosome. After decoding at the aminoacyl (A) site, the crystal structure of the anticodon stem-loop of tRNA^SufA6 bound in the peptidyl (P) site reveals ASL conformational changes that allow for recoding into the +1 mRNA frame. Furthermore, a crystal structure of full-length tRNA^SufA6 programmed in the P site shows extensive conformational rearrangements of the 30S head and body domains similar to what is observed in a translocation intermediate state containing elongation factor G (EF-G). The 30S movement positions tRNA^SufA6 toward the 30S exit (E) site disrupting key 16S rRNA-mRNA interactions that typically define the mRNA frame. In summary, this tRNA-induced 30S domain change in the absence of EF-G causes the ribosome to lose its grip on the mRNA and uncouples the canonical forward movement of the tRNAs during elongation.

How Messenger RNA and Nascent Chain Sequences Regulate Translation Elongation

Translation elongation is a highly coordinated, multistep, multifactor process that ensures accurate and efficient addition of amino acids to a growing nascent-peptide chain encoded in the sequence of translated messenger RNA (mRNA). Although translation elongation is heavily regulated by external factors, there is clear evidence that mRNA and nascent-peptide sequences control elongation dynamics, determining both the sequence and structure of synthesized proteins. Advances in methods have driven experiments that revealed the basic mechanisms of elongation as well as the mechanisms of regulation by mRNA and nascent-peptide sequences. In this review, we highlight how mRNA and nascent-peptide elements manipulate the translation machinery to alter the dynamics and pathway of elongation.

Alternative Mode of E-Site tRNA Binding in the Presence of a Downstream mRNA Stem Loop at the Entrance Channel

Structured mRNAs positioned downstream of the ribosomal decoding center alter gene expression by slowing protein synthesis. Here, we solved the cryo-EM structure of the bacterial ribosome bound to an mRNA containing a 3' stem loop that regulates translation. Unexpectedly, the E-site tRNA adopts two distinct orientations. In the first structure, normal interactions with the 50S and 30S E site are observed. However, in the second structure, although the E-site tRNA makes normal interactions with the 50S E site, its anticodon stem loop moves ∼54 Å away from the 30S E site to interact with the 30S head domain and 50S uL5. This position of the E-site tRNA causes the uL1 stalk to adopt a more open conformation that likely represents an intermediate state during E-site tRNA dissociation. These results suggest that structured mRNAs at the entrance channel restrict 30S subunit movement required during translation to slow E-site tRNA dissociation.

Programmed Ribosomal Frameshifting Generates a Copper Transporter and a Copper Chaperone from the Same Gene

Metal efflux pumps maintain ion homeostasis in the cell. The functions of the transporters are often supported by chaperone proteins, which scavenge the metal ions from the cytoplasm. Although the copper ion transporter CopA has been known in Escherichia coli, no gene for its chaperone had been identified. We show that the CopA chaperone is expressed in E. coli from the same gene that encodes the transporter. Some ribosomes translating copA undergo programmed frameshifting, terminate translation in the -1 frame, and generate the 70 aa-long polypeptide CopA(Z), which helps cells survive toxic copper concentrations. The high efficiency of frameshifting is achieved by the combined stimulatory action of a "slippery" sequence, an mRNA pseudoknot, and the CopA nascent chain. Similar mRNA elements are not only found in the copA genes of other bacteria but are also present in ATP7B, the human homolog of copA, and direct ribosomal frameshifting in vivo.

Conditional Switch between Frameshifting Regimes upon Translation of dnaX mRNA

Ribosome frameshifting during translation of bacterial dnaX can proceed via different routes, generating a variety of distinct polypeptides. Using kinetic experiments, we show that -1 frameshifting predominantly occurs during translocation of two tRNAs bound to the slippery sequence codons. This pathway depends on a stem-loop mRNA structure downstream of the slippery sequence and operates when aminoacyl-tRNAs are abundant. However, when aminoacyl-tRNAs are in short supply, the ribosome switches to an alternative frameshifting pathway that is independent of a stem-loop. Ribosome stalling at a vacant 0-frame A-site codon results in slippage of the P-site peptidyl-tRNA, allowing for -1-frame decoding. When the -1-frame aminoacyl-tRNA is lacking, the ribosomes switch into -2 frame. Quantitative mass spectrometry shows that the -2-frame product is synthesized in vivo. We suggest that switching between frameshifting routes may enrich gene expression at conditions of aminoacyl-tRNA limitation.

Comparison of error correction algorithms for Ion Torrent PGM data: application to hepatitis B virus

Ion Torrent Personal Genome Machine (PGM) technology is a mid-length read, low-cost and high-speed next-generation sequencing platform with a relatively high insertion and deletion (indel) error rate. A full systematic assessment of the effectiveness of various error correction algorithms in PGM viral datasets (e.g., hepatitis B virus (HBV)) has not been performed. We examined 19 quality-trimmed PGM datasets for the HBV reverse transcriptase (RT) region and found a total error rate of 0.48% ± 0.12%. Deletion errors were clearly present at the ends of homopolymer runs. Tests using both real and simulated data showed that the algorithms differed in their abilities to detect and correct errors and that the error rate and sequencing depth significantly affected the performance. Of the algorithms tested, Pollux showed a better overall performance but tended to over-correct 'genuine' substitution variants, whereas Fiona proved to be better at distinguishing these variants from sequencing errors. We found that the combined use of Pollux and Fiona gave the best results when error-correcting Ion Torrent PGM viral data.

Transposable elements in Drosophila

Transposable elements (TEs) are mobile genetic elements that can mobilize within host genomes. As TEs comprise more than 40% of the human genome and are linked to numerous diseases, understanding their mechanisms of mobilization and regulation is important. Drosophila melanogaster is an ideal model organism for the study of eukaryotic TEs as its genome contains a diverse array of active TEs. TEs universally impact host genome size via transposition and deletion events, but may also adopt unique functional roles in host organisms. There are 2 main classes of TEs: DNA transposons and retrotransposons. These classes are further divided into subgroups of TEs with unique structural and functional characteristics, demonstrating the significant variability among these elements. Despite this variability, D. melanogaster and other eukaryotic organisms utilize conserved mechanisms to regulate TEs. This review focuses on the transposition mechanisms and regulatory pathways of TEs, and their functional roles in D. melanogaster.

Anaerobically Grown Escherichia coli Has an Enhanced Mutation Rate and Distinct Mutational Spectra

Oxidative stress is a major cause of mutation but little is known about how growth in the absence of oxygen impacts the rate and spectrum of mutations. We employed long-term mutation accumulation experiments to directly measure the rates and spectra of spontaneous mutation events in Escherichia coli populations propagated under aerobic and anaerobic conditions. To detect mutations, whole genome sequencing was coupled with methods of analysis sufficient to identify a broad range of mutational classes, including structural variants (SVs) generated by movement of repetitive elements. The anaerobically grown populations displayed a mutation rate nearly twice that of the aerobic populations, showed distinct asymmetric mutational strand biases, and greater insertion element activity. Consistent with mutation rate and spectra observations, genes for transposition and recombination repair associated with SVs were up-regulated during anaerobic growth. Together, these results define differences in mutational spectra affecting the evolution of facultative anaerobes.

Understanding the evolution of holoparasitic plants: the complete plastid genome of the holoparasite Cytinus hypocistis (Cytinaceae)

Background and Aims Plant plastid genomes are highly conserved in size, gene content and structure; however, parasitic plants are a noticeable exception to this evolutionary stability. Although the evolution of parasites could help to better understand plastome evolution in general, complete plastomes of parasites have been sequenced only for some lineages so far. Here we contribute to filling this gap by providing and analysing the complete plastome sequence of Cytinus hypocistis, the first parasite sequenced for Malvales and a species suspected to have an extremely small genome. Methods We sequenced and assembled de novo the plastid genome of Cytinus hypocistis using a shotgun approach on genomic DNA. Phylogenomic analyses based on coding regions were performed on Malvidae. For each coding region present in Cytinus, we tested for relaxation or intensification of selective pressures in the Cytinus lineage compared with autotrophic Malvales. Key Results Cytinus hypocistis has an extremely divergent genome that is among the smallest sequenced to date (19·4 kb), with only 23 genes and no inverted repeat regions. Phylogenomic analysis confirmed the position of Cytinus within Malvales. All coding regions of Cytinus plastome presented very high substitution rates compared with non-parasitic Malvales. Conclusions Some regions were inferred to be under relaxed negative selection in Cytinus, suggesting that further plastome reduction is occurring due to relaxed purifying selection associated with the loss of photosynthetic activity. On the other hand, increased selection intensity and strong positive selection were detected for rpl22 in the Cytinus lineage, which might indicate an evolutionary role in the host-parasite arms race, a point that needs further research.

Characterization of new transposable element sub-families from white clover (Trifolium repens) using PCR amplification

Transposable elements (TEs) dominate the landscapes of most plant and animal genomes. Once considered junk DNA and genetic parasites, these interspersed, repetitive DNA elements are now known to play major roles in both genetic and epigenetic processes that sponsor genome variation and regulate gene expression. Knowledge of TE consensus sequences from elements in species whose genomes have not been sequenced is limited, and the individual TEs that are encountered in clones or short-reads rarely represent potentially canonical, let alone, functional representatives. In this study, we queried the Repbase database with eight BAC clones from white clover (Trifolium repens), identified a large number of candidate TEs, and used polymerase chain reaction and Sanger sequencing to create consensus sequences for three new TE families. The results show that TE family consensus sequences can be obtained experimentally in species for which just a single, full-length member of a TE family has been sequenced.

Rational design of a synthetic mammalian riboswitch as a ligand-responsive -1 ribosomal frame-shifting stimulator

Metabolite-responsive RNA pseudoknots derived from prokaryotic riboswitches have been shown to stimulate −1 programmed ribosomal frameshifting (PRF), suggesting −1 PRF as a promising gene expression platform to extend riboswitch applications in higher eukaryotes. However, its general application has been hampered by difficulty in identifying a specific ligand-responsive pseudoknot that also functions as a ligand-dependent -1 PRF stimulator. We addressed this problem by using the −1 PRF stimulation pseudoknot of SARS-CoV (SARS-PK) to build a ligand-dependent −1 PRF stimulator. In particular, the extra stem of SARS-PK was replaced by an RNA aptamer of theophylline and designed to couple theophylline binding with the stimulation of −1 PRF. Conformational and functional analyses indicate that the engineered theophylline-responsive RNA functions as a mammalian riboswitch with robust theophylline-dependent −1 PRF stimulation activity in a stable human 293T cell-line. Thus, RNA–ligand interaction repertoire provided by in vitro selection becomes accessible to ligand-specific −1 PRF stimulator engineering using SARS-PK as the scaffold for synthetic biology application.

A review on architecture of the gag-pol ribosomal frameshifting RNA in human immunodeficiency virus: a variability survey of virus genotypes

Programmed '-1' ribosomal frameshifting is necessary for expressing the pol gene overlapped from a gag of human immunodeficiency virus. A viral RNA structure that requires base pairing across the overlapping sequence region suggests a mechanism of regulating ribosome and helicase traffic during expression. To get precise roles of an element around the frameshift site, a review on architecture of the frameshifting RNA is performed in combination of reported information with augments of a representative set of 19 viral samples. In spite of a different length for the viral RNAs, a canonical comparison on the element sequence allocation is performed for viewing variability associations between virus genotypes. Additionally, recent and historical insights recognized in frameshifting regulation are looked back as for indel and single nucleotide polymorphism of RNA. As specially noted, structural changes at a frameshift site, the spacer sequence, and a three-helix junction element, as well as two Watson-Crick base pairs near a bulge and a C-G pair close a loop, are the most vital strategies for the virus frameshifting regulations. All of structural changes, which are dependent upon specific sequence variations, facilitate an elucidation about the RNA element conformation-dependent mechanism for frameshifting. These facts on disrupting base pair interactions also allow solving the problem of competition between ribosome and helicase on a same RNA template, common to single-stranded RNA viruses. In a broad perspective, each new insight of frameshifting regulation in the competition systems introduced by the RNA element construct changes will offer a compelling target for antiviral therapy.

Natural chymotrypsin-like-cleaved human mitochondrial peptides confirm tetra-, pentacodon, non-canonical RNA translations

Mass spectra of human mitochondrial peptides match non-canonical transcripts systematically (a) deleting mono/dinucleotides after trinucleotides (delRNA), (b) exchanging nucleotides (swinger RNA), translated according to tri, (c) tetra- and pentacodons (codons expanded by a 4th (and 5th) silent nucleotide(s)). Swinger transcriptions are 23 bijective transformations, nine symmetric (X<->Y, e.g. A<->C) and fourteen asymmetric exchanges (X->Y->Z->X, e.g. A->C->G->A). Here, proteomic analyses assuming cleavage after W,Y, F (chymotrypsin-like, for trypsinized samples) detect fewer chymotrypsinized than trypsinized peptides. Detected non-canonical peptides map preferentially on detected non-canonical RNAs for chymotrypsinized peptides, as previously found for trypsinized peptides. This suggests residual natural chymotrypsin-like digestion detectable within experimentally trypsinized peptide data. Some trypsinized peptides are detected twice, by analyses assuming trypsin, and those assuming chymotrypsin cleavages. They have higher spectra counts than peptides detected only once, meaning that abundant peptides are more frequently detected, but detection certainties resemble those for peptides detected only once. Analyses assuming 'incorrect' digestions are inadequate negative controls for digestion enzymes naturally active in biological samples. Chymotrypsin-analyses confirm non-canonical transcriptions/translations independently of results obtained assuming trypsinization, increase non-canonical peptidome coverage, indicating mitogenome-encoding of yet undetected proteins.

YopN and TyeA Hydrophobic Contacts Required for Regulating Ysc-Yop Type III Secretion Activity by Yersinia pseudotuberculosis

Yersinia bacteria target Yop effector toxins to the interior of host immune cells by the Ysc-Yop type III secretion system. A YopN-TyeA heterodimer is central to controlling Ysc-Yop targeting activity. A + 1 frameshift event in the 3-prime end of yopN can also produce a singular secreted YopN-TyeA polypeptide that retains some regulatory function even though the C-terminal coding sequence of this YopN differs greatly from wild type. Thus, this YopN C-terminal segment was analyzed for its role in type III secretion control. Bacteria producing YopN truncated after residue 278, or with altered sequence between residues 279 and 287, had lost type III secretion control and function. In contrast, YopN variants with manipulated sequence beyond residue 287 maintained full control and function. Scrutiny of the YopN-TyeA complex structure revealed that residue W₂₇₉ functioned as a likely hydrophobic contact site with TyeA. Indeed, a YopN_W279G mutant lost all ability to bind TyeA. The TyeA residue F₈ was also critical for reciprocal YopN binding. Thus, we conclude that specific hydrophobic contacts between opposing YopN and TyeA termini establishes a complex needed for regulating Ysc-Yop activity.

Adjustable under-expression of yeast mating pathway proteins in Saccharomyces cerevisiae using a programmed ribosomal frameshift

Experimental research in molecular biology frequently relies on the promotion or suppression of gene expression, an important tool in the study of its functions. Although yeast is among the most studied model systems with the ease of maintenance and manipulation, current experimental methods are mostly limited to gene deletion, suppression or overexpression of genes. Therefore, the ability to reduce protein expressions and then observing the effects would promote a better understanding of the exact functions and their interactions. Reducing protein expression is mainly limited by the difficulties associated with controlling the reduction level, and in some cases, the initial endogenous abundance is too low. For the under-expression to be useful as an experimental tool, repeatability and stability of reduced expression is important. We found that cis-elements in programmed -1 ribosomal frameshifting (-1RFS) of beet western yellow virus (BWYV) could be utilized to reduced protein expression in Saccharomyces cerevisiae. The two main advantages of using -1RFS are adjustable reduction rates and ease of use. To demonstrate the utility of this under-expression system, examples of reduced protein abundance were shown using yeast mating pathway components. The abundance of MAP kinase Fus3 was reduced to approximately 28-75 % of the wild-type value. Other MAP kinase mating pathway components, including Ste5, Ste11, and Ste7, were also under-expressed to verify that the -1RFS system works with different proteins. Furthermore, reduced Fus3 abundance altered the overall signal transduction outcome of the mating pathway, demonstrating the potential for further studies of signal transduction adjustment via under-expression.

Global translational impacts of the loss of the tRNA modification t6A in yeast

The universal tRNA modification t⁶A is found at position 37 of nearly all tRNAs decoding ANN codons. The absence of t⁶A₃₇ leads to severe growth defects in baker’s yeast, phenotypes similar to those caused by defects in mcm⁵s²U₃₄ synthesis. Mutants in mcm⁵s²U₃₄ can be suppressed by overexpression of tRNA^Lys_UUU, but we show t⁶A phenotypes could not be suppressed by expressing any individual ANN decoding tRNA, and t⁶A and mcm⁵s²U are not determinants for each other’s formation. Our results suggest that t⁶A deficiency, like mcm⁵s²U deficiency, leads to protein folding defects, and show that the absence of t⁶A led to stress sensitivities (heat, ethanol, salt) and sensitivity to TOR pathway inhibitors. Additionally, L-homoserine suppressed the slow growth phenotype seen in t⁶A-deficient strains, and proteins aggregates and Advanced Glycation End-products (AGEs) were increased in the mutants. The global consequences on translation caused by t⁶A absence were examined by ribosome profiling. Interestingly, the absence of t⁶A did not lead to global translation defects, but did increase translation initiation at upstream non-AUG codons and increased frame-shifting in specific genes. Analysis of codon occupancy rates suggests that one of the major roles of t⁶A is to homogenize the process of elongation by slowing the elongation rate at codons decoded by high abundance tRNAs and I₃₄:C₃ pairs while increasing the elongation rate of rare tRNAs and G₃₄:U₃ pairs. This work reveals that the consequences of t⁶A absence are complex and multilayered and has set the stage to elucidate the molecular basis of the observed phenotypes.

Translation of mitochondrial swinger RNAs according to tri-, tetra- and pentacodons

Transcriptomes and proteomes include RNA and protein fragments not matching regular transcription/translation. Some 'non-canonical' mitochondrial transcripts match mitogenomes after assuming one among 23 systematic exchanges between nucleotides, producing swinger RNAs (nine symmetric, X↔Y, example C↔T; 14 asymmetric, X→Y→Z→X, example A→T→G→A) in GenBank's EST database. Here, reanalyzes of (a) public human mitochondrial transcriptome data (Illumina: RNA-seq) allowed to detect mitochondrial swinger RNAs for all 23 exchanges and (b) independent public human mitochondrial trypsinized proteomic mass spectrometry data allowed to detect peptides predicted from translation of parts of swinger-transformed mitogenomes covered by detected swinger reads. RNA-seq and previous EST swinger transcript data converge. Swinger RNA translation frequently inserts various amino acids at stop codons. Swinger RNA-peptide associations exist also for peptides matching systematically frameshifting translation, peptides entirely coded by tetra- and pentacodons (regular codons expanded by silent mononucleotides at 4th, and silent dinucleotides at 4th and 5th position(s), respectively). Swinger peptides differ from regular mitochondrial proteins: not membrane embedded, reflect warmer, anaerobic, low resource conditions, reminding a free-living ancestor. Tetra- and pentacoded peptides associate with low, high GC contents, respectively, suggesting expanded codon translations associate with thermic stresses. Results confirm experimentally predicted swinger, tetra- and pentacoded mitochondrial peptides, increasing mitogenomic coding density.

Mimicking Ribosomal Unfolding of RNA Pseudoknot in a Protein Channel

Pseudoknots are a fundamental RNA tertiary structure with important roles in regulation of mRNA translation. Molecular force spectroscopic approaches such as optical tweezers can track the pseudoknot's unfolding intermediate states by pulling the RNA chain from both ends, but the kinetic unfolding pathway induced by this method may be different from that in vivo, which occurs during translation and proceeds from the 5' to 3' end. Here we developed a ribosome-mimicking, nanopore pulling assay for dissecting the vectorial unfolding mechanism of pseudoknots. The pseudoknot unfolding pathway in the nanopore, either from the 5' to 3' end or in the reverse direction, can be controlled by a DNA leader that is attached to the pseudoknot at the 5' or 3' ends. The different nanopore conductance between DNA and RNA translocation serves as a marker for the position and structure of the unfolding RNA in the pore. With this design, we provided evidence that the pseudoknot unfolding is a two-step, multistate, metal ion-regulated process depending on the pulling direction. Most notably, unfolding in both directions is rate-limited by the unzipping of the first helix domain (first step), which is Helix-1 in the 5' → 3' direction and Helix-2 in the 3' → 5' direction, suggesting that the initial unfolding step in either pulling direction needs to overcome an energy barrier contributed by the noncanonical triplex base-pairs and coaxial stacking interactions for the tertiary structure stabilization. These findings provide new insights into RNA vectorial unfolding mechanisms, which play an important role in biological functions including frameshifting.

Kinetics of Ribosome-Catalyzed Polymerization Using Artificial Aminoacyl-tRNA Substrates Clarifies Inefficiencies and Improvements

Ribosomal synthesis of polymers of unnatural amino acids (AAs) is limited by low incorporation efficiencies using the artificial AA-tRNAs, but the kinetics have yet to be studied. Here, kinetics were performed on five consecutive incorporations using various artificial AA-tRNAs with all intermediate products being analyzed. Yields within a few seconds displayed similar trends to our prior yields after 30 min without preincubation, demonstrating the relevance of fast kinetics to traditional long-incubation translations. Interestingly, the two anticodon swaps were much less inhibitory in the present optimized system, which should allow more flexibility in the engineering of artificial AA-tRNAs. The biggest kinetic defect was caused by the penultimate dC introduced from the standard, chemoenzymatic, charging method. This prompted peptidyl-tRNA drop-off, decreasing processivities during five consecutive AA incorporations. Indeed, two tRNA charging methods that circumvented the dC dramatically improved efficiencies of ribosomal, consecutive, unnatural AA incorporations to give near wild-type kinetics.

Codon expansion and systematic transcriptional deletions produce tetra-, pentacoded mitochondrial peptides

Genes include occasionally isolated codons with a fourth (and fifth) silent nucleotide(s). Assuming tetracodons, translated hypothetical peptides align with regular GenBank proteins; predicted tetracodons coevolve with predicted tRNAs with expanded anticodons in each mammal, Drosophila and Lepidosauria mitogenomes, GC contents and with lepidosaurian body temperatures, suggesting that expanded codons are an adaptation of translation to high temperature. Hypothetically, continuous stretches of tetra- and pentacodons code for peptides. Both systematic nucleotide deletions during transcription, and translation by tRNAs with expanded anticodons could produce these peptides. Reanalyses of human nanoLc mass spectrometry peptidome data detect numerous tetra- and pentapeptides translated from the human mitogenome. These map preferentially on (BLAST-detected) human RNAs matching the human mitogenome, assuming systematic mono- and dinucleotide deletions after each third nucleotide (delRNAs). Translation by expanded anticodons is incompatible with silent nucleotides in the midst rather than at codon 3' extremity. More than 1/3 of detected tetra- and pentapeptides assume silent positions at codon extremity, suggesting that both mechanisms, regular translation of delRNAs and translation of regular RNAs by expanded anticodons, produce this peptide subgroup. Results show that systematically deleting polymerization occurs, and confirm serial translation of expanded codons. Non-canonical transcriptions and translations considerably expand the coding potential of DNA and RNA sequences.

HIV-1 and Human PEG10 Frameshift Elements Are Functionally Distinct and Distinguished by Novel Small Molecule Modulators

Frameshifting during translation of viral or in rare cases cellular mRNA results in the synthesis of proteins from two overlapping reading frames within the same mRNA. In HIV-1 the protease, reverse transcriptase, and integrase enzymes are in a second reading frame relative to the structural group-specific antigen (gag), and their synthesis is dependent upon frameshifting. This ensures that a strictly regulated ratio of structural proteins and enzymes, which is critical for HIV-1 replication and viral infectivity, is maintained during protein synthesis. The frameshift element in HIV-1 RNA is an attractive target for the development of a new class of anti HIV-1 drugs. However, a number of examples are now emerging of human genes using −1 frameshifting, such as PEG10 and CCR5. In this study we have compared the HIV-1 and PEG10 frameshift elements and shown they have distinct functional characteristics. Frameshifting occurs at several points within each element. Moreover, frameshift modulators that were isolated by high-throughput screening of a library of 114,000 lead-like compounds behaved differently with the PEG10 frameshift element. The most effective compounds affecting the HIV-1 element enhanced frameshifting by 2.5-fold at 10 μM in two different frameshift reporter assay systems. HIV-1 protease:gag protein ratio was affected by a similar amount in a specific assay of virally-infected cultured cell, but the modulation of frameshifting of the first-iteration compounds was not sufficient to show significant effects on viral infectivity. Importantly, two compounds did not affect frameshifting with the human PEG10 element, while one modestly inhibited rather than enhanced frameshifting at the human element. These studies indicate that frameshift elements have unique characteristics that may allow targeting of HIV-1 and of other viruses specifically for development of antiviral therapeutic molecules without effect on human genes like PEG10 that use the same generic mechanism.

The role of wobble uridine modifications in +1 translational frameshifting in eukaryotes

In Saccharomyces cerevisiae, 11 out of 42 tRNA species contain 5-methoxycarbonylmethyl-2-thiouridine (mcm(5)s(2)U), 5-methoxycarbonylmethyluridine (mcm(5)U), 5-carbamoylmethyluridine (ncm(5)U) or 5-carbamoylmethyl-2'-O-methyluridine (ncm(5)Um) nucleosides in the anticodon at the wobble position (U34). Earlier we showed that mutants unable to form the side chain at position 5 (ncm(5) or mcm(5)) or lacking sulphur at position 2 (s(2)) of U34 result in pleiotropic phenotypes, which are all suppressed by overexpression of hypomodified tRNAs. This observation suggests that the observed phenotypes are due to inefficient reading of cognate codons or an increased frameshifting. The latter may be caused by a ternary complex (aminoacyl-tRNA*eEF1A*GTP) with a modification deficient tRNA inefficiently being accepted to the ribosomal A-site and thereby allowing an increased peptidyl-tRNA slippage and thus a frameshift error. In this study, we have investigated the role of wobble uridine modifications in reading frame maintenance, using either the Renilla/Firefly luciferase bicistronic reporter system or a modified Ty1 frameshifting site in a HIS4A::lacZ reporter system. We here show that the presence of mcm(5) and s(2) side groups at wobble uridines are important for reading frame maintenance and thus the aforementioned mutant phenotypes might partly be due to frameshift errors.

Mechanisms of mRNA frame maintenance and its subversion during translation of the genetic code

Important viral and cellular gene products are regulated by stop codon readthrough and mRNA frameshifting, processes whereby the ribosome detours from the reading frame defined by three nucleotide codons after initiation of translation. In the last few years, rapid progress has been made in mechanistically characterizing both processes and also revealing that trans-acting factors play important regulatory roles in frameshifting. Here, we review recent biophysical studies that bring new molecular insights to stop codon readthrough and frameshifting. Lastly, we consider whether there may be common mechanistic themes in -1 and +1 frameshifting based on recent X-ray crystal structures of +1 frameshift-prone tRNAs bound to the ribosome.

Investigating Bacterial Protein Synthesis Using Systems Biology Approaches

Protein synthesis is essential for bacterial growth and survival. Its study in Escherichia coli helped uncover features conserved among bacteria as well as universally. The pattern of discovery and the identification of some of the longest-known components of the protein synthesis machinery, including the ribosome itself, tRNAs, and translation factors proceeded through many stages of successively more refined biochemical purifications, finally culminating in the isolation to homogeneity, identification, and mapping of the smallest unit required for performing the given function. These early studies produced a wealth of information. However, many unknowns remained. Systems biology approaches provide an opportunity to investigate protein synthesis from a global perspective, overcoming the limitations of earlier ad hoc methods to gain unprecedented insights. This chapter reviews innovative systems biology approaches, with an emphasis on those designed specifically for investigating the protein synthesis machinery in E. coli.