Peptidyl-tRNA hydrolase as a key player in the liberation of truncated nascent chains from the ribosomal subunit.

Svetlov et al. identify the enzyme peptidyl-tRNA hydrolase as a ribosome-associated quality-control factor that promotes hydrolysis of the dislodged peptidyl-tRNA, which helps to recycle ribosomal subunits blocked by truncated nascent chains in bacteria.

Theoretical study of the interaction between the antibiotic linezolid and the active site of the 50S ribosomal subunit of the bacterium Haloarcula marismortui.

First antibiotic in the oxazolidinone class, linezolid fights gram-positive multiresistant bacteria by inhibiting protein synthesis through its interaction with the 50S subunit of the functional bacterial ribosome. For its antimicrobial action, it is necessary that its chiral carbon located in the oxazolidinone ring is in the S-conformation. Computational calculation at time-dependent density functional theory methodology, ultraviolet-visible (UV-Vis), and electronic circular dichroism spectra was obtained for noncomplexed and complexed forms of linezolid to verify the possible chirality of nitrogen atom in the acetamide group of the molecule. The molecular system has two chiral centers. So, there are now four possible configurations: RR, RS, SR, and SS. For a better understanding of the system, the electronic spectra at the PBE0/6-311++G(3df,2p) level of theory were obtained. The complexed form was obtained from the crystallographic data of the ribosome, containing the S-linezolid molecular system. The computational results obtained for the electronic properties are in good agreement with the experimental crystallographic data and available theoretical results.

Protocol for live-cell super-resolution imaging of transport of pre-ribosomal subunits through the nuclear pore complex.

Here, we present a protocol for single-molecule super-resolution imaging of the nuclear export of pre-ribosomal subunits pre-40S and pre-60S through nuclear pore complexes. We describe steps for plating cells and co-transfecting cells. We then detail steps for using single-point edge-excitation sub-diffraction microscopy, allowing visualization of real-time dynamics of the pre-ribosomal subunits. For complete details on the use and execution of this protocol, please refer to Junod et al. (2023).1.

Synthesis and evaluation of novel pleuromutilin derivatives targeting the 50S ribosomal subunit for antibacterial ability.

Multidrug-resistant bacteria, particularly methicillin-resistant Staphylococcus aureus, have become a major global public health concern. Therefore, developing new antibiotics that do not possess cross-resistance for the currently available antibiotics is critical. Herein, we synthesized a novel class of pleuromutilin derivatives containing substituted triazine with improved antibacterial activity. Among these derivatives, 6d, which contains 4-dimethylamino-1,3,5-triazine in the side chain of pleuromutilin, exhibited highly promising antimicrobial activity and mitigated antibiotic resistance. The high antibacterial potency of 6d was further supported by docking model analysis and green fluorescent protein inhibition assay. Additionally, cytotoxicity and acute oral toxicity evaluation and in vivo mouse systemic infection experiments revealed that 6d possessed tolerable toxicity and promising therapeutic efficacy.

Ribosome subunits are upregulated in brain samples of a subgroup of individuals with schizophrenia: A systematic gene expression meta-analysis.

One of the new theories accounting for the underlying pathophysiology of schizophrenia is excitation/inhibition imbalance. Interestingly, perturbation in protein synthesis machinery as well as oxidative stress can lead to excitation/inhibition imbalance. We thus performed a systematic meta-analysis of the expression of 79 ribosome subunit genes and two oxidative-stress related genes, HIF1A and NQO1, in brain samples of individuals with schizophrenia vs. healthy controls. We integrated 12 gene expression datasets, following the PRISMA guidelines (overall 511 samples, 253 schizophrenia and 258 controls). Five ribosome subunit genes were significantly upregulated in a subgroup of the patients with schizophrenia, while 24 (30%) showed a tendency for upregulation. HIF1A and NQO1 were also found to be significantly upregulated. Moreover, HIF1A and NQO1 showed positive correlation with the expression of the upregulated ribosome subunit genes. Our results, together with previous findings, suggest a possible role for altered mRNA translation in the pathogenesis of schizophrenia, in association with markers of increased oxidative stress in a subgroup of patients. Further studies should define whether the upregulation of ribosome subunits result in altered mRNA translation, which proteins are modulated and how it characterizes a subgroup of the patients with schizophrenia.

Molecular functions of RNA helicases during ribosomal subunit assembly.

During their biogenesis, the ribosomal subunits undergo numerous structural and compositional changes to achieve their final architecture. RNA helicases are a key driving force of such remodelling events but deciphering their particular functions has long been challenging due to lack of knowledge of their molecular functions and RNA substrates. Advances in the biochemical characterisation of RNA helicase activities together with new insights into RNA helicase binding sites on pre-ribosomes and structural snapshots of pre-ribosomal complexes containing RNA helicases now open the door to a deeper understanding of precisely how different RNA helicases contribute to ribosomal subunit maturation.

Antibiotic thermorubin tethers ribosomal subunits and impedes A-site interactions to perturb protein synthesis in bacteria.

Thermorubin (THB) is a long-known broad-spectrum ribosome-targeting antibiotic, but the molecular mechanism of its action was unclear. Here, our precise fast-kinetics assays in a reconstituted Escherichia coli translation system and 1.96 Å resolution cryo-EM structure of THB-bound 70S ribosome with mRNA and initiator tRNA, independently suggest that THB binding at the intersubunit bridge B2a near decoding center of the ribosome interferes with the binding of A-site substrates aminoacyl-tRNAs and class-I release factors, thereby inhibiting elongation and termination steps of bacterial translation. Furthermore, THB acts as an anti-dissociation agent that tethers the ribosomal subunits and blocks ribosome recycling, subsequently reducing the pool of active ribosomes. Our results show that THB does not inhibit translation initiation as proposed earlier and provide a complete mechanism of how THB perturbs bacterial protein synthesis. This in-depth characterization will hopefully spur efforts toward the design of THB analogs with improved solubility and effectivity against multidrug-resistant bacteria.

Structures of the holo CRISPR RNA-guided transposon integration complex.

CRISPR-associated transposons (CAST) are programmable mobile genetic elements that insert large DNA cargos using an RNA-guided mechanism1-3. CAST elements contain multiple conserved proteins: a CRISPR effector (Cas12k or Cascade), a AAA+ regulator (TnsC), a transposase (TnsA-TnsB) and a target-site-associated factor (TniQ). These components are thought to cooperatively integrate DNA via formation of a multisubunit transposition integration complex (transpososome). Here we reconstituted the approximately 1 MDa type V-K CAST transpososome from Scytonema hofmannii (ShCAST) and determined its structure using single-particle cryo-electon microscopy. The architecture of this transpososome reveals modular association between the components. Cas12k forms a complex with ribosomal subunit S15 and TniQ, stabilizing formation of a full R-loop. TnsC has dedicated interaction interfaces with TniQ and TnsB. Of note, we observe TnsC-TnsB interactions at the C-terminal face of TnsC, which contribute to the stimulation of ATPase activity. Although the TnsC oligomeric assembly deviates slightly from the helical configuration found in isolation, the TnsC-bound target DNA conformation differs markedly in the transpososome. As a consequence, TnsC makes new protein-DNA interactions throughout the transpososome that are important for transposition activity. Finally, we identify two distinct transpososome populations that differ in their DNA contacts near TniQ. This suggests that associations with the CRISPR effector can be flexible. This ShCAST transpososome structure enhances our understanding of CAST transposition systems and suggests ways to improve CAST transposition for precision genome-editing applications.

Ratchet, swivel, tilt and roll: a complete description of subunit rotation in the ribosome.

Protein synthesis by the ribosome requires large-scale rearrangements of the 'small' subunit (SSU; ∼1 MDa), including inter- and intra-subunit rotational motions. However, with nearly 2000 structures of ribosomes and ribosomal subunits now publicly available, it is exceedingly difficult to design experiments based on analysis of all known rotation states. To overcome this, we developed an approach where the orientation of each SSU head and body is described in terms of three angular coordinates (rotation, tilt and tilt direction) and a single translation. By considering the entire RCSB PDB database, we describe 1208 fully-assembled ribosome complexes and 334 isolated small subunits, which span >50 species. This reveals aspects of subunit rearrangements that are universal, and others that are organism/domain-specific. For example, we show that tilt-like rearrangements of the SSU body (i.e. 'rolling') are pervasive in both prokaryotic and eukaryotic (cytosolic and mitochondrial) ribosomes. As another example, domain orientations associated with frameshifting in bacteria are similar to those found in eukaryotic ribosomes. Together, this study establishes a common foundation with which structural, simulation, single-molecule and biochemical efforts can more precisely interrogate the dynamics of this prototypical molecular machine.

Translation-complex profiling of fission yeast cells reveals dynamic rearrangements of scanning ribosomal subunits upon nutritional stress.

Control of mRNA translation is key for stress responses. Translation initiation is usually rate-limiting and, in eukaryotes, involves mRNA scanning by the small ribosomal subunit. Despite its importance, many aspects of translation in vivo have not been explored fully, especially at the transcriptome-wide level. A recent method termed translation-complex profiling (TCP-seq) allows transcriptome-wide views of scanning ribosomal subunits. We applied TCP-seq to nutritional stress in the fission yeast Schizosaccharomyces pombe. At initiation sites, we observed multiple complexes resembling those of mammals, and consistent with queuing of scanning subunits. In 5' UTRs, small subunit accumulations were common and may reflect impediments to scanning. A key mediator of stress responses in S. pombe is the Fil1 transcription factor, which is regulated translationally by a poorly-understood mechanism involving upstream Open Reading Frames (uORFs). TCP-seq data of fil1 shows that stress allows scanning subunits to by-pass specific uORFs and reach the fil1 coding sequence. The integration of these observations with reporter assays revealed that fil1 translational control is mediated by a combination of scanning reinitiation-repressive and permissive uORFs, and establishes fil1 as a model for uORF-mediated translational control. Altogether, our transcriptome-wide study reveals general and gene-specific features of translation in a model eukaryote.

eIF5B and eIF1A reorient initiator tRNA to allow ribosomal subunit joining.

Translation initiation defines the identity and quantity of a synthesized protein. The process is dysregulated in many human diseases1,2. A key commitment step is when the ribosomal subunits join at a translation start site on a messenger RNA to form a functional ribosome. Here, we combined single-molecule spectroscopy and structural methods using an in vitro reconstituted system to examine how the human ribosomal subunits join. Single-molecule fluorescence revealed when the universally conserved eukaryotic initiation factors eIF1A and eIF5B associate with and depart from initiation complexes. Guided by single-molecule dynamics, we visualized initiation complexes that contained both eIF1A and eIF5B using single-particle cryo-electron microscopy. The resulting structure revealed how eukaryote-specific contacts between the two proteins remodel the initiation complex to orient the initiator aminoacyl-tRNA in a conformation compatible with ribosomal subunit joining. Collectively, our findings provide a quantitative and architectural framework for the molecular choreography orchestrated by eIF1A and eIF5B during translation initiation in humans.

Semisynthetic pleuromutilin antimicrobials with therapeutic potential against methicillin-resistant Staphylococcus aureus by targeting 50S ribosomal subunit.

A series of pleuromutilin analogs with a substituted 1,2,4-triazole were designed, synthesized and assessed for their in vitro and in vivo antibacterial activity. Initially, the MIC of the synthesized derivatives against five strains of Staphylococcus aureus (MRSA ATCC 43300, S. aureus ATCC 29213, clinical isolation of S. aureus AD3, S. aureus 144 and S. aureus SA17) were tested by the broth dilution method. Compounds 30a, 31b and 32a were the most active antibacterial agents in vitro against MRSA (MIC = 0.0625 µg/mL). The results of the time-kill curves showed that compounds 30a and 32a could reduce the amount of MRSA in vitro quickly (-7.70 log10 CFU/mL and -7.10 log10 CFU/mL reduction). In the experiment to further evaluate the in vivo antibacterial activity of compound 30a against MRSA, compound 30a (-1.71 log10 CFU/g) was effective in reducing MRSA load in thigh infected mice. Compound 30a (survival rate was 50%) displayed superior in vivo efficacy to that of tiamulin (survival rate was 30%) in the mouse systemic model. The results of further pharmacokinetic studies on compound 30a showed that the half-life (t1/2), clearance rate (Cl) and the area under the plasma concentration time curve (AUC0→∞) of compound 30a were 0.37 h, 5.43 L/h/kg and 1.84 µg h/mL, respectively. After affinity measurement by surface plasmon resonance (SPR), compound 30a exhibited high affinity with the 50S ribosome, with KD value of 1.95 × 10-6 M. Furthermore, the results of molecular docking studies revealed that compound 30a was successfully localized inside the binding pocket of 50S ribosomal subunit (ΔGb = -9.40 kcal/mol). Meanwhile, most of these compounds had no significant inhibitory effect on RAW 264.7 cells and 16HBE cells at the concentration of 8 µg/mL. The obtained outcomes showed that compound 30a could be utilized as an encouraging perspective in the development of a new therapeutic candidate for bacterial infection.

The human SKI complex regulates channeling of ribosome-bound RNA to the exosome via an intrinsic gatekeeping mechanism.

The superkiller (SKI) complex is the cytoplasmic co-factor and regulator of the RNA-degrading exosome. In human cells, the SKI complex functions mainly in co-translational surveillance-decay pathways, and its malfunction is linked to a severe congenital disorder, the trichohepatoenteric syndrome. To obtain insights into the molecular mechanisms regulating the human SKI (hSKI) complex, we structurally characterized several of its functional states in the context of 80S ribosomes and substrate RNA. In a prehydrolytic ATP form, the hSKI complex exhibits a closed conformation with an inherent gating system that effectively traps the 80S-bound RNA into the hSKI2 helicase subunit. When active, hSKI switches to an open conformation in which the gating is released and the RNA 3' end exits the helicase. The emerging picture is that the gatekeeping mechanism and architectural remodeling of hSKI underpin a regulated RNA channeling system that is mechanistically conserved among the cytoplasmic and nuclear helicase-exosome complexes.

Mitoribosomal small subunit maturation involves formation of initiation-like complexes.

Mitochondrial ribosomes (mitoribosomes) play a central role in synthesizing mitochondrial inner membrane proteins responsible for oxidative phosphorylation. Although mitoribosomes from different organisms exhibit considerable structural variations, recent insights into mitoribosome assembly suggest that mitoribosome maturation follows common principles and involves a number of conserved assembly factors. To investigate the steps involved in the assembly of the mitoribosomal small subunit (mt-SSU) we determined the cryoelectron microscopy structures of middle and late assembly intermediates of the Trypanosoma brucei mitochondrial small subunit (mt-SSU) at 3.6- and 3.7-Å resolution, respectively. We identified five additional assembly factors that together with the mitochondrial initiation factor 2 (mt-IF-2) specifically interact with functionally important regions of the rRNA, including the decoding center, thereby preventing premature mRNA or large subunit binding. Structural comparison of assembly intermediates with mature mt-SSU combined with RNAi experiments suggests a noncanonical role of mt-IF-2 and a stepwise assembly process, where modular exchange of ribosomal proteins and assembly factors together with mt-IF-2 ensure proper 9S rRNA folding and protein maturation during the final steps of assembly.

Cryo-EM structure of Mycobacterium tuberculosis 50S ribosomal subunit bound with clarithromycin reveals dynamic and specific interactions with macrolides.

Tuberculosis (TB) is the leading infectious disease caused by Mycobacterium tuberculosis (Mtb). Clarithromycin (CTY), an analog of erythromycin (ERY), is more potent against multidrug-resistance (MDR) TB. ERY and CTY were previously reported to bind to the nascent polypeptide exit tunnel (NPET) near peptidyl transferase center (PTC), but the only available CTY structure in complex with D. radiodurans (Dra) ribosome could be misinterpreted due to resolution limitation. To date, the mechanism of specificity and efficacy of CTY for Mtb remains elusive since the Mtb ribosome-CTY complex structure is still unknown. Here, we employed new sample preparation methods and solved the Mtb ribosome-CTY complex structure at 3.3Å with cryo-EM technique, where the crucial gate site A2062 (E. coli numbering) is located at the CTY binding site within NPET. Two alternative conformations of A2062, a novel syn-conformation as well as a swayed conformation bound with water molecule at interface, may play a role in coordinating the binding of specific drug molecules. The previously overlooked C-H hydrogen bond (H-bond) and π interaction may collectively contribute to the enhanced binding affinity. Together, our structure data provide a structural basis for the dynamic binding as well as the specificity of CTY and explain of how a single methyl group in CTY improves its potency, which provides new evidence to reveal previously unclear mechanism of translational modulation for future drug design and anti-TB therapy. Furthermore, our sample preparation method may facilitate drug discovery based on the complexes with low water solubility drugs by cryo-EM technique.

Caracterização do papel de Nop53 na montagem da subunidade ribossomal maior em Saccharomyces cerevisiae / Characterization of the role of Nop53 in the assembly of the large ribosomal subunit in Saccharomyces cerevisiae

Os ribossomos são complexos ribonucleoproteicos conservados formados por duas subunidades assimétricas (40S e 60S em eucariotos) responsáveis pela tradução da informação genética e catálise da síntese proteica. A montagem destes complexos em eucariotos é mais bem descrita em S. cerevisiae, constituindo um processo celular energeticamente dispendioso e com múltiplas etapas. Ela tem origem no nucléolo com a transcrição do pré-rRNA 35S e requer o recrutamento hierárquico e transiente de cerca de 200 fatores de montagem para garantir a formação correta dos centros funcionais aptos à tradução. Neste processo, que se estende no núcleo e citoplasma, 79 proteínas ribossomais associam-se gradativamente à medida que o prérRNA é dobrado, modificado e processado. O processamento do pré-rRNA 35S consiste na remoção progressiva de espaçadores internos (ITS1 e ITS2) e externos (5ETS e 3ETS), que separam e flanqueiam os rRNAs maduros componentes de ambas subunidades ribossomais. A clivagem do ITS1 separa as vias de maturação do pré-60S e do pré-40S. O ITS2, que, em associação a fatores de montagem, forma uma estrutura denominada ITS2-foot, é o último espaçador do pré-60S a ser removido. A composição do ITS2-foot permanece inalterada no nucléolo até a transição entre o estado E nucleolar e a formação da partícula Nog2 nuclear. Nesta etapa, a liberação do fator Erb1 permite o recrutamento do fator de montagem conservado e essencial Nop53. Na base do ITS2-foot, Nop53 recruta o exossomo via RNA helicase Mtr4 para a clivagem 3-5 exonucleolítica de parte do ITS2 levando à desmontagem do ITS2-foot. O fato de Nop53 atuar como ponte entre dois grandes complexos e apresentar uma estrutura flexível e estendida nos levou a aprofundar a caracterização de seu papel durante a maturação do pré60S. Neste trabalho, usando análise proteômica quantitativa label-free baseada em espectrometria de massas, caracterizou-se o interactoma de Nop53, e avaliou-se o impacto da depleção de Nop53 no interactoma da subunidade catalítica do exossomo Rrp6 e na composição de pré-ribossomos representativos de quase todas as etapas de maturação do pré-60S. Em paralelo, foram caracterizados mutantes truncados de Nop53 e avaliada por pull-down a interação de Nop53 com componentes do exossomo. Os resultados obtidos mostraram que Nop53 é capaz de interagir com o cofator do exossomo Mpp6, sugerindo pontos adicionais de interação durante o recrutamento do exossomo ao pré-60S. A análise do interactoma de Rrp6 mostrou uma associação precoce do exossomo aos intermediários pré-ribossomais nucleolares mais iniciais, anteriores aos previamente descritos. Mudanças na composição dos intermediários pré-60S revelaram que a depleção de Nop53 afeta a transição entre o estado E e a partícula Nog2, afetando eventos tardios de maturação como o recrutamento de Yvh1. Comparando-se o efeito da depleção de Nop53 com o de mutantes nop53 desprovidos da região de recrutamento do exossomo, obtivemos evidências bioquímicas do papel estrutural de Nop53 na base do ITS2- foot. Em conjunto, estas observações, à luz de estruturas de intermediários pré-ribossomais recentemente descritas, nos permitiram concluir que o recrutamento de Nop53 ao pré-60S contribui para a estabilização de eventos de remodelamento do rRNA que antecedem a formação da partícula Nog2

Ribosomes are conserved ribonucleoprotein complexes formed by two asymmetric subunits (the 40S and the 60S in eukaryotes) responsible for translating the genetic information and catalyzing protein synthesis. The assembly of these complexes in eukaryotes is best described in S. cerevisiae. It is an energetically demanding, multi-step cellular process, that starts in the nucleolus with the transcription of the 35S pre-rRNA. It requires the hierarchical and transient recruitment of about 200 assembly factors to ensure the correct formation of the functional centers suitable for translation. In this process, which extends into the nucleus and cytoplasm, 79 ribosomal proteins gradually associate as the pre-rRNA is folded, modified, and processed. The 35S pre-rRNA processing happens with the progressive removal of internal (ITS1 and ITS2) and external (5'ETS and 3'ETS) transcribed spacers, which separate and flank the mature rRNA components of both ribosomal subunits. The cleavage at the ITS1 separates the pre-60S and pre40S maturation pathways. The ITS2, which in association with assembly factors constitutes a structure called ITS2-foot, is the last pre-60S spacer to be removed. The composition of the ITS2- foot remains unchanged in the nucleolus until the transition between the nucleolar state E and the nuclear Nog2 particle. At this stage, the release of Erb1 allows the recruitment of the conserved and essential assembly factor Nop53. At the base of the ITS2-foot, Nop53 recruits the exosome via the RNA helicase Mtr4 for the ITS2 3'-5' exonucleolytic cleavage leading to the ITS2-foot disassembly. The fact that Nop53 acts as a bridge between these two large complexes and exhibits a flexible and extended structure led us to further characterize its role in the pre-60S maturation. In this work, using mass spectrometry-based label-free quantitative proteomics, we characterized the interactome of Nop53, as well as the impact of the depletion of Nop53 on the interactome of the exosome catalytic subunit Rrp6 and on the composition of pre-ribosomes representative of almost all pre-60S maturation stages. In parallel, we characterized nop53 truncated mutants and evaluated the interaction of Nop53 with exosome components by pulldown assays. The results showed that Nop53 can interact with the exosome cofactor Mpp6, suggesting the contribution of additional points of interaction during the exosome recruitment to the pre-60S. The analysis of the Rrp6 interactome revealed an early association of the exosome with pre-ribosomal intermediates at very early nucleolar stages, before those previously described. Changes in the composition of pre-60S intermediates revealed that Nop53 depletion affects the transition between the state E and the Nog2 particle, affecting late pre-60S maturation events, such as the Yvh1 recruitment. Comparing the effect of Nop53 depletion with that of nop53 mutants lacking the exosome interacting region, we obtained biochemical evidence of the structural role of Nop53 at the base of the ITS2-foot. Altogether, and in light of recently described structures of pre-ribosomal intermediates, these observations allowed us to conclude that the recruitment of Nop53 to the pre-60S contributes to the stabilization of rRNA remodeling events that precede the formation of the Nog2 particle

The Stringent Response Inhibits 70S Ribosome Formation in Staphylococcus aureus by Impeding GTPase-Ribosome Interactions.

During nutrient limitation, bacteria produce the alarmones (p)ppGpp as effectors of a stress signaling network termed the stringent response. RsgA, RbgA, Era, and HflX are four ribosome-associated GTPases (RA-GTPases) that bind to (p)ppGpp in Staphylococcus aureus. These enzymes are cofactors in ribosome assembly, where they cycle between the ON (GTP-bound) and OFF (GDP-bound) ribosome-associated states. Entry into the OFF state occurs upon hydrolysis of GTP, with GTPase activity increasing substantially upon ribosome association. When bound to (p)ppGpp, GTPase activity is inhibited, reducing 70S ribosome assembly and growth. Here, we determine how (p)ppGpp impacts RA-GTPase-ribosome interactions. We show that RA-GTPases preferentially bind to 5'-diphosphate-containing nucleotides GDP and ppGpp over GTP, which is likely exploited as a regulatory mechanism within the cell to shut down ribosome biogenesis during stress. Stopped-flow fluorescence and association assays reveal that when bound to (p)ppGpp, the association of RA-GTPases to ribosomal subunits is destabilized, both in vitro and within bacterial cells. Consistently, structural analysis of the ppGpp-bound RA-GTPase RsgA reveals an OFF-state conformation similar to the GDP-bound state, with the G2/switch I loop adopting a conformation incompatible with ribosome association. Altogether, we highlight (p)ppGpp-mediated inhibition of RA-GTPases as a major mechanism of stringent response-mediated ribosome assembly and growth control. IMPORTANCE The stringent response is a bacterial signaling network that utilizes the nucleotides pppGpp and ppGpp to reprogram cells in order to survive nutritional stresses. However, much about how these important nucleotides control cellular reprogramming is unknown. Our previous work revealed that (p)ppGpp can bind to and inhibit the enzymatic activity of four ribosome-associated GTPases (RA-GTPases), enzymes that facilitate maturation of the 50S and 30S ribosomal subunits. Here, we examine how this occurs mechanistically and demonstrate that this interaction prevents the accommodation of RA-GTPases on ribosomal subunits both in vitro and within bacterial cells, with the ppGpp-bound state structurally mimicking the inactive GDP-bound conformation of the enzyme. We additionally reveal that these GTPase enzymes have a greater affinity for OFF-state-inducing nucleotides, which is a mechanism likely to control ribosome assembly during growth. With this, we further our understanding of how ribosome function is controlled by (p)ppGpp, enabling bacterial survival during stress.

Rational Design of an Antimicrobial Peptide Based on Structural Insight into the Interaction of Pseudomonas aeruginosa Initiation Factor 1 with Its Cognate 30S Ribosomal Subunit.

Bacterial infections continue to represent a major worldwide health hazard following the emergence of drug-resistant pathogenic strains. Pseudomonas aeruginosa is an opportunistic pathogen causing nosocomial infections with increased morbidity and mortality. The increasing antibiotic resistance in P. aeruginosa has led to an unmet need for discovery of new antibiotic candidates. Bacterial protein synthesis is an essential metabolic process and a validated target for antibiotic development; however, the precise structural mechanism in P. aeruginosa remains unknown. In this work, the interaction of P. aeruginosa initiation factor 1 (IF1) with the 30S ribosomal subunit was studied by NMR, which enabled us to construct a structure of IF1-bound 30S complex. A short α-helix in IF1 was found to be critical for IF1 ribosomal binding and function. A peptide derived from this α-helix was tested and displayed a high ability to inhibit bacterial growth. These results provide a clue for rational design of new antimicrobials.

Studying the Structural Organization of Polyribosomes with Alexander S. Spirin.

"Would it be possible to analyze molecular mechanisms and structural organisation of polyribosome assemblies using cryo electron tomography?" - we asked through a longstanding collaboration between my research group and that of Alexander S. Spirin. Indeed, it was: we found that double-row polyribosomes can have both circular and linear arrangements of their mRNA [Afonina, Z. A., et al. (2013) Biochemistry (Moscow)], we figured out how eukaryotic ribosomes assemble on an mRNA to form supramolecular left-handed helices [Myasnikov, A. G., et al. (2014) Nat. Commun.], that the circularization of polyribosomes is poly-A and cap-independent [Afonina, Z. A., et al. (2014) Nucleic Acids Res.], and that intermediary polyribosomes with open structures exist after a transition from a juvenile phase to strongly translating polysomes of medium size [Afonina, Z. A., et al. (2015) Nucleic Acids Res.] until they form densely packed helical structures with reduced activity. Our joint fruitful exchanges, hence, led to major advances in the field, which are reviewed here from a personal and historical perspective in memory of Alexander S. Spirin.

Structured elements drive extensive circular RNA translation.

The human genome encodes tens of thousands circular RNAs (circRNAs) with mostly unknown functions. Circular RNAs require internal ribosome entry sites (IRES) if they are to undergo translation without a 5' cap. Here, we develop a high-throughput screen to systematically discover RNA sequences that can direct circRNA translation in human cells. We identify more than 17,000 endogenous and synthetic sequences as candidate circRNA IRES. 18S rRNA complementarity and a structured RNA element positioned on the IRES are important for driving circRNA translation. Ribosome profiling and peptidomic analyses show extensive IRES-ribosome association, hundreds of circRNA-encoded proteins with tissue-specific distribution, and antigen presentation. We find that circFGFR1p, a protein encoded by circFGFR1 that is downregulated in cancer, functions as a negative regulator of FGFR1 oncoprotein to suppress cell growth during stress. Systematic identification of circRNA IRES elements may provide important links among circRNA regulation, biological function, and disease.