A roadmap for rRNA folding and assembly during transcription

Rapid folding of nascent RNA regulates eukaryotic RNA biogenesis

An RNA's catalytic, regulatory, or coding potential depends on RNA structure formation. Because base pairing occurs during transcription, early structural states can govern RNA processing events and dictate the formation of functional conformations. These co-transcriptional states remain unknown. Here, we develop CoSTseq, which detects nascent RNA base pairing within and upon exit from RNA polymerases (Pols) transcriptome-wide in living yeast cells. By monitoring each nucleotide's base pairing activity during transcription, we identify distinct classes of behaviors. While 47% of rRNA nucleotides remain unpaired, rapid and delayed base pairing - with rates of 48.5 and 13.2 kb -1 of transcribed rDNA, respectively - typically completes when Pol I is only 25 bp downstream. We show that helicases act immediately to remodel structures across the rDNA locus and facilitate ribosome biogenesis. In contrast, nascent pre-mRNAs attain local structures indistinguishable from mature mRNAs, suggesting that refolding behind elongating ribosomes resembles co-transcriptional folding behind Pol II.

Beyond ligand binding: Single molecule observation reveals how riboswitches integrate multiple signals to balance bacterial gene regulation

Riboswitches are specialized RNA structures that orchestrate gene expression in response to sensing specific metabolite or ion ligands, mostly in bacteria. Upon ligand binding, these conformationally dynamic RNA motifs undergo structural changes that control critical gene expression processes such as transcription termination and translation initiation, thereby enabling cellular homeostasis and adaptation. Because RNA folds rapidly and co-transcriptionally, riboswitches make use of the low complexity of RNA sequences to adopt alternative, transient conformations on the heels of the transcribing RNA polymerase (RNAP), resulting in kinetic partitioning that defines the regulatory outcome. This review summarizes single molecule microscopy evidence that has begun to unveil a sophisticated network of dynamic, kinetically balanced interactions between riboswitch architecture and the gene expression machinery that, together, integrate diverse cellular signals.

Mapping snoRNA-target RNA interactions in an RNA binding protein-dependent manner with chimeric eCLIP

Small nucleolar RNAs (snoRNAs) are non-coding RNAs that function in ribosome and spliceosome biogenesis, primarily by guiding modifying enzymes to specific sites on ribosomal RNA (rRNA) and spliceosomal RNA (snRNA). However, many orphan snoRNAs remain uncharacterized, with unidentified or unvalidated targets, and studies on additional snoRNA-associated proteins are limited. We adapted an enhanced chimeric eCLIP approach to comprehensively profile snoRNA-target RNA interactions using both core and accessory snoRNA binding proteins as baits. Using core snoRNA binding proteins, we confirmed most annotated snoRNA-rRNA and snoRNA-snRNA interactions in mouse and human cell lines and called novel, high-confidence interactions for orphan snoRNAs. While some of these interactions result in chemical modification, others may have modification-independent functions. We then showed that snoRNA ribonucleoprotein complexes containing certain accessory proteins, like WDR43 and NOLC1, enriched for specific subsets of snoRNA-target RNA interactions with distinct roles in ribosome and spliceosome biogenesis. Notably, we discovered that SNORD89 guides 2'-O-methylation at two neighboring sites in U2 snRNA that are important for activating splicing, but also appear to ensure imperfect splicing for a subset of near-constitutive exons. Thus, chimeric eCLIP of snoRNA-associating proteins enables a comprehensive framework for studying snoRNA-target interactions in an RNA binding protein-dependent manner, revealing novel interactions and regulatory roles in RNA biogenesis.

Protein Fold Usages in Ribosomes: Another Glance to the Past

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

Putting It All Together: The Roles of Ribosomal Proteins in Nucleolar Stages of 60S Ribosomal Assembly in the Yeast Saccharomyces cerevisiae

Here we review the functions of ribosomal proteins (RPs) in the nucleolar stages of large ribosomal subunit assembly in the yeast Saccharomyces cerevisiae. We summarize the effects of depleting RPs on pre-rRNA processing and turnover, on the assembly of other RPs, and on the entry and exit of assembly factors (AFs). These results are interpreted in light of recent near-atomic-resolution cryo-EM structures of multiple assembly intermediates. Results are discussed with respect to each neighborhood of RPs and rRNA. We identify several key mechanisms related to RP behavior. Neighborhoods of RPs can assemble in one or more than one step. Entry of RPs can be triggered by molecular switches, in which an AF is replaced by an RP binding to the same site. To drive assembly forward, rRNA structure can be stabilized by RPs, including clamping rRNA structures or forming bridges between rRNA domains.

The DEAD-box ATPase Dbp10/DDX54 initiates peptidyl transferase center formation during 60S ribosome biogenesis

DEAD-box ATPases play crucial roles in guiding rRNA restructuring events during the biogenesis of large (60S) ribosomal subunits, but their precise molecular functions are currently unknown. In this study, we present cryo-EM reconstructions of nucleolar pre-60S intermediates that reveal an unexpected, alternate secondary structure within the nascent peptidyl-transferase-center (PTC). Our analysis of three sequential nucleolar pre-60S intermediates reveals that the DEAD-box ATPase Dbp10/DDX54 remodels this alternate base pairing and enables the formation of the rRNA junction that anchors the mature form of the universally conserved PTC A-loop. Post-catalysis, Dbp10 captures rRNA helix H61, initiating the concerted exchange of biogenesis factors during late nucleolar 60S maturation. Our findings show that Dbp10 activity is essential for the formation of the ribosome active site and reveal how this function is integrated with subsequent assembly steps to drive the biogenesis of the large ribosomal subunit.

Comprehensive map of ribosomal 2'-O-methylation and C/D box snoRNAs in Drosophila melanogaster

During their maturation, ribosomal RNAs (rRNAs) are decorated by hundreds of chemical modifications that participate in proper folding of rRNA secondary structures and therefore in ribosomal function. Along with pseudouridine, methylation of the 2'-hydroxyl ribose moiety (Nm) is the most abundant modification of rRNAs. The majority of Nm modifications in eukaryotes are placed by Fibrillarin, a conserved methyltransferase belonging to a ribonucleoprotein complex guided by C/D box small nucleolar RNAs (C/D box snoRNAs). These modifications impact interactions between rRNAs, tRNAs and mRNAs, and some are known to fine tune translation rates and efficiency. In this study, we built the first comprehensive map of Nm sites in Drosophila melanogaster rRNAs using two complementary approaches (RiboMethSeq and Nanopore direct RNA sequencing) and identified their corresponding C/D box snoRNAs by whole-transcriptome sequencing. We de novo identified 61 Nm sites, from which 55 are supported by both sequencing methods, we validated the expression of 106 C/D box snoRNAs and we predicted new or alternative rRNA Nm targets for 31 of them. Comparison of methylation level upon different stresses show only slight but specific variations, indicating that this modification is relatively stable in D. melanogaster. This study paves the way to investigate the impact of snoRNA-mediated 2'-O-methylation on translation and proteostasis in a whole organism.

Are non-protein coding RNAs junk or treasure?: An attempt to explain and reconcile opposing viewpoints of whether the human genome is mostly transcribed into non-functional or functional RNAs

The human genome project's lasting legacies are the emerging insights into human physiology and disease, and the ascendance of biology as the dominant science of the 21st century. Sequencing revealed that >90% of the human genome is not coding for proteins, as originally thought, but rather is overwhelmingly transcribed into non-protein coding, or non-coding, RNAs (ncRNAs). This discovery initially led to the hypothesis that most genomic DNA is "junk", a term still championed by some geneticists and evolutionary biologists. In contrast, molecular biologists and biochemists studying the vast number of transcripts produced from most of this genome "junk" often surmise that these ncRNAs have biological significance. What gives? This essay contrasts the two opposing, extant viewpoints, aiming to explain their bases, which arise from distinct reference frames of the underlying scientific disciplines. Finally, it aims to reconcile these divergent mindsets in hopes of stimulating synergy between scientific fields.

Costs of ribosomal RNA stabilization affect ribosome composition at maximum growth rate

Ribosomes are key to cellular self-fabrication and limit growth rate. While most enzymes are proteins, ribosomes consist of 1/3 protein and 2/3 ribonucleic acid (RNA) (in E. coli).Here, we develop a mechanistic model of a self-fabricating cell, validated across diverse growth conditions. Through resource balance analysis (RBA), we explore the variation in maximum growth rate with ribosome composition, assuming constant kinetic parameters.Our model highlights the importance of RNA instability. If we neglect it, RNA synthesis is always cheaper than protein synthesis, leading to an RNA-only ribosome at maximum growth rate. Upon accounting for RNA turnover, we find that a mixed ribosome composed of RNA and proteins maximizes growth rate. To account for RNA turnover, we explore two scenarios regarding the activity of RNases. In (a) degradation is proportional to RNA content. In (b) ribosomal proteins cooperatively mitigate RNA instability by protecting it from misfolding and subsequent degradation. In both cases, higher protein content elevates protein synthesis costs and simultaneously lowers RNA turnover expenses, resulting in mixed RNA-protein ribosomes. Only scenario (b) aligns qualitatively with experimental data across varied growth conditions.Our research provides fresh insights into ribosome biogenesis and evolution, paving the way for understanding protein-rich ribosomes in archaea and mitochondria.

The DEAD-box ATPase Dbp10/DDX54 initiates peptidyl transferase center formation during 60S ribosome biogenesis

DEAD-box ATPases play crucial roles in guiding rRNA restructuring events during the biogenesis of large (60S) ribosomal subunits, but their precise molecular functions are currently unknown. In this study, we present cryo-EM reconstructions of nucleolar pre-60S intermediates that reveal an unexpected, alternate secondary structure within the nascent peptidyl-transferase-center (PTC). Our analysis of three sequential nucleolar pre-60S intermediates reveals that the DEAD-box ATPase Dbp10/DDX54 remodels this alternate base pairing and enables the formation of the rRNA junction that anchors the mature form of the universally conserved PTC A-loop. Post-catalysis, Dbp10 captures rRNA helix H61, initiating the concerted exchange of biogenesis factors during late nucleolar 60S maturation. Our findings show that Dbp10 activity is essential for the formation of the ribosome active site and reveal how this function is integrated with subsequent assembly steps to drive the biogenesis of the large ribosomal subunit.

A disease associated mutant reveals how Ltv1 orchestrates RP assembly and rRNA folding of the small ribosomal subunit head

Ribosomes are complex macromolecules assembled from 4 rRNAs and 79 ribosomal proteins (RPs). Their assembly is organized in a highly hierarchical manner, which is thought to avoid dead-end pathways, thereby enabling efficient assembly of ribosomes in the large quantities needed for healthy cellular growth. Moreover, hierarchical assembly also can help ensure that each RP is included in the mature ribosome. Nonetheless, how this hierarchy is achieved remains unknown, beyond the examples that depend on direct RP-RP interactions, which account for only a fraction of the observed dependencies. Using assembly of the small subunit head and a disease-associated mutation in the assembly factor Ltv1 as a model system, we dissect here how the hierarchy in RP binding is constructed. A combination of data from yeast genetics, mass spectrometry, DMS probing and biochemical experiments demonstrate that the LIPHAK-disease-associated Ltv1 mutation leads to global defects in head assembly, which are explained by direct binding of Ltv1 to 5 out of 15 RPs, and indirect effects that affect 4 additional RPs. These indirect effects are mediated by conformational transitions in the nascent subunit that are regulated by Ltv1. Mechanistically, Ltv1 aids the recruitment of some RPs via direct protein-protein interactions, but surprisingly also delays the recruitment of other RPs. Delayed binding of key RPs also delays the acquisition of RNA structure that is stabilized by these proteins. Finally, our data also indicate direct roles for Ltv1 in chaperoning the folding of a key rRNA structural element, the three-helix junction j34-35-38. Thus, Ltv1 plays critical roles in organizing the order of both RP binding to rRNA and rRNA folding, thereby enabling efficient 40S subunit assembly.

KsgA facilitates ribosomal small subunit maturation by proofreading a key structural lesion

Ribosome assembly is orchestrated by many assembly factors, including ribosomal RNA methyltransferases, whose precise role is poorly understood. Here, we leverage the power of cryo-EM and machine learning to discover that the E. coli methyltransferase KsgA performs a 'proofreading' function in the assembly of the small ribosomal subunit by recognizing and partially disassembling particles that have matured but are not competent for translation. We propose that this activity allows inactive particles an opportunity to reassemble into an active state, thereby increasing overall assembly fidelity. Detailed structural quantifications in our datasets additionally enabled the expansion of the Nomura assembly map to highlight rRNA helix and r-protein interdependencies, detailing how the binding and docking of these elements are tightly coupled. These results have wide-ranging implications for our understanding of the quality-control mechanisms governing ribosome biogenesis and showcase the power of heterogeneity analysis in cryo-EM to unveil functionally relevant information in biological systems.

A disease associated mutant reveals how Ltv1 orchestrates RP assembly and rRNA folding of the small ribosomal subunit head

Ribosomes are complex macromolecules assembled from 4 rRNAs and 79 ribosomal proteins (RPs). Their assembly is organized in a highly hierarchical manner, which is thought to avoid dead-end pathways, thereby enabling efficient assembly of ribosomes in the large quantities needed for healthy cellular growth. Moreover, hierarchical assembly also can help ensure that each RP is included in the mature ribosome. Nonetheless, how this hierarchy is achieved remains unknown, beyond the examples that depend on direct RP-RP interactions, which account for only a fraction of the observed dependencies. Using assembly of the small subunit head and a disease-associated mutation in the assembly factor Ltv1 as a model system, we dissect here how the hierarchy in RP binding is constructed. Our data demonstrate that the LIPHAK-disease-associated Ltv1 mutation leads to global defects in head assembly, which are explained by direct binding of Ltv1 to 5 out of 15 RPs, and indirect effects that affect 4 additional RPs. These indirect effects are mediated by conformational transitions in the nascent subunit that are regulated by Ltv1. Mechanistically, Ltv1 aids the recruitment of some RPs via direct protein-protein interactions, but surprisingly also delays the recruitment of other RPs. Delayed binding of key RPs also delays the acquisition of RNA structure that is stabilized by these proteins. Finally, our data also indicate direct roles for Ltv1 in chaperoning the folding of a key rRNA structural element, the three-helix junction j34-35-38. Thus, Ltv1 plays critical roles in organizing the order of both RP binding to rRNA and rRNA folding, thereby enabling efficient 40S subunit assembly.

Tertiary structure of single-instant RNA molecule reveals folding landscape

The folding of RNA and protein molecules during their synthesis is a crucial self-assembly process that nature employs to convert genetic information into the complex molecular machinery that supports life. Misfolding events are the cause of several diseases, and the folding pathway of central biomolecules, such as the ribosome, is strictly regulated by programmed maturation processes and folding chaperones. However, the dynamic folding processes are challenging to study because current structure determination methods heavily rely on averaging, and existing computational methods do not efficiently simulate non-equilibrium dynamics. Here we utilize individual-particle cryo-electron tomography (IPET) to investigate the folding landscape of a rationally designed RNA origami 6-helix bundle that undergoes slow maturation from a "young" to "mature" conformation. By optimizing the IPET imaging and electron dose conditions, we obtain 3D reconstructions of 120 individual particles at resolutions ranging from 23-35 Å, enabling us first-time to observe individual RNA helices and tertiary structures without averaging. Statistical analysis of 120 tertiary structures confirms the two main conformations and suggests a possible folding pathway driven by helix-helix compaction. Studies of the full conformational landscape reveal both trapped states, misfolded states, intermediate states, and fully compacted states. The study provides novel insight into RNA folding pathways and paves the way for future studies of the energy landscape of molecular machines and self-assembly processes.

Emerging Quantitative Biochemical, Structural, and Biophysical Methods for Studying Ribosome and Protein-RNA Complex Assembly

Ribosome assembly is one of the most fundamental processes of gene expression and has served as a playground for investigating the molecular mechanisms of how protein-RNA complexes (RNPs) assemble. A bacterial ribosome is composed of around 50 ribosomal proteins, several of which are co-transcriptionally assembled on a ~4500-nucleotide-long pre-rRNA transcript that is further processed and modified during transcription, the entire process taking around 2 min in vivo and being assisted by dozens of assembly factors. How this complex molecular process works so efficiently to produce an active ribosome has been investigated over decades, resulting in the development of a plethora of novel approaches that can also be used to study the assembly of other RNPs in prokaryotes and eukaryotes. Here, we review biochemical, structural, and biophysical methods that have been developed and integrated to provide a detailed and quantitative understanding of the complex and intricate molecular process of bacterial ribosome assembly. We also discuss emerging, cutting-edge approaches that could be used in the future to study how transcription, rRNA processing, cellular factors, and the native cellular environment shape ribosome assembly and RNP assembly at large.

Small RNAs and Hfq capture unfolded RNA target sites during transcription

Small ribonucleoproteins (sRNPs) target nascent precursor RNAs to guide folding, modification, and splicing during transcription. Yet, rapid co-transcriptional folding of the RNA can mask sRNP sites, impeding target recognition and regulation. To examine how sRNPs target nascent RNAs, we monitored binding of bacterial Hfq⋅DsrA sRNPs to rpoS transcripts using single-molecule co-localization co-transcriptional assembly (smCoCoA). We show that Hfq⋅DsrA recursively samples the mRNA before transcription of the target site to poise it for base pairing with DsrA. We adapted smCoCoA to precisely measure when the target site is synthesized and revealed that Hfq⋅DsrA often binds the mRNA during target site synthesis close to RNA polymerase (RNAP). We suggest that targeting transcripts near RNAP allows an sRNP to capture a site before the transcript folds, providing a kinetic advantage over post-transcriptional targeting. We propose that other sRNPs may also use RNAP-proximal targeting to hasten recognition and regulation.

Cryo-EM captures early ribosome assembly in action

Ribosome biogenesis is a fundamental multi-step cellular process in all domains of life that involves the production, processing, folding, and modification of ribosomal RNAs (rRNAs) and ribosomal proteins. To obtain insights into the still unexplored early assembly phase of the bacterial 50S subunit, we exploited a minimal in vitro reconstitution system using purified ribosomal components and scalable reaction conditions. Time-limited assembly assays combined with cryo-EM analysis visualizes the structurally complex assembly pathway starting with a particle consisting of ordered density for only ~500 nucleotides of 23S rRNA domain I and three ribosomal proteins. In addition, our structural analysis reveals that early 50S assembly occurs in a domain-wise fashion, while late 50S assembly proceeds incrementally. Furthermore, we find that both ribosomal proteins and folded rRNA helices, occupying surface exposed regions on pre-50S particles, induce, or stabilize rRNA folds within adjacent regions, thereby creating cooperativity.

RBM3 suppresses stemness remodeling of prostate cancer in bone microenvironment by modulating N6-methyladenosine on CTNNB1 mRNA

Bone metastasis is the most happened metastatic event in prostate cancer (PCa) and needs a large effort in treatment. When PCa metastasizes to the bone, the new microenvironment can induce the epigenome reprogramming and stemness remodeling of cancer cells, thereby increasing the adaptability of cancer cells to the bone microenvironment, and this even leads to the occurrence of secondary tumor metastasis. Our group has previously found that RNA binding motif 3 (RBM3) affects the stem cell-like properties of PCa by interfering with alternative splicing of CD44. However, whether RBM3, as a stress-response protein, can resist microenvironmental remodeling of PCa particularly in bone metastasis remains unknown. By co-culturing PCa cells with osteoblasts to mimic PCa bone metastases, we found that RBM3 upregulates the N6-methyladenosine (m⁶A) methylation on the mRNA of catenin beta 1 (CTNNB1) in a manner dependent on methyltransferase 3 (METTL3), an N6-adenosine-methyltransferase complex catalytic subunit. Consequently, this modification results in a decreased stability of CTNNB1 mRNA and a followed inactivation of Wnt signaling, which ultimately inhibits the stemness remodeling of PCa cells by osteoblasts. Thus, the present study may extend our understanding of the inhibitory role of RBM3 on particularly bone metastasis of PCa.

DrTransformer: heuristic cotranscriptional RNA folding using the nearest neighbor energy model

MOTIVATION

Folding during transcription can have an important influence on the structure and function of RNA molecules, as regions closer to the 5' end can fold into metastable structures before potentially stronger interactions with the 3' end become available. Thermodynamic RNA folding models are not suitable to predict structures that result from cotranscriptional folding, as they can only calculate properties of the equilibrium distribution. Other software packages that simulate the kinetic process of RNA folding during transcription exist, but they are mostly applicable for short sequences.

RESULTS

We present a new algorithm that tracks changes to the RNA secondary structure ensemble during transcription. At every transcription step, new representative local minima are identified, a neighborhood relation is defined and transition rates are estimated for kinetic simulations. After every simulation, a part of the ensemble is removed and the remainder is used to search for new representative structures. The presented algorithm is deterministic (up to numeric instabilities of simulations), fast (in comparison with existing methods), and it is capable of folding RNAs much longer than 200 nucleotides.

AVAILABILITY AND IMPLEMENTATION

This software is open-source and available at https://github.com/ViennaRNA/drtransformer.

SUPPLEMENTARY INFORMATION

Supplementary data are available at Bioinformatics online.

Manipulation of topoisomerase expression inhibits cell division but not growth and reveals a distinctive promoter structure in Synechocystis

In cyanobacteria DNA supercoiling varies over the diurnal cycle and is integrated with temporal programs of transcription and replication. We manipulated DNA supercoiling in Synechocystis sp. PCC 6803 by CRISPRi-based knockdown of gyrase subunits and overexpression of topoisomerase I (TopoI). Cell division was blocked but cell growth continued in all strains. The small endogenous plasmids were only transiently relaxed, then became strongly supercoiled in the TopoI overexpression strain. Transcript abundances showed a pronounced 5'/3' gradient along transcription units, incl. the rRNA genes, in the gyrase knockdown strains. These observations are consistent with the basic tenets of the homeostasis and twin-domain models of supercoiling in bacteria. TopoI induction initially led to downregulation of G+C-rich and upregulation of A+T-rich genes. The transcriptional response quickly bifurcated into six groups which overlap with diurnally co-expressed gene groups. Each group shows distinct deviations from a common core promoter structure, where helically phased A-tracts are in phase with the transcription start site. Together, our data show that major co-expression groups (regulons) in Synechocystis all respond differentially to DNA supercoiling, and suggest to re-evaluate the long-standing question of the role of A-tracts in bacterial promoters.

The promise of cryo-EM to explore RNA structural dynamics

Conformational dynamics are essential to macromolecular function. This is certainly true of RNA, whose ability to undergo programmed conformational dynamics is essential to create and regulate complex biological processes. However, methods to easily and simultaneously interrogate both the structure and conformational dynamics of fully functional RNAs in isolation and in complex with proteins have not historically been available. Due to its ability to image and classify single particles, cryogenic electron microscopy (cryo-EM) has the potential to address this gap and may be particularly amenable to exploring structural dynamics within the three-dimensional folds of biologically active RNAs. We discuss the possibilities and current limitations of applying cryo-EM to simultaneously study RNA structure and conformational dynamics, and present one example that illustrates this (as of yet) not fully realized potential.

Nucleolus and Nucleolar Stress: From Cell Fate Decision to Disease Development

Besides the canonical function in ribosome biogenesis, there have been significant recent advances towards the fascinating roles of the nucleolus in stress response, cell destiny decision and disease progression. Nucleolar stress, an emerging concept describing aberrant nucleolar structure and function as a result of impaired rRNA synthesis and ribosome biogenesis under stress conditions, has been linked to a variety of signaling transductions, including but not limited to Mdm2-p53, NF-κB and HIF-1α pathways. Studies have uncovered that nucleolus is a stress sensor and signaling hub when cells encounter various stress conditions, such as nutrient deprivation, DNA damage and oxidative and thermal stress. Consequently, nucleolar stress plays a pivotal role in the determination of cell fate, such as apoptosis, senescence, autophagy and differentiation, in response to stress-induced damage. Nucleolar homeostasis has been involved in the pathogenesis of various chronic diseases, particularly tumorigenesis, neurodegenerative diseases and metabolic disorders. Mechanistic insights have revealed the indispensable role of nucleolus-initiated signaling in the progression of these diseases. Accordingly, the intervention of nucleolar stress may pave the path for developing novel therapies against these diseases. In this review, we systemically summarize recent findings linking the nucleolus to stress responses, signaling transduction and cell-fate decision, set the spotlight on the mechanisms by which nucleolar stress drives disease progression, and highlight the merit of the intervening nucleolus in disease treatment.

Cryo-EM reveals an entangled kinetic trap in the folding of a catalytic RNA

Functional RNAs fold through complex pathways that can contain misfolded "kinetic traps." A complete model of RNA folding requires understanding the formation of these misfolded states, but they are difficult to characterize because of their transient and potentially conformationally dynamic nature. We used cryo-electron microscopy (cryo-EM) to visualize a long-lived misfolded state in the folding pathway of the Tetrahymena thermophila group I intron, a paradigmatic RNA structure-function model system. The structure revealed how this state forms native-like secondary structure and tertiary contacts but contains two incorrectly crossed strands, consistent with a previous model. This incorrect topology mispositions a critical catalytic domain and cannot be resolved locally as extensive refolding is required. This work provides a structural framework for interpreting decades of biochemical and functional studies and demonstrates the power of cryo-EM for the exploration of RNA folding pathways.

Prp43/DHX15 exemplify RNA helicase multifunctionality in the gene expression network

Dynamic regulation of RNA folding and structure is critical for the biogenesis and function of RNAs and ribonucleoprotein (RNP) complexes. Through their nucleotide triphosphate-dependent remodelling functions, RNA helicases are key modulators of RNA/RNP structure. While some RNA helicases are dedicated to a specific target RNA, others are multifunctional and engage numerous substrate RNAs in different aspects of RNA metabolism. The discovery of such multitasking RNA helicases raises the intriguing question of how these enzymes can act on diverse RNAs but also maintain specificity for their particular targets within the RNA-dense cellular environment. Furthermore, the identification of RNA helicases that sit at the nexus between different aspects of RNA metabolism raises the possibility that they mediate cross-regulation of different cellular processes. Prominent and extensively characterized multifunctional DEAH/RHA-box RNA helicases are DHX15 and its Saccharomyces cerevisiae (yeast) homologue Prp43. Due to their central roles in key cellular processes, these enzymes have also served as prototypes for mechanistic studies elucidating the mode of action of this type of enzyme. Here, we summarize the current knowledge on the structure, regulation and cellular functions of Prp43/DHX15, and discuss the general concept and implications of RNA helicase multifunctionality.

Nucleic Acid Thermodynamics Derived from Mechanical Unzipping Experiments

Force-spectroscopy techniques have led to significant progress in studying the physicochemical properties of biomolecules that are not accessible in bulk assays. The application of piconewton forces with laser optical tweezers to single nucleic acids has permitted the characterization of molecular thermodynamics and kinetics with unprecedented accuracy. Some examples are the hybridization reaction between complementary strands in DNA and the folding of secondary, tertiary, and other heterogeneous structures, such as intermediate and misfolded states in RNA. Here we review the results obtained in our lab on deriving the nearest-neighbor free energy parameters in DNA and RNA duplexes from mechanical unzipping experiments. Remarkable nonequilibrium effects are also observed, such as the large irreversibility of RNA unzipping and the formation of non-specific secondary structures in single-stranded DNA. These features originate from forming stem-loop structures along the single strands of the nucleic acid. The recently introduced barrier energy landscape model quantifies kinetic trapping effects due to stem-loops being applicable to both RNA and DNA. The barrier energy landscape model contains the essential features to explain the many behaviors observed in heterogeneous nucleic-acid folding.

How does RNA fold dynamically?

Recent advances in interrogating RNA folding dynamics have shown the classical model of RNA folding to be incomplete. Here, we pose three prominent questions for the field that are at the forefront of our understanding of the importance of RNA folding dynamics for RNA function. The first centers on the most appropriate biophysical framework to describe changes to the RNA folding energy landscape that a growing RNA chain encounters during transcriptional elongation. The second focuses on the potential ubiquity of strand displacement - a process by which RNA can rapidly change conformations - and how this process may be generally present in broad classes of seemingly different RNAs. The third raises questions about the potential importance and roles of cellular protein factors in RNA conformational switching. Answers to these questions will greatly improve our fundamental knowledge of RNA folding and function, drive biotechnological advances that utilize engineered RNAs, and potentially point to new areas of biology yet to be discovered.

Cotranscriptional RNA Chemical Probing

Cotranscriptional folding is a fundamental step in RNA biogenesis and the basis for many RNA-mediated gene regulation systems. Understanding how RNA folds as it is synthesized requires experimental methods that can systematically identify intermediate RNA structures that form during transcription. Cotranscriptional RNA chemical probing experiments achieve this by applying high-throughput RNA structure probing to an in vitro transcribed array of cotranscriptionally folded intermediate transcripts. In this chapter, we present guidelines and procedures for integrating single-round in vitro transcription using E. coli RNA polymerase with high-throughput RNA chemical probing workflows. We provide an overview of key concepts including DNA template design, transcription roadblocking strategies, single-round in vitro transcription with E. coli RNA polymerase, and RNA chemical probing and describe procedures for DNA template preparation, cotranscriptional RNA chemical probing, RNA purification, and 3' adapter ligation. The end result of these procedures is a purified RNA library that can be prepared for Illumina sequencing using established high-throughput RNA structure probing library construction strategies.

RNA folding and functions of RNA helicases in ribosome biogenesis

Eukaryotic ribosome biogenesis involves the synthesis of ribosomal RNA (rRNA) and its stepwise folding into the unique structure present in mature ribosomes. rRNA folding starts already co-transcriptionally in the nucleolus and continues when pre-ribosomal particles further maturate in the nucleolus and upon their transit to the nucleoplasm and cytoplasm. While the approximate order of folding of rRNA subdomains is known, especially from cryo-EM structures of pre-ribosomal particles, the actual mechanisms of rRNA folding are less well understood. Both small nucleolar RNAs (snoRNAs) and proteins have been implicated in rRNA folding. snoRNAs hybridize to precursor rRNAs (pre-rRNAs) and thereby prevent premature folding of the respective rRNA elements. Ribosomal proteins (r-proteins) and ribosome assembly factors might have a similar function by binding to rRNA elements and preventing their premature folding. Besides that, a small group of ribosome assembly factors are thought to play a more active role in rRNA folding. In particular, multiple RNA helicases participate in individual ribosome assembly steps, where they are believed to coordinate RNA folding/unfolding events or the release of proteins from the rRNA. In this review, we summarize the current knowledge on mechanisms of RNA folding and on the specific function of the individual RNA helicases involved. As the yeast Saccharomyces cerevisiae is the organism in which ribosome biogenesis and the role of RNA helicases in this process is best studied, we focused our review on insights from this model organism, but also make comparisons to other organisms where applicable.

NAP1L5 Promotes Nucleolar Hypertrophy and Is Required for Translation Activation During Cardiomyocyte Hypertrophy

Pathological growth of cardiomyocytes during hypertrophy is characterized by excess protein synthesis; however, the regulatory mechanism remains largely unknown. Using a neonatal rat ventricular myocytes (NRVMs) model, here we find that the expression of nucleosome assembly protein 1 like 5 (Nap1l5) is upregulated in phenylephrine (PE)-induced hypertrophy. Knockdown of Nap1l5 expression by siRNA significantly blocks cell size enlargement and pathological gene induction after PE treatment. In contrast, Adenovirus-mediated Nap1l5 overexpression significantly aggravates the pro-hypertrophic effects of PE on NRVMs. RNA-seq analysis reveals that Nap1l5 knockdown reverses the pro-hypertrophic transcriptome reprogramming after PE treatment. Whereas, immune response is dominantly enriched in the upregulated genes, oxidative phosphorylation, cardiac muscle contraction and ribosome-related pathways are remarkably enriched in the down-regulated genes. Although Nap1l5-mediated gene regulation is correlated with PRC2 and PRC1, Nap1l5 does not directly alter the levels of global histone methylations at K4, K9, K27 or K36. However, puromycin incorporation assay shows that Nap1l5 is both necessary and sufficient to promote protein synthesis in cardiomyocyte hypertrophy. This is attributable to a direct regulation of nucleolus hypertrophy and subsequent ribosome assembly. Our findings demonstrate a previously unrecognized role of Nap1l5 in translation control during cardiac hypertrophy.