The RNA-bound proteome of MRSA reveals post-transcriptional roles for helix-turn-helix DNA-binding and Rossmann-fold proteins

R-DeeP/TripepSVM identifies the RNA-binding OB-fold-like protein PatR as regulator of heterocyst patterning

RNA-binding proteins (RBPs) are central components of gene regulatory networks. The differentiation of heterocysts in filamentous cyanobacteria is an example of cell differentiation in prokaryotes. Although multiple non-coding transcripts are involved in this process, no RBPs have been implicated thus far. Here we used quantitative mass spectrometry to analyze the differential fractionation of RNA-protein complexes after RNase treatment in density gradients yielding 333 RNA-associated proteins, while a bioinformatic prediction yielded 311 RBP candidates in Nostoc sp. PCC 7120. We validated in vivo the RNA-binding capacity of six RBP candidates. Some participate in essential physiological aspects, such as photosynthesis (Alr2890), thylakoid biogenesis (Vipp1) or heterocyst differentiation (PrpA, PatU3), but their association with RNA was unknown. Validated RBPs Asl3888 and Alr1700 were not previously characterized. Alr1700 is an RBP with two oligonucleotide/oligosaccharide-binding (OB)-fold-like domains that is differentially expressed in heterocysts and interacts with non-coding regulatory RNAs. Deletion of alr1700 led to complete deregulation of the cell differentiation process, a striking increase in the number of heterocyst-like cells, and was ultimately lethal in the absence of combined nitrogen. These observations characterize this RBP as a master regulator of the heterocyst patterning and differentiation process, leading us to rename Alr1700 to PatR.

pyRBDome: a comprehensive computational platform for enhancing RNA-binding proteome data

High-throughput proteomics approaches have revolutionised the identification of RNA-binding proteins (RBPome) and RNA-binding sequences (RBDome) across organisms. Yet, the extent of noise, including false positives, associated with these methodologies, is difficult to quantify as experimental approaches for validating the results are generally low throughput. To address this, we introduce pyRBDome, a pipeline for enhancing RNA-binding proteome data in silico. It aligns the experimental results with RNA-binding site (RBS) predictions from distinct machine-learning tools and integrates high-resolution structural data when available. Its statistical evaluation of RBDome data enables quick identification of likely genuine RNA-binders in experimental datasets. Furthermore, by leveraging the pyRBDome results, we have enhanced the sensitivity and specificity of RBS detection through training new ensemble machine-learning models. pyRBDome analysis of a human RBDome dataset, compared with known structural data, revealed that although UV-cross-linked amino acids were more likely to contain predicted RBSs, they infrequently bind RNA in high-resolution structures. This discrepancy underscores the limitations of structural data as benchmarks, positioning pyRBDome as a valuable alternative for increasing confidence in RBDome datasets.

Comprehensive Profiling of Roquin Binding Preferences for RNA Stem-Loops

The cellular levels of mRNAs are controlled post-transcriptionally by cis-regulatory elements located in the 3'-untranslated region. These linear or structured elements are recognized by RNA-binding proteins (RBPs) to modulate mRNA stability. The Roquin-1 and -2 proteins specifically recognize RNA stem-loop motifs, the trinucleotide loop-containing constitutive decay elements (CDEs) and the hexanucleotide loop-containing alternative decay elements (ADEs), with their unique ROQ domain to initiate mRNA degradation. However, the RNA-binding capacity of Roquin towards different classes of stem-loops has not been rigorously characterized, leaving its exact binding preferences unclear. Here, we map the RNA-binding preference of the ROQ domain at nucleotide resolution introducing sRBNS (structured RNA Bind-n-Seq), a customized RBNS workflow with pre-structured RNA libraries. We found a clear preference of Roquin towards specific loop sizes and extended the consensus motifs for CDEs and ADEs. The newly identified motifs are recognized with nanomolar affinity through the canonical RNA-ROQ interface. Using these new stem-loop variants as blueprints, we predicted novel Roquin target mRNAs and verified the expanded target space in cells. The study demonstrates the power of high-throughput assays including RNA structure formation for the systematic investigation of (structural) RNA-binding preferences to comprehensively identify mRNA targets and elucidate the biological function of RBPs.

Chemical crosslinking extends and complements UV crosslinking in analysis of RNA/DNA nucleic acid-protein interaction sites by mass spectrometry

UV (ultra-violet) crosslinking with mass spectrometry (XL-MS) has been established for identifying RNA-and DNA-binding proteins along with their domains and amino acids involved. Here, we explore chemical XL-MS for RNA-protein, DNA-protein, and nucleotide-protein complexes in vitro and in vivo . We introduce a specialized nucleotide-protein-crosslink search engine, NuXL, for robust and fast identification of such crosslinks at amino acid resolution. Chemical XL-MS complements UV XL-MS by generating different crosslink species, increasing crosslinked protein yields in vivo almost four-fold and thus it expands the structural information accessible via XL-MS. Our workflow facilitates integrative structural modelling of nucleic acid-protein complexes and adds spatial information to the described RNA-binding properties of enzymes, for which crosslinking sites are often observed close to their cofactor-binding domains. In vivo UV and chemical XL-MS data from E. coli cells analysed by NuXL establish a comprehensive nucleic acid-protein crosslink inventory with crosslink sites at amino acid level for more than 1500 proteins. Our new workflow combined with the dedicated NuXL search engine identified RNA crosslinks that cover most RNA-binding proteins, with DNA and RNA crosslinks detected in transcriptional repressors and activators.

Phage anti-CRISPR control by an RNA- and DNA-binding helix-turn-helix protein

In all organisms, regulation of gene expression must be adjusted to meet cellular requirements and frequently involves helix-turn-helix (HTH) domain proteins1. For instance, in the arms race between bacteria and bacteriophages, rapid expression of phage anti-CRISPR (acr) genes upon infection enables evasion from CRISPR-Cas defence; transcription is then repressed by an HTH-domain-containing anti-CRISPR-associated (Aca) protein, probably to reduce fitness costs from excessive expression2-5. However, how a single HTH regulator adjusts anti-CRISPR production to cope with increasing phage genome copies and accumulating acr mRNA is unknown. Here we show that the HTH domain of the regulator Aca2, in addition to repressing Acr synthesis transcriptionally through DNA binding, inhibits translation of mRNAs by binding conserved RNA stem-loops and blocking ribosome access. The cryo-electron microscopy structure of the approximately 40 kDa Aca2-RNA complex demonstrates how the versatile HTH domain specifically discriminates RNA from DNA binding sites. These combined regulatory modes are widespread in the Aca2 family and facilitate CRISPR-Cas inhibition in the face of rapid phage DNA replication without toxic acr overexpression. Given the ubiquity of HTH-domain-containing proteins, it is anticipated that many more of them elicit regulatory control by dual DNA and RNA binding.

A Fur family protein BosR is a novel RNA-binding protein that controls rpoS RNA stability in the Lyme disease pathogen

The σ54-σS sigma factor cascade plays a central role in regulating differential gene expression during the enzootic cycle of Borreliella burgdorferi, the Lyme disease pathogen. In this pathway, the primary transcription of rpoS (which encodes σS) is under the control of σ54 which is activated by a bacterial enhancer-binding protein (EBP), Rrp2. The σ54-dependent activation in B. burgdorferi has long been thought to be unique, requiring an additional factor, BosR, a homologue of classical Fur/PerR repressor/activator. However, how BosR is involved in this σ54-dependent activation remains unclear and perplexing. In this study, we demonstrate that BosR does not function as a regulator for rpoS transcriptional activation. Instead, it functions as a novel RNA-binding protein that governs the turnover rate of rpoS mRNA. We further show that BosR directly binds to the 5' untranslated region (UTR) of rpoS mRNA, and the binding region overlaps with a region required for rpoS mRNA degradation. Mutations within this 5'UTR region result in BosR-independent RpoS production. Collectively, these results uncover a novel role of Fur/PerR family regulators as RNA-binding proteins and redefine the paradigm of the σ54-σS pathway in B. burgdorferi.

Interrogation of RNA-protein interaction dynamics in bacterial growth

Characterising RNA-protein interaction dynamics is fundamental to understand how bacteria respond to their environment. In this study, we have analysed the dynamics of 91% of the Escherichia coli expressed proteome and the RNA-interaction properties of 271 RNA-binding proteins (RBPs) at different growth phases. We find that 68% of RBPs differentially bind RNA across growth phases and characterise 17 previously unannotated proteins as bacterial RBPs including YfiF, a ncRNA-binding protein. While these new RBPs are mostly present in Proteobacteria, two of them are orthologs of human mitochondrial proteins associated with rare metabolic disorders. Moreover, we reveal novel RBP functions for proteins such as the chaperone HtpG, a new stationary phase tRNA-binding protein. For the first time, the dynamics of the bacterial RBPome have been interrogated, showcasing how this approach can reveal the function of uncharacterised proteins and identify critical RNA-protein interactions for cell growth which could inform new antimicrobial therapies.

A global survey of small RNA interactors identifies KhpA and KhpB as major RNA-binding proteins in Fusobacterium nucleatum

The common oral microbe Fusobacterium nucleatum has recently drawn attention after it was found to colonize tumors throughout the human body. Fusobacteria are also interesting study systems for bacterial RNA biology as these early-branching species encode many small noncoding RNAs (sRNAs) but lack homologs of the common RNA-binding proteins (RBPs) CsrA, Hfq and ProQ. To search for alternate sRNA-associated RBPs in F. nucleatum, we performed a systematic mass spectrometry analysis of proteins that co-purified with 19 different sRNAs. This approach revealed strong enrichment of the KH domain proteins KhpA and KhpB with nearly all tested sRNAs, including the σE-dependent sRNA FoxI, a regulator of several envelope proteins. KhpA/B act as a dimer to bind sRNAs with low micromolar affinity and influence the stability of several of their target transcripts. Transcriptome studies combined with biochemical and genetic analyses suggest that KhpA/B have several physiological functions, including being required for ethanolamine utilization. Our RBP search and the discovery of KhpA/B as major RBPs in F. nucleatum are important first steps in identifying key players of post-transcriptional control at the root of the bacterial phylogenetic tree.

Improved RNA stability estimation through Bayesian modeling reveals most Salmonella transcripts have subminute half-lives

RNA decay is a crucial mechanism for regulating gene expression in response to environmental stresses. In bacteria, RNA-binding proteins (RBPs) are known to be involved in posttranscriptional regulation, but their global impact on RNA half-lives has not been extensively studied. To shed light on the role of the major RBPs ProQ and CspC/E in maintaining RNA stability, we performed RNA sequencing of Salmonella enterica over a time course following treatment with the transcription initiation inhibitor rifampicin (RIF-seq) in the presence and absence of these RBPs. We developed a hierarchical Bayesian model that corrects for confounding factors in rifampicin RNA stability assays and enables us to identify differentially decaying transcripts transcriptome-wide. Our analysis revealed that the median RNA half-life in Salmonella in early stationary phase is less than 1 min, a third of previous estimates. We found that over half of the 500 most long-lived transcripts are bound by at least one major RBP, suggesting a general role for RBPs in shaping the transcriptome. Integrating differential stability estimates with cross-linking and immunoprecipitation followed by RNA sequencing (CLIP-seq) revealed that approximately 30% of transcripts with ProQ binding sites and more than 40% with CspC/E binding sites in coding or 3' untranslated regions decay differentially in the absence of the respective RBP. Analysis of differentially destabilized transcripts identified a role for ProQ in the oxidative stress response. Our findings provide insights into posttranscriptional regulation by ProQ and CspC/E, and the importance of RBPs in regulating gene expression.

Global Identification of RNA-Binding Proteins in Bacteria

RNA-binding proteins (RBPs) are at the heart of many biological processes and are therefore essential for cellular life. Following identification of single RBPs by classical genetics and molecular biology methods, approaches for RBP discovery on a systems level have recently emerged. For instance, RNA interactome capture (RIC) enables the global purification of RBPs cross-linked to polyadenylated RNA using oligo(dT) probes. RIC was originally developed for eukaryotic organisms but was recently established for capturing RBPs in bacteria. In this chapter, we provide a detailed step-by-step protocol for performing RIC in bacteria. The protocol is based on its application to Escherichia coli but should be amenable for charting other genetically tractable bacterial species.

RNA interactome capture in Escherichia coli globally identifies RNA-binding proteins

RNA-binding proteins (RPBs) are deeply involved in fundamental cellular processes in bacteria and are vital for their survival. Despite this, few studies have so far been dedicated to direct and global identification of bacterial RBPs. We have adapted the RNA interactome capture (RIC) technique, originally developed for eukaryotic systems, to globally identify RBPs in bacteria. RIC takes advantage of the base pairing potential of poly(A) tails to pull-down RNA-protein complexes. Overexpressing poly(A) polymerase I in Escherichia coli drastically increased transcriptome-wide RNA polyadenylation, enabling pull-down of crosslinked RNA-protein complexes using immobilized oligo(dT) as bait. With this approach, we identified 169 putative RBPs, roughly half of which are already annotated as RNA-binding. We experimentally verified the RNA-binding ability of a number of uncharacterized RBPs, including YhgF, which is exceptionally well conserved not only in bacteria, but also in archaea and eukaryotes. We identified YhgF RNA targets in vivo using CLIP-seq, verified specific binding in vitro, and reveal a putative role for YhgF in regulation of gene expression. Our findings present a simple and robust strategy for RBP identification in bacteria, provide a resource of new bacterial RBPs, and lay the foundation for further studies of the highly conserved RBP YhgF.

Profiling the in vivo RNA interactome associated with the endoribonuclease RNase III in Staphylococcus aureus

Regulatory small RNA (sRNA) have been extensively studied in model Gram-negative bacteria, but the functional characterisation of these post-transcriptional gene regulators in Gram-positives remains a major challenge. Our previous work in enterohaemorrhagic E. coli utilised the proximity-dependant ligation technique termed CLASH (UV-crosslinking, ligation, and sequencing of hybrids) for direct high-throughput sequencing of the regulatory sRNA-RNA interactions within the cell. Recently, we adapted the CLASH technique and demonstrated that UV-crosslinking and RNA proximity-dependant ligation can be applied to Staphylococcus aureus, which uncovered the first RNA-RNA interaction network in a Gram-positive bacterium. In this chapter, we describe modifications to the CLASH technique that were developed to capture the RNA interactome associated with the double-stranded endoribonuclease RNase III in two clinical isolates of S. aureus. To briefly summarise our CLASH methodology, regulatory RNA-RNA interactions were first UV-crosslinked in vivo to the RNase III protein and protein-RNA complexes were affinity-purified using the His6-TEV-FLAG tags. Linkers were ligated to RNase III-bound RNA during library preparation and duplexed RNA-RNA species were ligated together to form a single contiguous RNA 'hybrid'. The RNase III-RNA binding sites and RNA-RNA interactions occurring on RNase III (RNA hybrids) were then identified by paired-end sequencing technology. RNase III-CLASH represents a step towards a systems-level understanding of regulatory RNA in Gram-positive bacteria.

Global profiling of the RNA and protein complexes of Escherichia coli by size exclusion chromatography followed by RNA sequencing and mass spectrometry (SEC-seq)

New methods for the global identification of RNA-protein interactions have led to greater recognition of the abundance and importance of RNA-binding proteins (RBPs) in bacteria. Here, we expand this tool kit by developing SEC-seq, a method based on a similar concept as the established Grad-seq approach. In Grad-seq, cellular RNA and protein complexes of a bacterium of interest are separated in a glycerol gradient, followed by high-throughput RNA-sequencing and mass spectrometry analyses of individual gradient fractions. New RNA-protein complexes are predicted based on the similarity of their elution profiles. In SEC-seq, we have replaced the glycerol gradient with separation by size exclusion chromatography, which shortens operation times and offers greater potential for automation. Applying SEC-seq to Escherichia coli, we find that the method provides a higher resolution than Grad-seq in the lower molecular weight range up to ~500 kDa. This is illustrated by the ability of SEC-seq to resolve two distinct, but similarly sized complexes of the global translational repressor CsrA with either of its antagonistic small RNAs, CsrB and CsrC. We also characterized changes in the SEC-seq profiles of the small RNA MicA upon deletion of its RNA chaperones Hfq and ProQ and investigated the redistribution of these two proteins upon RNase treatment. Overall, we demonstrate that SEC-seq is a tractable and reproducible method for the global profiling of bacterial RNA-protein complexes that offers the potential to discover yet-unrecognized associations between bacterial RNAs and proteins.

TripDesign: A DNA Triplex Design Approach Based on Interaction Forces

A DNA triplex has the advantages of improved nanostructure stability and pH environment responsiveness compared with single-stranded and double-stranded nucleic acids. However, sequence stability and low design efficiency hinder the application of DNA triplexes. Therefore, a DNA triplex design approach (TripDesign) based on interaction forces is proposed. First, we present the stacking force constraint, torsional stress constraint, and G-quadruplex motif constraint and then use an improved memetic algorithm to design triplex sequences under combinatorial constraints. Finally, to quantify the process of triplex formation, we also explore the minimum length of the triplex-forming oligos (TFOs) required to form the triplex and the factors that produce depletion in cyclic pH-jump experiments. The experimental results show that the sequences produced by TripDesign have high stability and reversibility, and the proposed approach achieves efficient and automatic sequence design. In addition, this study characterizes multiple basic parameters of DNA triplex formation and promotes the wider application of DNA triplexes in nanotechnology.