RNase E in the γ-Proteobacteria: conservation of intrinsically disordered noncatalytic region and molecular evolution of microdomains

Cooperation of regulatory RNA and the RNA degradosome in transcript surveillance

The ompD transcript, encoding an outer membrane porin in Salmonella, harbors a controlling element in its coding region that base-pairs imperfectly with a 'seed' region of the small regulatory RNA (sRNA) MicC. When tagged with the sRNA, the ompD mRNA is cleaved downstream of the pairing site by the conserved endoribonuclease RNase E, leading to transcript destruction. We observe that the sRNA-induced cleavage site is accessible to RNase E in vitro upon recruitment of ompD into the 30S translation pre-initiation complex (PIC) in the presence of the degradosome components. Evaluation of substrate accessibility suggests that the paused 30S PIC presents the mRNA for targeted recognition and degradation. Ribonuclease activity on PIC-bound ompD is critically dependent on the recruitment of RNase E into the multi-enzyme RNA degradosome, and our data suggest a process of substrate capture and handover to catalytic sites within the degradosome, in which sequential steps of seed matching and duplex remodelling contribute to cleavage efficiency. Our findings support a putative mechanism of surveillance at translation that potentially terminates gene expression efficiently and rapidly in response to signals provided by regulatory RNA.

Progress toward Proteome-Wide Photo-Cross-Linking to Enable Residue-Level Visualization of Protein Structures and Networks In Vivo

Cross-linking mass spectrometry (XL-MS) is emerging as a method at the crossroads of structural and cellular biology, uniquely capable of identifying protein-protein interactions with residue-level resolution and on the proteome-wide scale. With the development of cross-linkers that can form linkages inside cells and easily cleave during fragmentation on the mass spectrometer (MS-cleavable cross-links), it has become increasingly facile to identify contacts between any two proteins in complex samples, including in live cells or tissues. Photo-cross-linkers possess the advantages of high temporal resolution and high reactivity, thereby engaging all residue-types (rather than just lysine); nevertheless, photo-cross-linkers have not enjoyed widespread use and are yet to be employed for proteome-wide studies because their products are challenging to identify. Here, we demonstrate the synthesis and application of two heterobifunctional photo-cross-linkers that feature diazirines and N-hydroxy-succinimidyl carbamate groups, the latter of which unveil doubly fissile MS-cleavable linkages upon acyl transfer to protein targets. Moreover, these cross-linkers demonstrate high water-solubility and cell-permeability. Using these compounds, we demonstrate the feasibility of proteome-wide photo-cross-linking in cellulo. These studies elucidate a small portion of Escherichia coli's interaction network, albeit with residue-level resolution. With further optimization, these methods will enable the detection of protein quinary interaction networks in their native environment at residue-level resolution, and we expect that they will prove useful toward the effort to explore the molecular sociology of the cell.

Attachment of the RNA degradosome to the bacterial inner cytoplasmic membrane prevents wasteful degradation of rRNA in ribosome assembly intermediates

RNA processing and degradation shape the transcriptome by generating stable molecules that are necessary for translation (rRNA and tRNA) and by facilitating the turnover of mRNA, which is necessary for the posttranscriptional control of gene expression. In bacteria and the plant chloroplast, RNA degradosomes are multienzyme complexes that process and degrade RNA. In many bacterial species, the endoribonuclease RNase E is the central component of the RNA degradosome. RNase E-based RNA degradosomes are inner membrane proteins in a large family of gram-negative bacteria (β- and γ-Proteobacteria). Until now, the reason for membrane localization was not understood. Here, we show that a mutant strain of Escherichia coli, in which the RNA degradosome is localized to the interior of the cell, has high levels of 20S and 40S particles that are defective intermediates in ribosome assembly. These particles have aberrant protein composition and contain rRNA precursors that have been cleaved by RNase E. After RNase E cleavage, rRNA fragments are degraded to nucleotides by exoribonucleases. In vitro, rRNA in intact ribosomes is resistant to RNase E cleavage, whereas protein-free rRNA is readily degraded. We conclude that RNA degradosomes in the nucleoid of the mutant strain interfere with cotranscriptional ribosome assembly. We propose that membrane-attached RNA degradosomes in wild-type cells control the quality of ribosome assembly after intermediates are released from the nucleoid. That is, the compact structure of mature ribosomes protects rRNA against cleavage by RNase E. Turnover of a proportion of intermediates in ribosome assembly explains slow growth of the mutant strain. Competition between mRNA and rRNA degradation could be the cause of slower mRNA degradation in the mutant strain. We conclude that attachment of the RNA degradosome to the bacterial inner cytoplasmic membrane prevents wasteful degradation of rRNA precursors, thus explaining the reason for conservation of membrane-attached RNA degradosomes throughout the β- and γ-Proteobacteria.

Structural Insights into the Dimeric Form of Bacillus subtilis RNase Y Using NMR and AlphaFold

RNase Y is a crucial component of genetic translation, acting as the key enzyme initiating mRNA decay in many Gram-positive bacteria. The N-terminal domain of Bacillus subtilis RNase Y (Nter-BsRNaseY) is thought to interact with various protein partners within a degradosome complex. Bioinformatics and biophysical analysis have previously shown that Nter-BsRNaseY, which is in equilibrium between a monomeric and a dimeric form, displays an elongated fold with a high content of α-helices. Using multidimensional heteronuclear NMR and AlphaFold models, here, we show that the Nter-BsRNaseY dimer is constituted of a long N-terminal parallel coiled-coil structure, linked by a turn to a C-terminal region composed of helices that display either a straight or bent conformation. The structural organization of the N-terminal domain is maintained within the AlphaFold model of the full-length RNase Y, with the turn allowing flexibility between the N- and C-terminal domains. The catalytic domain is globular, with two helices linking the KH and HD modules, followed by the C-terminal region. This latter region, with no function assigned up to now, is most likely involved in the dimerization of B. subtilis RNase Y together with the N-terminal coiled-coil structure.

Compartmentalization of RNA Degradosomes in Bacteria Controls Accessibility to Substrates and Ensures Concerted Degradation of mRNA to Nucleotides

RNA degradosomes are multienzyme complexes composed of ribonucleases, RNA helicases, and metabolic enzymes. RNase E-based degradosomes are widespread in Proteobacteria. The Escherichia coli RNA degradosome is sequestered from transcription in the nucleoid and translation in the cytoplasm by localization to the inner cytoplasmic membrane, where it forms short-lived clusters that are proposed to be sites of mRNA degradation. In Caulobacter crescentus, RNA degradosomes localize to ribonucleoprotein condensates in the interior of the cell [bacterial ribonucleoprotein-bodies (BR-bodies)], which have been proposed to drive the concerted degradation of mRNA to nucleotides. The turnover of mRNA in growing cells is important for maintaining pools of nucleotides for transcription and DNA replication. Membrane attachment of the E. coli RNA degradosome is necessary to avoid wasteful degradation of intermediates in ribosome assembly. Sequestering RNA degradosomes to C. crescentus BR-bodies, which exclude structured RNA, could have a similar role in protecting intermediates in ribosome assembly from degradation. Expected final online publication date for the Annual Review of Microbiology, Volume 76 is September 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Uncovering Non-random Binary Patterns Within Sequences of Intrinsically Disordered Proteins

Sequence-ensemble relationships of intrinsically disordered proteins (IDPs) are governed by binary patterns such as the linear clustering or mixing of specific residues or residue types with respect to one another. To enable the discovery of potentially important, shared patterns across sequence families, we describe a computational method referred to as NARDINI for Non-random Arrangement of Residues in Disordered Regions Inferred using Numerical Intermixing. This work was partially motivated by the observation that parameters that are currently in use for describing different binary patterns are not interoperable across IDPs of different amino acid compositions and lengths. In NARDINI, we generate an ensemble of scrambled sequences to set up a composition-specific null model for the patterning parameters of interest. We then compute a series of pattern-specific z-scores to quantify how each pattern deviates from a null model for the IDP of interest. The z-scores help in identifying putative non-random linear sequence patterns within an IDP. We demonstrate the use of NARDINI derived z-scores by identifying sequence patterns in three well-studied IDP systems. We also demonstrate how NARDINI can be deployed to study archetypal IDPs across homologs and orthologs. Overall, NARDINI is likely to aid in designing novel IDPs with a view toward engineering new sequence-function relationships or uncovering cryptic ones. We further propose that the z-scores introduced here are likely to be useful for theoretical and computational descriptions of sequence-ensemble relationships across IDPs of different compositions and lengths.

Regulator of ribonuclease activity modulates the pathogenicity of Vibrio vulnificus

RraA, a protein regulator of RNase E activity, plays a unique role in modulating the mRNA abundance in Escherichia coli. The marine pathogenic bacterium Vibrio vulnificus also possesses homologs of RNase E (VvRNase E) and RraA (VvRraA1 and VvRraA2). However, their physiological roles have not yet been investigated. In this study, we demonstrated that VvRraA1 expression levels affect the pathogenicity of V. vulnificus. Compared to the wild-type strain, the VvrraA1-deleted strain (ΔVvrraA1) showed decreased motility, invasiveness, biofilm formation ability as well as virulence in mice; these phenotypic changes of ΔVvrraA1 were restored by the exogenous expression of VvrraA1. Transcriptomic analysis indicated that VvRraA1 expression levels affect the abundance of a large number of mRNA species. Among them, the half-lives of mRNA species encoding virulence factors (e.g., smcR and htpG) that have been previously shown to affect VvrraA1 expression-dependent phenotypes were positively correlated with VvrraA1 expression levels. These findings suggest that VvRraA1 modulates the pathogenicity of V. vulnificus by regulating the abundance of a subset of mRNA species.

The world of ribonucleases from pseudomonads: a short trip through the main features and singularities

The development of synthetic biology has brought an unprecedented increase in the number molecular tools applicable into a microbial chassis. The exploration of such tools into different bacteria revealed not only the challenges of context dependency of biological functions but also the complexity and diversity of regulatory layers in bacterial cells. Most of the standardized genetic tools and principles/functions have been mostly based on model microorganisms, namely Escherichia coli. In contrast, the non-model pseudomonads lack a deeper understanding of their regulatory layers and have limited molecular tools. They are resistant pathogens and promising alternative bacterial chassis, making them attractive targets for further studies. Ribonucleases (RNases) are key players in the post-transcriptional control of gene expression by degrading or processing the RNA molecules in the cell. These enzymes act according to the cellular requirements and can also be seen as the recyclers of ribonucleotides, allowing a continuous input of these cellular resources. This makes these post-transcriptional regulators perfect candidates to regulate microbial physiology. This review summarizes the current knowledge and unique properties of ribonucleases in the world of pseudomonads, taking into account genomic context analysis, biological function and strategies to use ribonucleases to improve biotechnological processes.

Impact of pseudouridylation, substrate fold, and degradosome organization on the endonuclease activity of RNase E

The conserved endoribonuclease RNase E dominates the dynamic landscape of RNA metabolism and underpins control mediated by small regulatory RNAs in diverse bacterial species. We explored the enzyme's hydrolytic mechanism, allosteric activation, and interplay with partner proteins in the multicomponent RNA degradosome assembly of Escherichia coli. RNase E cleaves single-stranded RNA with preference to attack the phosphate located at the 5' nucleotide preceding uracil, and we corroborate key interactions that select that base. Unexpectedly, RNase E activity is impeded strongly when the recognized uracil is isomerized to 5-ribosyluracil (pseudouridine), from which we infer the detailed geometry of the hydrolytic attack process. Kinetics analyses support models for recognition of secondary structure in substrates by RNase E and for allosteric autoregulation. The catalytic power of the enzyme is boosted when it is assembled into the multienzyme RNA degradosome, most likely as a consequence of substrate capture and presentation. Our results rationalize the origins of substrate preferences of RNase E and illuminate its catalytic mechanism, supporting the roles of allosteric domain closure and cooperation with other components of the RNA degradosome complex.

Polyribosome-Dependent Clustering of Membrane-Anchored RNA Degradosomes To Form Sites of mRNA Degradation in Escherichia coli

The essential endoribonuclease RNase E, which is a component of the Escherichia coli multienzyme RNA degradosome, has a global role in RNA processing and degradation. RNase E localizes to the inner cytoplasmic membrane in small, short-lived clusters (puncta). Rifampin, which arrests transcription, inhibits RNase E clustering and increases its rate of diffusion. Here, we show that inhibition of clustering is due to the arrest of transcription using a rifampin-resistant control strain. Two components of the RNA degradosome, the 3′ exoribonuclease polynucleotide phosphorylase (PNPase) and the DEAD box RNA helicase RhlB, colocalize with RNase E in puncta. Clustering of PNPase and RhlB is inhibited by rifampin, and their diffusion rates increase, as evidenced by in vivo photobleaching measurements. Results with rifampin treatment reported here show that RNA degradosome diffusion is constrained by interaction with RNA substrate. Kasugamycin, which arrests translation initiation, inhibits formation of puncta and increases RNA degradosome diffusion rates. Since kasugamycin treatment results in continued synthesis and turnover of ribosome-free mRNA but inhibits polyribosome formation, RNA degradosome clustering is therefore polyribosome dependent. Chloramphenicol, which arrests translation elongation, results in formation of large clusters (foci) of RNA degradosomes that are distinct from puncta. Since chloramphenicol-treated ribosomes are stable, the formation of RNA degradosome foci could be part of a stress response that protects inactive polyribosomes from degradation. Our results strongly suggest that puncta are sites where translationally active polyribosomes are captured by membrane-associated RNA degradosomes. These sites could be part of a scanning process that is an initial step in mRNA degradation.

The DEAD-box RNA helicase RhlE2 is a global regulator of Pseudomonas aeruginosa lifestyle and pathogenesis

RNA helicases perform essential housekeeping and regulatory functions in all domains of life by binding and unwinding RNA molecules. The bacterial RhlE-like DEAD-box RNA helicases are among the least well studied of these enzymes. They are widespread especially among Proteobacteria, whose genomes often encode multiple homologs. The significance of the expansion and diversification of RhlE-like proteins for bacterial fitness has not yet been established. Here, we study the two RhlE homologs present in the opportunistic pathogen Pseudomonas aeruginosa. We show that, in the course of evolution, RhlE1 and RhlE2 have diverged in their biological functions, molecular partners and RNA-dependent enzymatic activities. Whereas RhlE1 is mainly needed for growth in the cold, RhlE2 also acts as global post-transcriptional regulator, affecting the level of hundreds of cellular transcripts indispensable for both environmental adaptation and virulence. The global impact of RhlE2 is mediated by its unique C-terminal extension, which supports the RNA unwinding activity of the N-terminal domain as well as an RNA-dependent interaction with the RNase E endonuclease and the cellular RNA degradation machinery. Overall, our work reveals how the functional and molecular divergence between two homologous RNA helicases can contribute to bacterial fitness and pathogenesis.

Escherichia coli RNase E can efficiently replace RNase Y in Bacillus subtilis

RNase Y and RNase E are disparate endoribonucleases that govern global mRNA turnover/processing in the two evolutionary distant bacteria Bacillus subtilis and Escherichia coli, respectively. The two enzymes share a similar in vitro cleavage specificity and subcellular localization. To evaluate the potential equivalence in biological function between the two enzymes in vivo we analyzed whether and to what extent RNase E is able to replace RNase Y in B. subtilis. Full-length RNase E almost completely restores wild type growth of the rny mutant. This is matched by a surprising reversal of transcript profiles both of individual genes and on a genome-wide scale. The single most important parameter to efficient complementation is the requirement for RNase E to localize to the inner membrane while truncation of the C-terminal sequences corresponding to the degradosome scaffold has only a minor effect. We also compared the in vitro cleavage activity for the major decay initiating ribonucleases Y, E and J and show that no conclusions can be drawn with respect to their activity in vivo. Our data confirm the notion that RNase Y and RNase E have evolved through convergent evolution towards a low specificity endonuclease activity universally important in bacteria.

Ribonucleases control distinct traits of Pseudomonas putida lifestyle

The role of archetypal ribonucleases (RNases) in the physiology and stress endurance of the soil bacterium and metabolic engineering platform Pseudomonas putida KT2440 has been inspected. To this end, variants of this strain lacking each of the most important RNases were constructed. Each mutant lacked either one exoribonuclease (PNPase, RNase R) or one endoribonuclease (RNase E, RNase III, RNase G). The global physiological and metabolic costs of the absence of each of these enzymes were then analysed in terms of growth, motility and morphology. The effects of different oxidative chemicals that mimic the stresses endured by this microorganism in its natural habitats were studied as well. The results highlighted that each ribonuclease is specifically related with different traits of the environmental lifestyle that distinctively characterizes this microorganism. Interestingly, the physiological responses of P. putida to the absence of each enzyme diverged significantly from those known previously in Escherichia coli. This exposed not only species-specific regulatory functions for otherwise known RNase activities but also expanded the panoply of post-transcriptional adaptation devices that P. putida can make use of for facing hostile environments.

Phase-separated bacterial ribonucleoprotein bodies organize mRNA decay

In bacteria, mRNA decay is controlled by megadalton scale macromolecular assemblies called, "RNA degradosomes," composed of nucleases and other RNA decay associated proteins. Recent advances in bacterial cell biology have shown that RNA degradosomes can assemble into phase-separated structures, termed bacterial ribonucleoprotein bodies (BR-bodies), with many analogous properties to eukaryotic processing bodies and stress granules. This review will highlight the functional role that BR-bodies play in the mRNA decay process through its organization into a membraneless organelle in the bacterial cytoplasm. This review will also highlight the phylogenetic distribution of BR-bodies across bacterial species, which suggests that these phase-separated structures are broadly distributed across bacteria, and in evolutionarily related mitochondria and chloroplasts. This article is categorized under: RNA Turnover and Surveillance > Turnover/Surveillance Mechanisms RNA Interactions with Proteins and Other Molecules > RNA-Protein Complexes RNA Export and Localization > RNA Localization RNA Turnover and Surveillance > Regulation of RNA Stability.

The RNase J-Based RNA Degradosome Is Compartmentalized in the Gastric Pathogen Helicobacter pylori

Posttranscriptional regulation is a major level of gene expression control in any cell. In bacteria, multiprotein machines called RNA degradosomes are central for RNA processing and degradation, and some were reported to be compartmentalized inside these organelleless cells. The minimal RNA degradosome of the important gastric pathogen Helicobacter pylori is composed of the essential ribonuclease RNase J and RhpA, its sole DEAD box RNA helicase, and plays a major role in the regulation of mRNA decay and adaptation to gastric colonization. Here, the subcellular localization of the H. pylori RNA degradosome was investigated using cellular fractionation and both confocal and superresolution microscopy. We established that RNase J and RhpA are peripheral inner membrane proteins and that this association was mediated neither by ribosomes nor by RNA nor by the RNase Y membrane protein. In live H. pylori cells, we observed that fluorescent RNase J and RhpA protein fusions assemble into nonpolar foci. We identified factors that regulate the formation of these foci without affecting the degradosome membrane association. Flotillin, a bacterial membrane scaffolding protein, and free RNA promote focus formation in H. pylori Finally, RNase J-GFP (RNase J-green fluorescent protein) molecules and foci in cells were quantified by three-dimensional (3D) single-molecule fluorescence localization microscopy. The number and size of the RNase J foci were found to be scaled with growth phase and cell volume as previously reported for eukaryotic ribonucleoprotein granules. In conclusion, we propose that membrane compartmentalization and the regulated clustering of RNase J-based degradosome hubs represent important levels of control of their activity and specificity.IMPORTANCE Helicobacter pylori is a bacterial pathogen that chronically colonizes the stomach of half of the human population worldwide. Infection by H. pylori can lead to the development of gastric pathologies such as ulcers and adenocarcinoma, which causes up to 800,000 deaths in the world each year. Persistent colonization by H. pylori relies on regulation of the expression of adaptation-related genes. One major level of such control is posttranscriptional regulation, which, in H. pylori, largely relies on a multiprotein molecular machine, an RNA degradosome, that we previously discovered. In this study, we established that the two protein partners of this machine are associated with the membrane of H. pylori Using cutting-edge microscopy, we showed that these complexes assemble into hubs whose formation is regulated by free RNA and scaled with bacterial size and growth phase. Organelleless cellular compartmentalization of molecular machines into hubs emerges as an important regulatory level in bacteria.

Function analysis of RNase E in the filamentous cyanobacterium Anabaena sp. PCC 7120

RNase E is an endoribonuclease and plays a central role in RNA metabolism. Cyanobacteria, as ancient oxygen-producing photosynthetic bacteria, also contain RNase E homologues. Here, we introduced mutations into the S1 subdomain (F53A), the 5'-sensor subdomain (R160A), and the DNase I subdomain (D296A) according to the key activity sites of Escherichia coli RNase E. The results of degradation assays demonstrated that Asp296 is important to RNase E activity in Anabaena sp. PCC 7120 (hereafter PCC 7120). The docking model of RNase E in PCC 7120 (AnaRne) and RNA suggested a possible recognition mechanism of AnaRne to RNA. Moreover, overexpression of AnaRne and its N-terminal catalytic domain (AnaRneN) in vivo led to the abnormal cell division and inhibited the growth of PCC 7120. The quantitative analysis showed a significant decrease of ftsZ transcription in the case of overexpression of AnaRne or AnaRneN and ftsZ mRNA could be directly degraded by AnaRne through degradation assays in vitro, indicating that AnaRne was related to the expression of ftsZ and eventually affected cell division. In essence, our studies expand the understanding of the structural and functional evolutionary basis of RNase E and lay a foundation for further analysis of RNA metabolism in cyanobacteria.

Making the Case for Disordered Proteins and Biomolecular Condensates in Bacteria

Intrinsically disordered proteins/regions (IDPs/IDRs) contribute to a diverse array of molecular functions in eukaryotic systems. There is also growing recognition that membraneless biomolecular condensates, many of which are organized or regulated by IDPs/IDRs, can enable spatial and temporal regulation of complex biochemical reactions in eukaryotes. Motivated by these findings, we assess if (and how) membraneless biomolecular condensates and IDPs/IDRs are functionally involved in key cellular processes and molecular functions in bacteria. We summarize the conceptual underpinnings of condensate assembly and leverage these concepts by connecting them to recent findings that implicate specific types of condensates and IDPs/IDRs in important cellular level processes and molecular functions in bacterial systems.

Interaction between the scaffold proteins CBP by IQGAP1 provides an interface between gene expression and cytoskeletal activity

Crosstalk between cellular pathways is often mediated through scaffold proteins that function as platforms for the assembly of signaling complexes. Based on yeast two-hybrid analysis, we report here the interaction between two complex scaffold proteins, CREB-binding protein (CBP) and the Ras GTPase-activating-like protein 1 (IQGAP1). Dissection of the interaction between the two proteins reveals that the central, thus far uncharacterized, region of IQGAP1 interacts with the HAT domain and the C-terminal intrinsically disordered region of CBP (termed ID5). Structural analysis of ID5 by solution NMR spectroscopy and SAXS reveals the presence of two regions with pronounced helical propensity. The ID5 region(s) involved in the interaction of nanomolar affinity were delineated by solution NMR titrations and pull-down assays. Moreover, we found that IQGAP1 acts as an inhibitor of the histone acetyltransferase (HAT) activity of CBP. In in vitro assays, the CBP-binding region of IQGAP1 positively and negatively regulates the function of HAT proteins of different families including CBP, KAT5 and PCAF. As many signaling pathways converge on CBP and IQGAP1, their interaction provides an interface between transcription regulation and the coordination of cytoskeleton. Disruption or alteration of the interaction between these scaffold proteins may lead to cancer development or metastatic processes, highlighting the importance of this interaction.

Dissecting the Functional Contributions of the Intrinsically Disordered C-terminal Tail of Bacillus subtilis FtsZ

FtsZ is a bacterial GTPase that is central to the spatial and temporal control of cell division. It is a filament-forming enzyme that encompasses a well-folded core domain and a disordered C-terminal tail (CTT). The CTT is essential for ensuring proper assembly of the cytokinetic ring, and its deletion leads to mis-localization of FtsZ, aberrant assembly, and cell death. In this work, we dissect the contributions of modules within the disordered CTT to assembly and enzymatic activity of Bacillus subtilis FtsZ (Bs-FtsZ). The CTT features a hypervariable C-terminal linker (CTL) and a conserved C-terminal peptide (CTP). Our in vitro studies show that the CTL weakens the driving forces for forming single-stranded active polymers and suppresses lateral associations of these polymers, whereas the CTP promotes the formation of alternative assemblies. Accordingly, in full-length Bs-FtsZ, the CTL acts as a spacer that spatially separates the CTP sticker from the core, thus ensuring filament formation through core-driven polymerization and lateral associations through CTP-mediated interactions. We also find that the CTL weakens GTP binding while enhancing the catalytic rate, whereas the CTP has opposite effects. The joint contributions of the CTL and CTP make Bs-FtsZ, an enzyme that is only half as efficient as a truncated version that lacks the CTT. Overall, our data suggest that the CTT acts as an auto-regulator of Bs-FtsZ assembly and as an auto-inhibitor of enzymatic activity. Based on our results, we propose hypotheses regarding the hypervariability of CTLs and compare FtsZs to other bacterial proteins with tethered IDRs.

RNA lifetime control, from stereochemistry to gene expression

Through the activities of various multi-component assemblies, protein-coding transcripts can be chaperoned toward protein synthesis or nudged into a funnel of rapid destruction. The capacity of these machine-like assemblies to tune RNA lifetime underpins the harmony of gene expression in all cells. Some of the molecular machines that mediate transcript turnover also contribute to on-the-fly surveillance of aberrant mRNAs and non-coding RNAs. How these dynamic assemblies distinguish functional RNAs from those that must be degraded is an intriguing puzzle for understanding the regulation of gene expression and dysfunction associated with disease. Recent data illuminate what the machines look like, and how they find, recognise and operate on transcripts to sculpt the dynamic regulatory landscape. This review captures current structural and mechanistic insights into the key enzymes and their effector assemblies that contribute to the fate-determining decision points for RNA in post-transcriptional control of genetic information.

A DEAD-box protein regulates ribosome assembly through control of ribosomal protein synthesis

DEAD-box proteins (DBPs) comprise a large family of proteins that most commonly have been identified as regulators of ribosome assembly. The Escherichia coli DBP, SrmB, represents a model bacterial DBP whose absence impairs formation of the large ribosomal subunit (LSU). To define the basis for SrmB function, suppressors of the ribosomal defect of ΔsrmB strains were isolated. The major class of suppressors was found to map to the 5′ untranslated region (UTR) of the rplM-rpsI operon, which encodes the ribosomal proteins (r-proteins) L13 and S9. An analysis of protein abundance indicated that both r-proteins are under-produced in the ΔsrmB strain, but are increased in these suppressors, implicating r-protein underproduction as the molecular basis for the observed ribosomal defects. Reduced r-protein synthesis was determined to be caused by intrinsic transcription termination within the rplM 5′ UTR that is abrogated by SrmB. These results reveal a specific mechanism for DBP regulation of ribosomal assembly, indirectly mediated through its effects on r-protein expression.

Bacterial RNA Degradosomes: Molecular Machines under Tight Control

Bacterial RNA degradosomes are multienzyme molecular machines that act as hubs for post-transcriptional regulation of gene expression. The ribonuclease activities of these complexes require tight regulation, as they are usually essential for cell survival while potentially destructive. Recent studies have unveiled a wide variety of regulatory mechanisms including autoregulation, post-translational modifications, and protein compartmentalization. Recently, the subcellular organization of bacterial RNA degradosomes was found to present similarities with eukaryotic messenger ribonucleoprotein (mRNP) granules, membraneless compartments that are also involved in mRNA and protein storage and/or mRNA degradation. In this review, we present the current knowledge on the composition and targets of RNA degradosomes, the most recent developments regarding the regulation of these machineries, and their similarities with the eukaryotic mRNP granules.

Dip-a-Dee-Doo-Dah: Bacteriophage-Mediated Rescoring of a Harmoniously Orchestrated RNA Metabolism

RNA turnover and processing in bacteria are governed by the structurally divergent but functionally convergent RNA degradosome, and the mechanisms have been researched extensively in Gram-positive and Gram-negative bacteria. An emerging research field focuses on how bacterial viruses hijack all aspects of the bacterial metabolism, including the host machinery of RNA metabolism. This review addresses research on phage-based influence on RNA turnover, which can act either indirectly or via dedicated effector molecules that target degradosome assemblies. The structural divergence of host RNA turnover mechanisms likely explains the limited number of phage proteins directly targeting these specialized, host-specific complexes. The unique and nonconserved structure of DIP, a phage-encoded inhibitor of the Pseudomonas degradosome, illustrates this hypothesis. However, the natural occurrence of phage-encoded mechanisms regulating RNA turnover indicates a clear evolutionary benefit for this mode of host manipulation. Further exploration of the viral dark matter of unknown phage proteins may reveal more structurally novel interference strategies that, in turn, could be exploited for biotechnological applications.

Detachment of the RNA degradosome from the inner membrane of Escherichia coli results in a global slowdown of mRNA degradation, proteolysis of RNase E and increased turnover of ribosome-free transcripts

The reason for RNase E attachment to the inner membrane is largely unknown. To understand the cell biology of RNA degradation, we have characterized a strain expressing RNase E lacking the membrane attachment site (cytoplasmic RNase E). Genome‐wide data show a global slowdown in mRNA degradation. There is no correlation between mRNA stabilization and the function or cellular location of encoded proteins. The activity of cRNase E is comparable to the wild‐type enzyme in vitro, but the mutant protein is unstable in vivo. Autoregulation of cRNase E synthesis compensates for protein instability. cRNase E associates with other proteins to assemble a cytoplasmic RNA degradosome. CsrB/C sRNAs, whose stability is regulated by membrane‐associated CsrD, are stabilized. Membrane attachment of RNase E is thus necessary for CsrB/C turnover. In contrast to mRNA stability, ribosome‐free transcripts are sensitive to inactivation by cRNase E. Our results show that effects on RNA degradation are not due to the differences in the activity or level of cRNase E, or failure to assemble the RNA degradosome. We propose that membrane attachment is necessary for RNase E stability, functional interactions with membrane‐associated regulatory factors and protection of ribosome‐free transcripts from premature interactions with RNase E in the nucleoid.

Predicting Functions of Disordered Proteins with MoRFpred

Intrinsically disordered proteins and regions are involved in a wide range of cellular functions, and they often facilitate protein-protein interactions. Molecular recognition features (MoRFs) are segments of intrinsically disordered regions that bind to partner proteins, where binding is concomitant with a transition to a structured conformation. MoRFs facilitate translation, transport, signaling, and regulatory processes and are found across all domains of life. A popular computational tool, MoRFpred, accurately predicts MoRFs in protein sequences. MoRFpred is implemented as a user-friendly web server that is freely available at http://biomine.cs.vcu.edu/servers/MoRFpred/ . We describe this predictor, explain how to run the web server, and show how to interpret the results it generates. We also demonstrate the utility of this web server based on two case studies, focusing on the relevance of evolutionary conservation of MoRF regions.

RNase E and the High-Fidelity Orchestration of RNA Metabolism

The bacterial endoribonuclease RNase E occupies a pivotal position in the control of gene expression, as its actions either commit transcripts to an irreversible fate of rapid destruction or unveil their hidden functions through specific processing. Moreover, the enzyme contributes to quality control of rRNAs. The activity of RNase E can be directed and modulated by signals provided through regulatory RNAs that guide the enzyme to specific transcripts that are to be silenced. Early in its evolutionary history, RNase E acquired a natively unfolded appendage that recruits accessory proteins and RNA. These accessory factors facilitate the activity of RNase E and include helicases that remodel RNA and RNA-protein complexes, and polynucleotide phosphorylase, a relative of the archaeal and eukaryotic exosomes. RNase E also associates with enzymes from central metabolism, such as enolase and aconitase. RNase E-based complexes are diverse in composition, but generally bear mechanistic parallels with eukaryotic machinery involved in RNA-induced gene regulation and transcript quality control. That these similar processes arose independently underscores the universality of RNA-based regulation in life. Here we provide a synopsis and perspective of the contributions made by RNase E to sustain robust gene regulation with speed and accuracy.

Co-Evolution of Intrinsically Disordered Proteins with Folded Partners Witnessed by Evolutionary Couplings

Although improved strategies for the detection and analysis of evolutionary couplings (ECs) between protein residues already enable the prediction of protein structures and interactions, they are mostly restricted to conserved and well-folded proteins. Whereas intrinsically disordered proteins (IDPs) are central to cellular interaction networks, due to the lack of strict structural constraints, they undergo faster evolutionary changes than folded domains. This makes the reliable identification and alignment of IDP homologs difficult, which led to IDPs being omitted in most large-scale residue co-variation analyses. By preforming a dedicated analysis of phylogenetically widespread bacterial IDP–partner interactions, here we demonstrate that partner binding imposes constraints on IDP sequences that manifest in detectable interprotein ECs. These ECs were not detected for interactions mediated by short motifs, rather for those with larger IDP–partner interfaces. Most identified coupled residue pairs reside close (<10 Å) to each other on the interface, with a third of them forming multiple direct atomic contacts. EC-carrying interfaces of IDPs are enriched in negatively charged residues, and the EC residues of both IDPs and partners preferentially reside in helices. Our analysis brings hope that IDP–partner interactions difficult to study could soon be successfully dissected through residue co-variation analysis.

Substrate Recognition and Autoinhibition in the Central Ribonuclease RNase E

The endoribonuclease RNase E is a principal factor in RNA turnover and processing that helps to exercise fine control of gene expression in bacteria. While its catalytic activity can be strongly influenced by the chemical identity of the 5′ end of RNA substrates, the enzyme can also cleave numerous substrates irrespective of the chemistry of their 5′ ends through a mechanism that has remained largely unexplained. We report structural and functional data illuminating details of both operational modes. Our crystal structure of RNase E in complex with the sRNA RprA reveals a duplex recognition site that saddles an inter-protomer surface to help present substrates for cleavage. Our data also reveal an autoinhibitory pocket that modulates the overall activity of the ribonuclease. Taking these findings together, we propose how RNase E uses versatile modes of RNA recognition to achieve optimal activity and specificity.

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RNase E recognizes RNA secondary structure

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Signature on the substrate 5′ end recognizes and activates RNase E

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RNase E intrinsic activity is repressed by a conserved autoinhibition motif

RNA helicases in RNA decay

RNA molecules have the tendency to fold into complex structures or to associate with complementary RNAs that exoribonucleases have difficulties processing or degrading. Therefore, degradosomes in bacteria and organelles as well as exosomes in eukaryotes have teamed-up with RNA helicases. Whereas bacterial degradosomes are associated with RNA helicases from the DEAD-box family, the exosomes and mitochondrial degradosome use the help of Ski2-like and Suv3 RNA helicases.

Analysis of the natively unstructured RNA/protein-recognition core in the Escherichia coli RNA degradosome and its interactions with regulatory RNA/Hfq complexes

The RNA degradosome is a multi-enzyme assembly that plays a central role in the RNA metabolism of Escherichia coli and numerous other bacterial species including pathogens. At the core of the assembly is the endoribonuclease RNase E, one of the largest E. coli proteins and also one that bears the greatest region predicted to be natively unstructured. This extensive unstructured region, situated in the C-terminal half of RNase E, is punctuated with conserved short linear motifs that recruit partner proteins, direct RNA interactions, and enable association with the cytoplasmic membrane. We have structurally characterized a subassembly of the degradosome-comprising a 248-residue segment of the natively unstructured part of RNase E, the DEAD-box helicase RhlB and the glycolytic enzyme enolase, and provide evidence that it serves as a flexible recognition centre that can co-recruit small regulatory RNA and the RNA chaperone Hfq. Our results support a model in which the degradosome captures substrates and regulatory RNAs through the recognition centre, facilitates pairing to cognate transcripts and presents the target to the ribonuclease active sites of the greater assembly for cooperative degradation or processing.

To be disordered or not to be disordered: is that still a question for proteins in the cell?

There is ample evidence that many proteins or regions of proteins lack a well-defined folded structure under native-like conditions. These are called intrinsically disordered proteins (IDPs) or intrinsically disordered regions (IDRs). Whether this intrinsic disorder is also their main structural characteristic in living cells has been a matter of intense debate. The structural analysis of IDPs became an important challenge also because of their involvement in a plethora of human diseases, which made IDPs attractive targets for therapeutic development. Therefore, biophysical approaches are increasingly being employed to probe the structural and dynamical state of proteins, not only in isolation in a test tube, but also in a complex biological environment and even within intact cells. Here, we survey direct and indirect evidence that structural disorder is in fact the physiological state of many proteins in the proteome. The paradigmatic case of α-synuclein is used to illustrate the controversial nature of this topic.

Linking functions: an additional role for an intrinsically disordered linker domain in the transcriptional coactivator CBP

The multi-domain transcriptional coactivators CBP/p300 integrate a multitude of signaling inputs, interacting with more than 400 proteins via one or more of their globular domains. While CBP/p300 function is typically considered in terms of these structured domains, about half of the protein consists of intrinsically disordered regions (IDRs) of varying length. However, these IDRs have only been thought of as linkers that allow flexible spatial arrangement of the structured domains, but recent studies have shown that similar IDRs mediate specific and critical interactions in other proteins. To examine the roles of IDRs in CBP, we performed yeast-two-hybrid screenings of placenta and lung cancer cDNA libraries, which demonstrated that the long IDR linking the KIX domain and bromodomain of CBP (termed ID3) can potentially bind to several proteins. The RNA-binding Zinc-finger protein 106 (ZFP106) detected in both libraries was identified as a novel substrate for CBP-mediated acetylation. Nuclear magnetic resonance (NMR) spectroscopy combined with cross-linking experiments and competition-binding assays showed that the fully disordered isolated ID3 transiently interacts with an IDR of ZFP106 in a fashion that disorder of both regions is maintained. These findings demonstrate that beside the linking function, ID3 can also interact with acetylation substrates of CBP.

RNase E-based degradosome modulates polyadenylation of mRNAs after Rho-independent transcription terminators in Escherichia coli

Here we demonstrate that the RNase E-based degradosome is required for poly(A) polymerase I (PAP I)-dependent polyadenylation after Rho-independent transcription terminators for both mono- and polycistronic transcripts. Disruption of degradosome assembly in mutants lacking the polynucleotide phosphorylase (PNPase) binding domain led to a significant increase in the level of PNPase synthesized polynucleotide tails in the rpsJ and rpsM polycistronic transcripts and the lpp monocistronic transcript. The polynucleotide tails were mostly located within the coding sequences in the degradosome mutants compared to the wild type control where the majority of the PAP I synthesized poly(A) tails were after the Rho-independent transcription terminators. For the Rho terminated metNIQ operon, the tails for all three mRNAs were predominately polynucleotide and were located within the coding sequences in both wild type and degradosome mutant strains. Furthermore, by employing a pnp-R100D point mutant that encodes a catalytically inactive PNPase protein that still forms intact degradosomes, we show that a catalytically active PNPase is required for normal mRNA polyadenylation by PAP I. Our data suggest that polyadenylation requires a functional degradosome to maintain an equilibrium between free PNPase and the PAP I polyadenylation complex.

Structural elucidation of a novel mechanism for the bacteriophage-based inhibition of the RNA degradosome

In all domains of life, the catalysed degradation of RNA facilitates rapid adaptation to changing environmental conditions, while destruction of foreign RNA is an important mechanism to prevent host infection. We have identified a virus-encoded protein termed gp37/Dip, which directly binds and inhibits the RNA degradation machinery of its bacterial host. Encoded by giant phage фKZ, this protein associates with two RNA binding sites of the RNase E component of the Pseudomonas aeruginosa RNA degradosome, occluding them from substrates and resulting in effective inhibition of RNA degradation and processing. The 2.2 Å crystal structure reveals that this novel homo-dimeric protein has no identifiable structural homologues. Our biochemical data indicate that acidic patches on the convex outer surface bind RNase E. Through the activity of Dip, фKZ has evolved a unique mechanism to down regulate a key metabolic process of its host to allow accumulation of viral RNA in infected cells.

RNA degradosomes in bacteria and chloroplasts: classification, distribution and evolution of RNase E homologs

Ribonuclease E (RNase E) of Escherichia coli, which is the founding member of a widespread family of proteins in bacteria and chloroplasts, is a fascinating enzyme that still has not revealed all its secrets. RNase E is an essential single-strand specific endoribonuclease that is involved in the processing and degradation of nearly every transcript in E. coli. A striking enzymatic property is a preference for substrates with a 5' monophosphate end although recent work explains how RNase E can overcome the protection afforded by the 5' triphosphate end of a primary transcript. Other features of E. coli RNase E include its interaction with enzymes involved in RNA degradation to form the multienzyme RNA degradosome and its localization to the inner cytoplasmic membrane. The N-terminal catalytic core of the RNase E protomer associates to form a tetrameric holoenzyme. Each RNase E protomer has a large C-terminal intrinsically disordered (ID) noncatalytic region that contains sites for interactions with protein components of the RNA degradosome as well as RNA and phospholipid bilayers. In this review, RNase E homologs have been classified into five types based on their primary structure. A recent analysis has shown that type I RNase E in the γ-proteobacteria forms an orthologous group of proteins that has been inherited vertically. The RNase E catalytic core and a large ID noncatalytic region containing an RNA binding motif and a membrane targeting sequence are universally conserved features of these orthologs. Although the ID noncatalytic region has low composition and sequence complexity, it is possible to map microdomains, which are short linear motifs that are sites of interaction with protein and other ligands. Throughout bacteria, the composition of the multienzyme RNA degradosome varies with species, but interactions with exoribonucleases (PNPase, RNase R), glycolytic enzymes (enolase, aconitase) and RNA helicases (DEAD-box proteins, Rho) are common. Plasticity in RNA degradosome composition is due to rapid evolution of RNase E microdomains. Characterization of the RNase E-PNPase interaction in α-proteobacteria, γ-proteobacteria and cyanobacteria suggests that it arose independently several times during evolution, thus conferring an advantage in control and coordination of RNA processing and degradation.