Co-transcriptional folding of the glmS ribozyme enables a rapid response to metabolite

The glmS ribozyme riboswitch, located in the 5' untranslated region of the Bacillus subtilis glmS messenger RNA (mRNA), regulates cell wall biosynthesis through ligand-induced self-cleavage and decay of the glmS mRNA. Although self-cleavage of the refolded glmS ribozyme has been studied extensively, it is not known how early the ribozyme folds and self-cleaves during transcription. Here, we combine single-molecule fluorescence with kinetic modeling to show that self-cleavage can occur during transcription before the ribozyme is fully synthesized. Moreover, co-transcriptional folding of the RNA at a physiological elongation rate allows the ribozyme catalytic core to react without the downstream peripheral stability domain. Dimethyl sulfate footprinting further revealed how slow sequential folding favors formation of the native core structure through fraying of misfolded helices and nucleation of a native pseudoknot. Ribozyme self-cleavage at an early stage of transcription may benefit glmS regulation in B. subtilis, as it exposes the mRNA to exoribonuclease before translation of the open reading frame can begin. Our results emphasize the importance of co-transcriptional folding of RNA tertiary structure for cis-regulation of mRNA stability.

In Silico Design, In Vitro Construction, and In Vivo Application of Synthetic Small Regulatory RNAs in Bacteria

Small regulatory RNAs (sRNAs) are short non-coding RNAs in bacteria capable of post-transcriptional regulation. sRNAs have recently gained attention as tools in basic and applied sciences, for example, to fine-tune genetic circuits or biotechnological processes. Even though sRNAs often have a rather simple and modular structure, the design of functional synthetic sRNAs is not necessarily trivial. This protocol outlines how to use computational predictions and synthetic biology approaches to design, construct, and validate synthetic sRNA functionality for their application in bacteria. The computational tool, SEEDling, matches the optimal seed region with the user-selected sRNA scaffold for repression of target mRNAs. The synthetic sRNAs are assembled using Golden Gate cloning and their functionality is subsequently validated. The protocol uses the acrA mRNA as an exemplary proof-of-concept target in Escherichia coli. Since AcrA is part of a multidrug efflux pump, acrA repression can be revealed by assessing oxacillin susceptibility in a phenotypic screen. However, in case target repression does not result in a screenable phenotype, an alternative validation of synthetic sRNA functionality based on a fluorescence reporter is described.

Crossing bacterial boundaries: The carbon catabolite repression system Crc-Hfq of Pseudomonas putida KT2440 as a tool to control translation in E. coli

As a global regulatory mechanism, carbon catabolite repression allows bacteria and eukaryal microbes to preferentially utilize certain substrates from a mixture of carbon sources. The mechanism varies among different species. In Pseudomonas spp., it is mainly mediated by the Crc-Hfq complex which binds to the 5' region of the target mRNAs, thereby inhibiting their translation. This molecular mechanism enables P. putida to rapidly adjust and fine-tune gene expression in changing environments. Hfq is an RNA-binding protein that is ubiquitous and highly conserved in bacterial species. Considering the characteristics of Hfq, and the widespread use and rapid response of Crc-Hfq in P. putida, this complex has the potential to become a general toolbox for post-transcriptional multiplex regulation. In this study, we demonstrate for the first time that transplanting the pseudomonal catabolite repression protein, Crc, into E. coli causes multiplex gene repression. Under the control of Crc, the production of a diester and its precursors was significantly reduced. The effects of Crc introduction on cell growth in both minimal and rich media were evaluated. Two potential factors - off-target effects and Hfq-sequestration - could explain negative effects on cell growth. Simultaneous reduction of off-targeting and increased sequestration of Hfq by the introduction of the small RNA CrcZ, indicated that Hfq sequestration plays a more prominent role in the negative side-effects. This suggests that the negative growth effect can be mitigated by well-controlled expression of Hfq. This study reveals the feasibility of controlling gene expression using heterologous regulation systems.

Mechanism of Fluoride Ion Encapsulation by Magnesium Ions in a Bacterial Riboswitch

Riboswitches sense various ions in bacteria and activate gene expression to synthesize proteins that help maintain ion homeostasis. The crystal structure of the aptamer domain (AD) of the fluoride riboswitch shows that the F- ion is encapsulated by three Mg2+ ions bound to the ligand-binding domain (LBD) located at the core of the AD. The assembly mechanism of this intricate structure is unknown. To this end, we performed computer simulations using coarse-grained and all-atom RNA models to bridge multiple time scales involved in riboswitch folding and ion binding. We show that F- encapsulation by the Mg2+ ions bound to the riboswitch involves multiple sequential steps. Broadly, two Mg2+ ions initially interact with the phosphate groups of the LBD using water-mediated outer-shell coordination and transition to a direct inner-shell interaction through dehydration to strengthen their interaction with the LBD. We propose that the efficient binding mode of the third Mg2+ and F- is that they form a water-mediated ion pair and bind to the LBD simultaneously to minimize the electrostatic repulsion between three Mg2+ bound to the LBD. The tertiary stacking interactions among the LBD nucleobases alone are insufficient to stabilize the alignment of the phosphate groups to facilitate Mg2+ binding. We show that the stability of the whole assembly is an intricate balance of the interactions among the five phosphate groups, three Mg2+, and the encapsulated F- ion aided by the Mg2+ solvated water. These insights are helpful in the rational design of RNA-based ion sensors and fast-switching logic gates.

RNA Post-transcriptional Modifications of an Early-Stage Large-Subunit Ribosomal Intermediate

Protein production by ribosomes is fundamental to life, and proper assembly of the ribosome is required for protein production. The RNA, which is post-transcriptionally modified, provides the platform for ribosome assembly. Thus, a complete understanding of ribosome assembly requires the determination of the RNA post-transcriptional modifications in all of the ribosome assembly intermediates and on each pathway. There are 26 RNA post-transcriptional modifications in 23S RNA of the mature Escherichia coli (E. coli) large ribosomal subunit. The levels of these modifications have been investigated extensively only for a small number of large subunit intermediates and under a limited number of cellular and environmental conditions. In this study, we determined the level of incorporations of 2-methyl adenosine, 3-methyl pseudouridine, 5-hydroxycytosine, and seven pseudouridines in an early-stage E. coli large-subunit assembly intermediate with a sedimentation coefficient of 27S. The 27S intermediate is one of three large subunit intermediates accumulated in E. coli cells lacking the DEAD-box RNA helicase DbpA and expressing the helicase inactive R331A DbpA construct. The majority of the investigated modifications are incorporated into the 27S large subunit intermediate to similar levels to those in the mature 50S large subunit, indicating that these early modifications or the enzymes that incorporate them play important roles in the initial events of large subunit ribosome assembly.

Interactions between terminal ribosomal RNA helices stabilize the E. coli large ribosomal subunit

The ribosome is a large ribonucleoprotein assembly that uses diverse and complex molecular interactions to maintain proper folding. In vivo assembled ribosomes have been isolated using MS2 tags installed in either the 16S or 23S ribosomal RNAs (rRNAs), to enable studies of ribosome structure and function in vitro. RNA tags in the Escherichia coli 50S subunit have commonly been inserted into an extended helix H98 in 23S rRNA, as this addition does not affect cellular growth or in vitro ribosome activity. Here, we find that E. coli 50S subunits with MS2 tags inserted in H98 are destabilized compared to wild-type (WT) 50S subunits. We identify the loss of RNA-RNA tertiary contacts that bridge helices H1, H94, and H98 as the cause of destabilization. Using cryogenic electron microscopy (cryo-EM), we show that this interaction is disrupted by the addition of the MS2 tag and can be restored through the insertion of a single adenosine in the extended H98 helix. This work establishes ways to improve MS2 tags in the 50S subunit that maintain ribosome stability and investigates a complex RNA tertiary structure that may be important for stability in various bacterial ribosomes.

Structural basis for control of bacterial RNA polymerase pausing by a riboswitch and its ligand

Folding of nascent transcripts can be modulated by the RNA polymerase (RNAP) that carries out their transcription, and vice versa. A pause of RNAP during transcription of a preQ1 riboswitch (termed que-PEC) is stabilized by a previously characterized template consensus sequence and the ligand-free conformation of the nascent RNA. Ligand binding to the riboswitch induces RNAP pause release and downstream transcription termination; however, the mechanism by which riboswitch folding modulates pausing is unclear. Here, we report single-particle cryo-electron microscopy reconstructions of que-PEC in ligand-free and ligand-bound states. In the absence of preQ1, the RNA transcript is in an unexpected hyper-translocated state, preventing downstream nucleotide incorporation. Strikingly, on ligand binding, the riboswitch rotates around its helical axis, expanding the surrounding RNAP exit channel and repositioning the transcript for elongation. Our study reveals the tight coupling by which nascent RNA structures and their ligands can functionally regulate the macromolecular transcription machinery.

The Small RNA NcS25 Regulates Biological Amine-Transporting Outer Membrane Porin BCAL3473 in Burkholderia cenocepacia

Regulation of porin expression in bacteria is complex and often involves small-RNA regulators. Several small-RNA regulators have been described for Burkholderia cenocepacia, and this study aimed to characterize the biological role of the conserved small RNA NcS25 and its cognate target, outer membrane protein BCAL3473. The B. cenocepacia genome carries a large number of genes encoding porins with yet-uncharacterized functions. Expression of the porin BCAL3473 is strongly repressed by NcS25 and activated by other factors, such as a LysR-type regulator and nitrogen-depleted growth conditions. The porin is involved in transport of arginine, tyrosine, tyramine, and putrescine across the outer membrane. Porin BCAL3473, with NcS25 as a major regulator, plays an important role in the nitrogen metabolism of B. cenocepacia. IMPORTANCE Burkholderia cenocepacia is a Gram-negative bacterium which causes infections in immunocompromised individuals and in people with cystic fibrosis. A low outer membrane permeability is one of the factors giving it a high level of innate resistance to antibiotics. Porins provide selective permeability for nutrients, and antibiotics can also traverse the outer membrane by this means. Knowing the properties and specificities of porin channels is therefore important for understanding resistance mechanisms and for developing new antibiotics and could help in overcoming permeability issues in antibiotic treatment.

RNA-activated protein cleavage with a CRISPR-associated endopeptidase

In prokaryotes, CRISPR-Cas systems provide adaptive immune responses against foreign genetic elements through RNA-guided nuclease activity. Recently, additional genes with non-nuclease functions have been found in genetic association with CRISPR systems, suggesting that there may be other RNA-guided non-nucleolytic enzymes. One such gene from Desulfonema ishimotonii encodes the TPR-CHAT protease Csx29, which is associated with the CRISPR effector Cas7-11. Here, we demonstrate that this CRISPR-associated protease (CASP) exhibits programmable RNA-activated endopeptidase activity against a sigma factor inhibitor to regulate a transcriptional response. Cryo-electron microscopy of an active and substrate-bound CASP complex reveals an allosteric activation mechanism that reorganizes Csx29 catalytic residues upon target RNA binding. This work reveals an RNA-guided function in nature that can be leveraged for RNA-sensing applications in vitro and in human cells.

Upstream Flanking Sequence Assists Folding of an RNA Thermometer

Many heat shock genes in bacteria are regulated through a class of temperature-sensitive stem-loop (SL) RNAs called RNA thermometers (RNATs). One of the most widely studied RNATs is the Repression Of heat Shock Expression (ROSE) element associated with expression of heat shock proteins. Located in the 5'UTR, the RNAT contains one to three auxiliary hairpins upstream of it. Herein, we address roles of these upstream SLs in the folding and function of an RNAT. Bradyrhizobium japonicum is a nitrogen-fixing bacterium that experiences a wide range of temperatures in the soil and contains ROSE elements, each having multiple upstream SLs. The 5'UTR of the messenger (mRNA) for heat shock protein A (hspA) in B. japonicum has an intricate secondary structure containing three SLs upstream of the RNAT SL. While structure-function studies of the hspA RNAT itself have been reported, it has been unclear if these auxiliary SLs contribute to the temperature-sensing function of the ROSE elements. Herein, we show that the full length (FL) sequence has several melting transitions indicating that the ROSE element unfolds in a non-two-state manner. The upstream SLs are more stable than the RNAT itself, and a variant with disrupted base pairing in the SL immediately upstream of the RNAT has little influence on the melting of the RNAT. On the basis of these results and modeling of the co-transcriptional folding of the ROSE element, we propose that the upstream SLs function to stabilize the transcript and aid proper folding and dynamics of the RNAT.

Structure and engineering of the type III-E CRISPR-Cas7-11 effector complex

The type III-E CRISPR-Cas effector Cas7-11, with dual RNase activities for precursor CRISPR RNA (pre-crRNA) processing and crRNA-guided target RNA cleavage, is a new platform for bacterial and mammalian RNA targeting. We report the 2.5-Å resolution cryoelectron microscopy structure of Cas7-11 in complex with a crRNA and its target RNA. Cas7-11 adopts a modular architecture comprising seven domains (Cas7.1-Cas7.4, Cas11, INS, and CTE) and four interdomain linkers. The crRNA 5' tag is recognized and processed by Cas7.1, whereas the crRNA spacer hybridizes with the target RNA. Consistent with our biochemical data, the catalytic residues for programmable cleavage in Cas7.2 and Cas7.3 neighbor the scissile phosphates before the flipped-out fourth and tenth nucleotides in the target RNA, respectively. Using structural insights, we rationally engineered a compact Cas7-11 variant (Cas7-11S) for single-vector AAV packaging for transcript knockdown in human cells, enabling in vivo Cas7-11 applications.

Nanopore sequencing of RNA and cDNA molecules in Escherichia coli

High-throughput sequencing dramatically changed our view of transcriptome architectures and allowed for ground-breaking discoveries in RNA biology. Recently, sequencing of full-length transcripts based on the single-molecule sequencing platform from Oxford Nanopore Technologies (ONT) was introduced and is widely used to sequence eukaryotic and viral RNAs. However, experimental approaches implementing this technique for prokaryotic transcriptomes remain scarce. Here, we present an experimental and bioinformatic workflow for ONT RNA-seq in the bacterial model organism Escherichia coli, which can be applied to any microorganism. Our study highlights critical steps of library preparation and computational analysis and compares the results to gold standards in the field. Furthermore, we comprehensively evaluate the applicability and advantages of different ONT-based RNA sequencing protocols, including direct RNA, direct cDNA, and PCR-cDNA. We find that (PCR)-cDNA-seq offers improved yield and accuracy compared to direct RNA sequencing. Notably, (PCR)-cDNA-seq is suitable for quantitative measurements and can be readily used for simultaneous and accurate detection of transcript 5' and 3' boundaries, analysis of transcriptional units, and transcriptional heterogeneity. In summary, based on our comprehensive study, we show nanopore RNA-seq to be a ready-to-use tool allowing rapid, cost-effective, and accurate annotation of multiple transcriptomic features. Thereby nanopore RNA-seq holds the potential to become a valuable alternative method for RNA analysis in prokaryotes.

Discovery and initial characterization of YloC, a novel endoribonuclease in Bacillus subtilis

The Bacillus subtilis genome is predicted to encode numerous ribonucleases, including four 3' exoribonucleases that have been characterized to some extent. A strain containing gene knockouts of all four known 3' exoribonucleases is viable, suggesting that one or more additional RNases remain to be discovered. A protein extract from the quadruple RNase mutant strain was fractionated and RNase activity was followed, resulting in the identification of an enzyme activity catalyzed by the YloC protein. YloC is an endoribonuclease and is a member of the highly conserved "YicC family" of proteins that is widespread in bacteria. YloC is a metal-dependent enzyme that catalyzes the cleavage of single-stranded RNA, preferentially at U residues, and exists in an oligomeric form, most likely a hexamer. As such, YloC shares some characteristics with the SARS-CoV Nsp15 endoribonuclease. While the in vivo function of YloC in B. subtilis is yet to be determined, YloC was found to act similarly to YicC in an Escherichia coli in vivo assay that assesses decay of the small RNA, RyhB. Thus, YloC may play a role in small RNA regulation.

OperonSEQer: A set of machine-learning algorithms with threshold voting for detection of operon pairs using short-read RNA-sequencing data

Operon prediction in prokaryotes is critical not only for understanding the regulation of endogenous gene expression, but also for exogenous targeting of genes using newly developed tools such as CRISPR-based gene modulation. A number of methods have used transcriptomics data to predict operons, based on the premise that contiguous genes in an operon will be expressed at similar levels. While promising results have been observed using these methods, most of them do not address uncertainty caused by technical variability between experiments, which is especially relevant when the amount of data available is small. In addition, many existing methods do not provide the flexibility to determine the stringency with which genes should be evaluated for being in an operon pair. We present OperonSEQer, a set of machine learning algorithms that uses the statistic and p-value from a non-parametric analysis of variance test (Kruskal-Wallis) to determine the likelihood that two adjacent genes are expressed from the same RNA molecule. We implement a voting system to allow users to choose the stringency of operon calls depending on whether your priority is high recall or high specificity. In addition, we provide the code so that users can retrain the algorithm and re-establish hyperparameters based on any data they choose, allowing for this method to be expanded as additional data is generated. We show that our approach detects operon pairs that are missed by current methods by comparing our predictions to publicly available long-read sequencing data. OperonSEQer therefore improves on existing methods in terms of accuracy, flexibility, and adaptability.

Bacteria and archaea, single-cell organisms collectively known as prokaryotes, live in all imaginable environments and comprise the majority of living organisms on this planet. Prokaryotes play a critical role in the homeostasis of multicellular organisms (such as animals and plants) and ecosystems. In addition, bacteria can be pathogenic and cause a variety of diseases in these same hosts and ecosystems. In short, understanding the biology and molecular functions of bacteria and archaea and devising mechanisms to engineer and optimize their properties are critical scientific endeavors with significant implications in healthcare, agriculture, manufacturing, and climate science among others. One major molecular difference between unicellular and multicellular organisms is the way they express genes–multicellular organisms make individual RNA molecules for each gene while, prokaryotes express operons (i.e., a group of genes coding functionally related proteins) in contiguous polycistronic RNA molecules. Understanding which genes exist within operons is critical for elucidating basic biology and for engineering organisms. In this work, we use a combination of statistical and machine learning-based methods to use next-generation sequencing data to predict operon structure across a range of prokaryotes. Our method provides an easily implemented, robust, accurate, and flexible way to determine operon structure in an organism-agnostic manner using readily available data.

RNA sequence and structure control assembly and function of RNA condensates

Intracellular condensates formed through liquid-liquid phase separation (LLPS) primarily contain proteins and RNA. Recent evidence points to major contributions of RNA self-assembly in the formation of intracellular condensates. As the majority of previous studies on LLPS have focused on protein biochemistry, effects of biological RNAs on LLPS remain largely unexplored. In this study, we investigate the effects of crowding, metal ions, and RNA structure on formation of RNA condensates lacking proteins. Using bacterial riboswitches as a model system, we first demonstrate that LLPS of RNA is promoted by molecular crowding, as evidenced by formation of RNA droplets in the presence of polyethylene glycol (PEG 8K). Crowders are not essential for LLPS, however. Elevated Mg²⁺ concentrations promote LLPS of specific riboswitches without PEG. Calculations identify key RNA structural and sequence elements that potentiate the formation of PEG-free condensates; these calculations are corroborated by key wet-bench experiments. Based on this, we implement structure-guided design to generate condensates with novel functions including ligand binding. Finally, we show that RNA condensates help protect their RNA components from degradation by nucleases, suggesting potential biological roles for such higher-order RNA assemblies in controlling gene expression through RNA stability. By utilizing both natural and artificial RNAs, our study provides mechanistic insight into the contributions of intrinsic RNA properties and extrinsic environmental conditions to the formation and regulation of condensates comprised of RNAs.

The RNA chaperone StpA enables fast RNA refolding by destabilization of mutually exclusive base pairs within competing secondary structure elements

In bacteria RNA gene regulatory elements refold dependent on environmental clues between two or more long-lived conformational states each associated with a distinct regulatory state. The refolding kinetics are strongly temperature-dependent and especially at lower temperatures they reach timescales that are biologically not accessible. To overcome this problem, RNA chaperones have evolved. However, the precise molecular mechanism of how these proteins accelerate RNA refolding reactions remains enigmatic. Here we show how the RNA chaperone StpA of Escherichia coli leads to an acceleration of a bistable RNA's refolding kinetics through the selective destabilization of key base pairing interactions. We find in laser assisted real-time NMR experiments on photocaged bistable RNAs that the RNA chaperone leads to a two-fold increase in refolding rates at low temperatures due to reduced stability of ground state conformations. Further, we can show that upon interaction with StpA, base pairing interactions in the bistable RNA are modulated to favor refolding through the dominant pseudoknotted transition pathway. Our results shed light on the molecular mechanism of the interaction between RNA chaperones and bistable RNAs and are the first step into a functional classification of chaperones dependent on their biophysical mode of operation.

Structural insights into dpCoA-RNA decapping by NudC

Various kinds of cap structures, such as m7G, triphosphate groups, NAD and dpCoA, protect the 5' terminus of RNA. The cap structures bond covalently to RNA and affect its stability, translation, and transport. The removal of the caps is mainly executed by Nudix hydrolase family proteins, including Dcp2, RppH and NudC. Numerous efforts have been made to elucidate the mechanism underlying the removal of m7G, triphosphate group, and NAD caps. In contrast, few studies related to the cleavage of the RNA dpCoA cap have been conducted. Here, we report the hydrolytic activity of Escherichia coli NudC towards dpCoA and dpCoA-capped RNA in vitro. We also determined the crystal structure of dimeric NudC in complex with dpCoA at 2.0 Å resolution. Structural analysis revealed that dpCoA is recognized and hydrolysed in a manner similar to NAD. In addition, NudC may also remove other dinucleotide derivative caps of RNA, which comprise the AMP moieties. NudC homologs in Saccharomyces cerevisiae and Arabidopsis thaliana exhibited similar dpCoA decapping (deCoAping) activity. These results together indicate a conserved mechanism underpinning the hydrolysis of dpCoA-capped RNA in both prokaryotes and eukaryotes.

Comparative study on tertiary contacts and folding of RNase P RNAs from a psychrophilic, a mesophilic/radiation-resistant, and a thermophilic bacterium

In most bacterial type A RNase P RNAs (P RNAs), two major loop-helix tertiary contacts (L8-P4 and L18-P8) help to orient the two independently folding S- and C-domains for concerted recognition of precursor tRNA substrates. Here, we analyze the effects of mutations in these tertiary contacts in P RNAs from three different species: (i) the psychrophilic bacterium Pseudoalteromonas translucida (Ptr), (ii) the mesophilic radiation-resistant bacterium Deinococcus radiodurans (Dra), and (iii) the thermophilic bacterium Thermus thermophilus (Tth). We show by UV melting experiments that simultaneous disruption of these two interdomain contacts has a stabilizing effect on all three P RNAs. This can be inferred from reduced RNA unfolding at lower temperatures and a more concerted unfolding at higher temperatures. Thus, when the two domains tightly interact via the tertiary contacts, one domain facilitates structural transitions in the other. P RNA mutants with disrupted interdomain contacts showed severe kinetic defects that were most pronounced upon simultaneous disruption of the L8-P4 and L18-P8 contacts. At 37°C, the mildest effects were observed for the thermostable Tth RNA. A third interdomain contact, L9-P1, makes only a minor contribution to P RNA tertiary folding. Furthermore, D. radiodurans RNase P RNA forms an additional pseudoknot structure between the P9 and P12 of its S-domain. This interaction was found to be particularly crucial for RNase P holoenzyme activity at near-physiological Mg²⁺ concentrations (2 mM). We further analyzed an exceptionally stable folding trap of the G,C-rich Tth P RNA.

High-Quality Nucleic Acid Isolation from Hard-to-Lyse Bacterial Strains Using PMAP-36, a Broad-Spectrum Antimicrobial Peptide

The efficiency of existing cell lysis methods to isolate nucleic acids from diverse bacteria varies depending on cell wall structures. This study tested a novel idea of using broad-spectrum antimicrobial peptides to improve the lytic efficiency of hard-to-lyse bacteria and characterized their differences. The lysis conditions of Staphylococcus aureus using recombinant porcine myeloid antimicrobial peptide 36 (PMAP-36), a broad-spectrum pig cathelicidin, was optimized, and RNA isolation was performed with cultured pellets of ten bacterial species using various membranolytic proteins. Additionally, three other antimicrobial peptides, protegrin-1 (PG-1), melittin, and nisin, were evaluated for their suitability as the membranolytic agents of bacteria. However, PMAP-36 use resulted in the most successful outcomes in RNA isolation from diverse bacterial species. The amount of total RNA obtained using PMAP-36 increased by ~2-fold compared to lysozyme in Salmonella typhimurium. Streptococci species were refractory to all lytic proteins tested, although the RNA yield from PMAP-36 treatment was slightly higher than that from other methods. PMAP-36 use produced high-quality RNA, and reverse transcription PCR showed the efficient amplification of the 16S rRNA gene from all tested strains. Additionally, the results of genomic DNA isolation were similar to those of RNA isolation. Thus, our findings present an additional option for high quality and unbiased nucleic acid isolation from microbiomes or challenging bacterial strains.

Cryo-EM structure of the RNA-guided ribonuclease Cas12g

Cas12g, the type V-G CRISPR-Cas effector, is an RNA-guided ribonuclease that targets single-stranded RNA substrate. The CRISPR-Cas12g system offers a potential platform for transcriptome engineering and diagnostic applications. We determined the structures of Cas12g-guide RNA complexes in the absence and presence of target RNA by cryo-EM to a resolution of 3.1 Å and 4.8 Å, respectively. Cas12g adopts a bilobed structure with miniature REC2 and Nuc domains, whereas the guide RNAs fold into a flipped 'F' shape, which is primarily recognized by the REC lobe. Target RNA and the CRISPR RNA (crRNA) guide form a duplex that inserts into the central cavity between the REC and NUC lobes, inducing conformational changes in both lobes to activate Cas12g. The structural insights would facilitate the development of Cas12g-based applications.

Beyond Plug and Pray: Context Sensitivity and in silico Design of Artificial Neomycin Riboswitches

Gene regulation in prokaryotes often depends on RNA elements such as riboswitches or RNA thermometers located in the 5' untranslated region of mRNA. Rearrangements of the RNA structure in response, e.g., to the binding of small molecules or ions control translational initiation or premature termination of transcription and thus mRNA expression. Such structural responses are amenable to computational modelling, making it possible to rationally design synthetic riboswitches for a given aptamer. Starting from an artificial aptamer, we construct the first synthetic transcriptional riboswitches that respond to the antibiotic neomycin. We show that the switching behaviour in vivo critically depends not only on the sequence of the riboswitch itself, but also on its sequence context. We therefore developed in silico methods to predict the impact of the context, making it possible to adapt the design and to rescue non-functional riboswitches. We furthermore analyse the influence of 5' hairpins with varying stability on neomycin riboswitch activity. Our data highlight the limitations of a simple plug-and-play approach in the design of complex genetic circuits and demonstrate that detailed computational models significantly simplify, improve, and automate the design of transcriptional circuits. Our design software is available under a free licence on GitHub (https://github.com/xileF1337/riboswitch_design).

The Serine Biosynthesis of Paenibacillus polymyxa WLY78 Is Regulated by the T-Box Riboswitch

Serine is important for nearly all microorganisms in protein and downstream amino acids synthesis, however, the effect of serine on growth and nitrogen fixation was not completely clear in many bacteria, besides, the regulatory mode of serine remains to be fully established. In this study, we demonstrated that L-serine is essential for growth and nitrogen fixation of Paenibacillus polymyxa WLY78, but high concentrations of L-serine inhibit growth, nitrogenase activity, and nifH expression. Then, we revealed that expression of the serA whose gene product catalyzes the first reaction in the serine biosynthetic pathway is regulated by the T-box riboswitch regulatory system. The 508 bp mRNA leader region upstream of the serA coding region contains a 280 bp T-box riboswitch. The secondary structure of the T-box riboswitch with several conserved features: three stem-loop structures, a 14-bp T-box sequence, and an intrinsic transcriptional terminator, is predicted. Mutation and the transcriptional leader-lacZ fusions experiments revealed that the specifier codon of serine is AGC (complementary to the anticodon sequence of tRNA^ser). qRT-PCR showed that transcription of serA is induced by serine starvation, whereas deletion of the specifier codon resulted in nearly no expression of serA. Deletion of the terminator sequence or mutation of the continuous seven T following the terminator led to constitutive expression of serA. The data indicated that the T-box riboswitch, a noncoding RNA segment in the leader region, regulates expression of serA by a transcription antitermination mechanism.

RSwitch: A Novel Bioinformatics Database on Riboswitches as Antibacterial Drug Targets

The RSwitch is a MySQL database implemented on a PHP-based server, which also provides various useful tools for analyses of DNA, RNA, and protein sequences applying a user-friendly interface. The RSwitch database currently contains information and annotations of 215 bacterial riboswitches from 16 different types found in 50 human pathogenic bacteria. The riboswitch classes include those sensing FMN, glmS, Cobalamin, Lysine, SAH, SAM, Purine, TPP, c-di-GMPI, c-di-GMPII, Moco, PreQ1, Fluoride, Glycine, Mg²⁺, and Mn²⁺ type of bacterial riboswitches. The database provides information about the riboswitch aptamer sequences, the thermodynamic ensemble of the RNA structures on partition function and on free minimum energy function. Additionally, the database presents the centroid structure and the positional entropy for each position of the aptamer sequences. The database also provides the biochemical pathways in which the riboswitches are involved in, as well as multiple sequence alignments, multi-drug resistance bacterial strains and consensus motifs for each type of the switches. The RSwitch database is permanently available online without any restrictions. This bioinformatics database provides for the first time all information needed for assessing the suitability of the presented riboswitches as antibacterial drug targets.

Expansion segments in bacterial and archaeal 5S ribosomal RNAs

The large ribosomal RNAs of eukaryotes frequently contain expansion sequences that add to the size of the rRNAs but do not affect their overall structural layout and are compatible with major ribosomal function as an mRNA translation machine. The expansion of prokaryotic ribosomal RNAs is much less explored. In order to obtain more insight into the structural variability of these conserved molecules, we herein report the results of a comprehensive search for the expansion sequences in prokaryotic 5S rRNAs. Overall, 89 expanded 5S rRNAs of 15 structural types were identified in 15 archaeal and 36 bacterial genomes. Expansion segments ranging in length from 13 to 109 residues were found to be distributed among 17 insertion sites. The strains harboring the expanded 5S rRNAs belong to the bacterial orders Clostridiales, Halanaerobiales, Thermoanaerobacterales, and Alteromonadales as well as the archael order Halobacterales When several copies of a 5S rRNA gene are present in a genome, the expanded versions may coexist with normal 5S rRNA genes. The insertion sequences are typically capable of forming extended helices, which do not seemingly interfere with folding of the conserved core. The expanded 5S rRNAs have largely been overlooked in 5S rRNA databases.

RASP: an atlas of transcriptome-wide RNA secondary structure probing data

RNA molecules fold into complex structures that are important across many biological processes. Recent technological developments have enabled transcriptome-wide probing of RNA secondary structure using nucleases and chemical modifiers. These approaches have been widely applied to capture RNA secondary structure in many studies, but gathering and presenting such data from very different technologies in a comprehensive and accessible way has been challenging. Existing RNA structure probing databases usually focus on low-throughput or very specific datasets. Here, we present a comprehensive RNA structure probing database called RASP (RNA Atlas of Structure Probing) by collecting 161 deduplicated transcriptome-wide RNA secondary structure probing datasets from 38 papers. RASP covers 18 species across animals, plants, bacteria, fungi, and also viruses, and categorizes 18 experimental methods including DMS-seq, SHAPE-Seq, SHAPE-MaP, and icSHAPE, etc. Specially, RASP curates the up-to-date datasets of several RNA secondary structure probing studies for the RNA genome of SARS-CoV-2, the RNA virus that caused the on-going COVID-19 pandemic. RASP also provides a user-friendly interface to query, browse, and visualize RNA structure profiles, offering a shortcut to accessing RNA secondary structures grounded in experimental data. The database is freely available at http://rasp.zhanglab.net.

Essential biochemical, biophysical and computational inputs on efficient functioning of Mycobacterium tuberculosis H37Rv FtsY

Mycobacterium tuberculosis (M. tuberculosis H₃₇Rv) utilizes the signal recognition particle pathway (SRP pathway) system for secretion of various proteins from ribosomes to the extracellular surface which plays an important role in the machinery running inside the bacterium. This system comprises of three major components FtsY, FfH and 4.5S rRNA. This manuscript highlights essential factors responsible for the optimized enzymatic activity of FtsY. Kinetic parameters include V_max and K_m for the hydrolysis of GTP by ftsY which were 20.25±5.16 μM/min/mg and 39.95±7.7 μM respectively. k_cat and catalytic efficiency of the reaction were 0.012±0.003 s^-1 and 0.00047±0.0001 μM/s^-1 respectively. These values were affected upon changing the standard conditions. Cations (Mg²⁺ and Mn²⁺) play important role in FtsY enzymatic activity as increasing Mg²⁺ decrease the activity. Mn²⁺on the other hand is required at higher concentration around 60 mM for carrying optimum GTPase activity. FtsY is hydrolyzing ATP and GDP as well and GDP acts as an inhibitor of the reaction. MD simulation shows effective binding and stabilization of the FtsY complexed structure with GTP, GDP and ATP. Mutational analysis was done at two important residues of GTP binding motif of FtsY, namely, GXXXXGK (K236) and DXXG (D367) and showed that these mutations significantly decrease FtsY GTPase activity. FtsY is comprised of α helices, but this structural pattern was shown to change with increasing concentrations of GTP and ATP which symbolize that these ligands cause significant conformational change by variating the secondary structure to transduce signals required by downstream effectors. This binding favors the functional stabilization of FtsY by destabilization of α-helix integrity. Revealing the hidden aspects of the functioning of FtsY might be an essential part for the understanding of the SRP pathway which is one of the important contributors of M. tuberculosis virulence.

A Quick Primer in Fluorescence-Based Equilibrium and Pre-steady State Methods for Determining Protein-Nucleotide Affinities

Biomolecular interactions facilitate the biochemical processes that sustain life. Proteins, RNAs, and ribonucleoprotein complexes perform cellular functions that range from catalyzing the formation or cleavage of bonds to being structural building blocks, both of which are only possible through the interaction with their respective biomolecular partner(s). Having access to the parameters that describe these interactions is important for our understanding of the principles that underlie enzymatic and nonenzymatic processes. Here we describe two fluorescence-based approaches to determine two key parameters, the affinity and the rate of association/dissociation of a protein and a ligand. Considerations are provided to expand the described approach to other experimental systems.

RNA-based fluorescent biosensors for live cell detection of bacterial sRNA

Bacteria contain a diverse set of RNAs to provide tight regulation of gene expression in response to environmental stimuli. Bacterial small RNAs (sRNAs) work in conjunction with protein cofactors to bind complementary mRNA sequences in the cell, leading to up- or downregulation of protein synthesis. In vivo imaging of sRNAs can aid in understanding their spatiotemporal dynamics in real time, which inspires new ways to manipulate these systems for a variety of applications including synthetic biology and therapeutics. Current methods for sRNA imaging are quite limited in vivo and do not provide real-time information about fluctuations in sRNA levels. Herein, we describe our efforts toward the development of an RNA-based fluorescent biosensor for bacterial sRNA both in vitro and in vivo. We validated these sensors for three different bacterial sRNAs in Escherichia coli and demonstrated that the designs provide a bright, sequence-specific signal output in response to exogenous and endogenous RNA targets.

Characterization of Protein-Nucleic Acid Complexes by Size-Exclusion Chromatography Coupled with Light Scattering, Absorbance, and Refractive Index Detectors

Size-exclusion chromatography (SEC) coupled with multiangle light scattering detection (SEC/MALS) enables determination of the molecular weight, oligomeric state, and stoichiometry of protein-nucleic acid complexes in solution. Often such complexes show anomalous behavior on SEC, thus presenting a challenge in determination of molecular weight and stoichiometry based solely on the elution position from SEC. In contrast to analytical ultracentrifugation, the SEC/MALS analysis is not affected by the shape of the complex. Here we describe the use of SEC/MALS for characterization of the stoichiometry of the complex between the reverse transcriptase (RT) domain from group II intron-maturase from Eubacterium rectale and intron RNA, and for monitoring protein dimerization that is driven by interaction between single-stranded DNA upstream of the P1 promoter, known as FUSE and FUSE binding protein-interacting repressor (FIR).

An RNA pseudoknot is essential for standby-mediated translation of the tisB toxin mRNA in Escherichia coli

In response to DNA damage, Escherichia coli cells activate the expression of the toxin gene tisB of the toxin-antitoxin system tisB-istR1. Of three isoforms, only the processed, highly structured +42 tisB mRNA is active. Translation requires a standby site, composed of two essential elements: a single-stranded region located 100 nucleotides upstream of the sequestered RBS, and a structure near the 5'-end of the active mRNA. Here, we propose that this 5'-structure is an RNA pseudoknot which is required for 30S and protein S1-alone binding to the mRNA. Point mutations that prevent formation of this pseudoknot inhibit formation of translation initiation complexes, impair S1 and 30S binding to the mRNA, and render the tisB mRNA non-toxic in vivo. A set of mutations created in either the left or right arm of stem 2 of the pseudoknot entailed loss of toxicity upon overexpression of the corresponding mRNA variants. Combining the matching right-left arm mutations entirely restored toxicity levels to that of the wild-type, active mRNA. Finally, since many pseudoknots have high affinity for S1, we predicted similar pseudoknots in non-homologous type I toxin-antitoxin systems that exhibit features similar to that of tisB-IstR1, suggesting a shared requirement for standby acting at great distances.

Requirements for efficient ligand-gated co-transcriptional switching in designed variants of the B. subtilis pbuE adenine-responsive riboswitch in E. coli

Riboswitches, generally located in the 5'-leader of bacterial mRNAs, direct expression via a small molecule-dependent structural switch that informs the transcriptional or translational machinery. While the structure and function of riboswitch effector-binding (aptamer) domains have been intensely studied, only recently have the requirements for efficient linkage between small molecule binding and the structural switch in the cellular and co-transcriptional context begun to be actively explored. To address this aspect of riboswitch function, we have performed a structure-guided mutagenic analysis of the B. subtilis pbuE adenine-responsive riboswitch, one of the simplest riboswitches that employs a strand displacement switching mechanism to regulate transcription. Using a cell-based fluorescent protein reporter assay to assess ligand-dependent regulatory activity in E. coli, these studies revealed previously unrecognized features of the riboswitch. Within the aptamer domain, local and long-range conformational dynamics influenced by sequences within helices have a significant effect upon efficient regulatory switching. Sequence features of the expression platform including the pre-aptamer leader sequence, a toehold helix and an RNA polymerase pause site all serve to promote strong ligand-dependent regulation. By optimizing these features, we were able to improve the performance of the B. subtilis pbuE riboswitch in E. coli from 5.6-fold induction of reporter gene expression by the wild type riboswitch to over 120-fold in the top performing designed variant. Together, these data point to sequence and structural features distributed throughout the riboswitch required to strike a balance between rates of ligand binding, transcription and secondary structural switching via a strand exchange mechanism and yield new insights into the design of artificial riboswitches.

Solution structure and RNA-binding of a minimal ProQ-homolog from Legionella pneumophila (Lpp1663)

Small regulatory RNAs (sRNAs) play an important role for posttranscriptional gene regulation in bacteria. sRNAs recognize their target messenger RNAs (mRNAs) by base-pairing, which is often facilitated by interactions with the bacterial RNA-binding proteins Hfq or ProQ. The FinO/ProQ RNA-binding protein domain was first discovered in the bacterial repressor of conjugation, FinO. Since then, the functional role of FinO/ProQ-like proteins in posttranscriptional gene regulation was extensively studied in particular in the enterobacteria E. coli and Salmonella enterica and a wide range of sRNA-targets was identified for these proteins. In addition, enterobacterial ProQ homologs also recognize and protect the 3'-ends of a number of mRNAs from exonucleolytic degradation. However, the RNA-binding properties of FinO/ProQ proteins with regard to the recognition of different RNA targets are not yet fully understood. Here, we present the solution NMR structure of the so far functionally uncharacterized ProQ homolog Lpp1663 from Legionella pneumophila as a newly confirmed member and a minimal model system of the FinO/ProQ protein family. In addition, we characterize the RNA-binding preferences of Lpp1663 with high resolution NMR spectroscopy and isothermal titration calorimetry (ITC). Our results suggest a binding preference for single-stranded uridine-rich RNAs in the vicinity of stable stem-loop structures. According to chemical shift perturbation experiments, the single-stranded U-rich RNAs interact mainly with a conserved RNA-binding surface on the concave site of Lpp1663.

A rare bacterial RNA motif is implicated in the regulation of the purF gene whose encoded enzyme synthesizes phosphoribosylamine

The Fibro-purF motif is a putative structured noncoding RNA domain that was discovered previously in species of Fibrobacter by using comparative sequence analysis methods. An updated bioinformatics search yielded a total of only 30 unique-sequence representatives, exclusively found upstream of the purF gene that codes for the enzyme amidophosphoribosyltransferase. This enzyme synthesizes the compound 5-phospho-D-ribosylamine (PRA), which is the first committed step in purine biosynthesis. The consensus model for Fibro-purF motif RNAs includes a predicted three-stem junction that carries numerous conserved nucleotide positions within the regions joining the stems. This architecture appears to be of sufficient size and complexity for the formation of the ligand-binding aptamer portion of a riboswitch. In this study, we conducted biochemical analyses of a representative Fibro-purF motif RNA to confirm that the RNA generally folds according to the predicted consensus model. However, due to the instability of PRA, binding of this ligand candidate by the RNA could not be directly assessed. Genetic analyses were used to demonstrate that Fibro-purF motif RNAs regulate gene expression in accordance with predicted PRA concentrations. These findings indicate that Fibro-purF motif RNAs are genetic regulation elements that likely suppress PRA biosynthesis when sufficient levels of this purine precursor are present.

Structural basis for assembly of non-canonical small subunits into type I-C Cascade

Bacteria and archaea employ CRISPR (clustered, regularly, interspaced, short palindromic repeats)-Cas (CRISPR-associated) systems as a type of adaptive immunity to target and degrade foreign nucleic acids. While a myriad of CRISPR-Cas systems have been identified to date, type I-C is one of the most commonly found subtypes in nature. Interestingly, the type I-C system employs a minimal Cascade effector complex, which encodes only three unique subunits in its operon. Here, we present a 3.1 Å resolution cryo-EM structure of the Desulfovibrio vulgaris type I-C Cascade, revealing the molecular mechanisms that underlie RNA-directed complex assembly. We demonstrate how this minimal Cascade utilizes previously overlooked, non-canonical small subunits to stabilize R-loop formation. Furthermore, we describe putative PAM and Cas3 binding sites. These findings provide the structural basis for harnessing the type I-C Cascade as a genome-engineering tool.

The Thermus thermophilus DEAD-box protein Hera is a general RNA binding protein and plays a key role in tRNA metabolism

RNA helicases catalyze the ATP-dependent destabilization of RNA duplexes. DEAD-box helicases share a helicase core that mediates ATP binding and hydrolysis, RNA binding and unwinding. Most members of this family contain domains flanking the core that can confer RNA substrate specificity and guide the helicase to a specific RNA. However, the in vivo RNA substrates of most helicases are currently not defined. The DEAD-box helicase Hera from Thermus thermophilus contains a helicase core, followed by a dimerization domain and an RNA binding domain that folds into an RNA recognition motif (RRM). The RRM mediates high affinity binding to an RNA hairpin, and an adjacent duplex is then unwound by the helicase core. Hera is a cold-shock protein, and has been suggested to act as an RNA chaperone under cold-shock conditions. Using crosslinking immunoprecipitation of Hera/RNA complexes and sequencing, we show that Hera binds to a large fraction of T. thermophilus RNAs under normal-growth and cold-shock conditions without a strong sequence preference, in agreement with a structure-specific recognition of RNAs and a general function in RNA metabolism. Under cold-shock conditions, Hera is recruited to RNAs with high propensities to form stable secondary structures. We show that selected RNAs identified, including a set of tRNAs, bind to Hera in vitro, and activate the Hera helicase core. Gene ontology analysis reveals an enrichment of genes related to translation, including mRNAs of ribosomal proteins, tRNAs, tRNA ligases, and tRNA-modifying enzymes, consistent with a key role of Hera in ribosome and tRNA metabolism.

Synthesis and Metabolic Fate of 4-Methylthiouridine in Bacterial tRNA

Ribonucleic acid (RNA) is central to many life processes and, to fulfill its function, it has a substantial chemical variety in its building blocks. Enzymatic thiolation of uridine introduces 4-thiouridine (s4 U) into many bacterial transfer RNAs (tRNAs), which is used as a sensor for UV radiation. A similar modified nucleoside, 2-thiocytidine, was recently found to be sulfur-methylated especially in bacteria exposed to antibiotics and simple methylating reagents. Herein, we report the synthesis of 4-methylthiouridine (ms4 U) and confirm its presence and additional formation under stress in Escherichia coli. We used the synthetic ms4 U for isotope dilution mass spectrometry and compared its abundance to other reported tRNA damage products. In addition, we applied sophisticated stable-isotope pulse chase studies (NAIL-MS) and showed its AlkB-independent removal in vivo. Our findings reveal the complex nature of bacterial RNA damage repair.

A scoutRNA Is Required for Some Type V CRISPR-Cas Systems

CRISPR-Cas12c/d proteins share limited homology with Cas12a and Cas9 bacterial CRISPR RNA (crRNA)-guided nucleases used widely for genome editing and DNA detection. However, Cas12c (C2c3)- and Cas12d (CasY)-catalyzed DNA cleavage and genome editing activities have not been directly observed. We show here that a short-complementarity untranslated RNA (scoutRNA), together with crRNA, is required for Cas12d-catalyzed DNA cutting. The scoutRNA differs in secondary structure from previously described tracrRNAs used by CRISPR-Cas9 and some Cas12 enzymes, and in Cas12d-containing systems, scoutRNA includes a conserved five-nucleotide sequence that is essential for activity. In addition to supporting crRNA-directed DNA recognition, biochemical and cell-based experiments establish scoutRNA as an essential cofactor for Cas12c-catalyzed pre-crRNA maturation. These results define scoutRNA as a third type of transcript encoded by a subset of CRISPR-Cas genomic loci and explain how Cas12c/d systems avoid requirements for host factors including ribonuclease III for bacterial RNA-mediated adaptive immunity.

Lead-seq: transcriptome-wide structure probing in vivo using lead(II) ions

The dynamic conformation of RNA molecules within living cells is key to their function. Recent advances in probing the RNA structurome in vivo, including the use of SHAPE (Selective 2'-Hydroxyl Acylation analyzed by Primer Extension) or kethoxal reagents or DMS (dimethyl sulfate), provided unprecedented insights into the architecture of RNA molecules in the living cell. Here, we report the establishment of lead probing in a global RNA structuromics approach. In order to elucidate the transcriptome-wide RNA landscape in the enteric pathogen Yersinia pseudotuberculosis, we combined lead(II) acetate-mediated cleavage of single-stranded RNA regions with high-throughput sequencing. This new approach, termed 'Lead-seq', provides structural information independent of base identity. We show that the method recapitulates secondary structures of tRNAs, RNase P RNA, tmRNA, 16S rRNA and the rpsT 5'-untranslated region, and that it reveals global structural features of mRNAs. The application of Lead-seq to Y. pseudotuberculosis cells grown at two different temperatures unveiled the first temperature-responsive in vivo RNA structurome of a bacterial pathogen. The translation of candidate genes derived from this approach was confirmed to be temperature regulated. Overall, this study establishes Lead-seq as complementary approach to interrogate intracellular RNA structures on a global scale.

RNA-binding activity and regulatory functions of the emerging sRNA-binding protein ProQ

Regulatory small RNAs (sRNAs) ubiquitously impact bacterial physiology through antisense-mediated control of mRNA translation and stability. In Gram negative bacteria, sRNAs often associate with RNA-binding proteins (RBPs), both to gain cellular stability and to enable regulatory efficiency. The Hfq and CsrA proteins were for long the only known global RBPs implicated in sRNA biology. During the last five years, the FinO domain-containing protein ProQ has emerged as another global RBP with a broad spectrum of sRNA and mRNA ligands. This review provides a summary of the current knowledge of enterobacterial ProQ, with a special focus on RNA binding activity, RNA ligand preferences, influence on RNA stability and gene expression, and impact on bacterial physiology. Considering that characterization of ProQ is still in its infancy, we highlight aspects that, when addressed, will provide important clues to the physiological functions and regulatory mechanisms of this globally acting RBP.

Quantification of the affinities of CRISPR-Cas9 nucleases for cognate protospacer adjacent motif (PAM) sequences

The CRISPR/Cas9 nucleases have been widely applied for genome editing in various organisms. Cas9 nucleases complexed with a guide RNA (Cas9-gRNA) find their targets by scanning and interrogating the genomic DNA for sequences complementary to the gRNA. Recognition of the DNA target sequence requires a short protospacer adjacent motif (PAM) located outside this sequence. Given that the efficiency of target location may depend on the strength of interactions that promote target recognition, here we sought to compare affinities of different Cas9 nucleases for their cognate PAM sequences. To this end, we measured affinities of Cas9 nucleases from Streptococcus pyogenes, Staphylococcus aureus, and Francisella novicida complexed with guide RNAs (gRNAs) (SpCas9-gRNA, SaCas9-gRNA, and FnCas9-gRNA, respectively) and of three engineered SpCas9-gRNA variants with altered PAM specificities for short, PAM-containing DNA probes. We used a "beacon" assay that measures the relative affinities of DNA probes by determining their ability to competitively affect the rate of Cas9-gRNA binding to fluorescently labeled target DNA derivatives called "Cas9 beacons." We observed significant differences in the affinities for cognate PAM sequences among the studied Cas9 enzymes. The relative affinities of SpCas9-gRNA and its engineered variants for canonical and suboptimal PAMs correlated with previous findings on the efficiency of these PAM sequences in genome editing. These findings suggest that high affinity of a Cas9 nuclease for its cognate PAM promotes higher genome-editing efficiency.

Formation and Stoichiometry of CRISPR-Cascade Complexes with Varying Spacer Lengths Revealed by Native Mass Spectrometry

The adaptive immune system of bacteria and archaea against viral DNA is based on clustered, regularly interspaced, short palindromic repeats (CRISPRs) which are encoded in the host genome and translated into CRISPR RNAs (crRNAs) containing single spacer sequences complementary to foreign DNA. crRNAs assemble with CRISPR-associated (Cas) proteins forming surveillance complexes that base-pair with viral DNA and mediate its degradation. As specificity of degradation is provided by the crRNA spacer sequence, genetic engineering of the CRISPR system has emerged as a popular molecular tool, for instance, in gene silencing and programmed DNA degradation. Elongating or shortening the crRNA spacer sequence are therefore promising ventures to modify specificity toward the target DNA. However, even though the stoichiometry of wild-type complexes is well established, it is unknown how variations in crRNA spacer length affect their stoichiometry. The CRISPR-associated antiviral defense surveillance complexes of Streptococcus thermophilus (StCascade complexes) contain crRNA and five protein subunits. Using native mass spectrometry, we studied the formation and stoichiometry of StCascade complexes assembled on a set of crRNAs with different spacer lengths. We assigned all relevant complexes and gained insights into the stoichiometry of the complexes as well as their preferred assembly. We found that stable complexes, which incorporate or lose a (Cas7)₂(Cse2)₁-module, assemble on crRNA varied in length by 12-nucleotide units, while varying crRNA length in six-nucleotide units results in heterogeneous mixtures of complexes. Combining our results from the various variants, we generated an assembly pathway revealing general features of I-E type Cascade complex formation.

De novo transcriptome analysis of halotolerant bacterium Staphylococcus sp. strain P-TSB-70 isolated from East coast of India: In search of salt stress tolerant genes

In the present study, we identified salt stress tolerant genes from the marine bacterium Staphylococcus sp. strain P-TSB-70 through transcriptome sequencing. In favour of whole-genome transcriptome profiling of Staphylococcus sp. strain P-TSB-70 (GenBank Accn. No. KP117091) which tolerated upto 20% NaCl stress, the strain was cultured in the laboratory condition with 20% NaCl stress. Transcriptome analyses were performed by SOLiD4.0 sequencing technology from which 10280 and 9612 transcripts for control and treated, respectively, were obtained. The coverage per base (CPB) statistics were analyzed for both the samples. Gene ontology (GO) analysis has been categorized at varied graph levels based on three primary ontology studies viz. cellular components, biological processes, and molecular functions. The KEGG analysis of the assembled transcripts using KAAS showed presumed components of metabolic pathways which perhaps implicated in diverse metabolic pathways responsible for salt tolerance viz. glycolysis/gluconeogenesis, oxidative phosphorylation, glutathione metabolism, etc. further involving in salt tolerance. Overall, 90 salt stress tolerant genes were identified as of 186 salt-related transcripts. Several genes have been found executing normally in the TCA cycle pathway, integral membrane proteins, generation of the osmoprotectants, enzymatic pathway associated with salt tolerance. Recognized genes fit diverse groups of salt stress genes viz. abc transporter, betaine, sodium antiporter, sodium symporter, trehalose, ectoine, and choline, that belong to different families of genes involved in the pathway of salt stress. The control sample of the bacterium showed elevated high proportion of transcript contigs (29%) while upto 20% salt stress treated sample of the bacterium showed a higher percentage of transcript contigs (31.28%). A total of 1,288 and 1,133 transcript contigs were measured entirely as novel transcript contigs in both control and treated samples, respectively. The structure and function of 10 significant salt stress tolerant genes of Staphylococcus sp. have been analyzed in this study. The information acquired in the present study possibly used to recognize and clone the salt stress tolerant genes and support in developing the salt stress-tolerant plant varieties to expand the agricultural productivity in the saline system.

An RNA thermometer dictates production of a secreted bacterial toxin

Frequent transitions of bacterial pathogens between their warm-blooded host and external reservoirs are accompanied by abrupt temperature shifts. A temperature of 37°C serves as reliable signal for ingestion by a mammalian host, which induces a major reprogramming of bacterial gene expression and metabolism. Enteric Yersiniae are Gram-negative pathogens accountable for self-limiting gastrointestinal infections. Among the temperature-regulated virulence genes of Yersinia pseudotuberculosis is cnfY coding for the cytotoxic necrotizing factor (CNF_Y), a multifunctional secreted toxin that modulates the host’s innate immune system and contributes to the decision between acute infection and persistence. We report that the major determinant of temperature-regulated cnfY expression is a thermo-labile RNA structure in the 5’-untranslated region (5’-UTR). Various translational gene fusions demonstrated that this region faithfully regulates translation initiation regardless of the transcription start site, promoter or reporter strain. RNA structure probing revealed a labile stem-loop structure, in which the ribosome binding site is partially occluded at 25°C but liberated at 37°C. Consistent with translational control in bacteria, toeprinting (primer extension inhibition) experiments in vitro showed increased ribosome binding at elevated temperature. Point mutations locking the 5’-UTR in its 25°C structure impaired opening of the stem loop, ribosome access and translation initiation at 37°C. To assess the in vivo relevance of temperature control, we used a mouse infection model. Y. pseudotuberculosis strains carrying stabilized RNA thermometer variants upstream of cnfY were avirulent and attenuated in their ability to disseminate into mesenteric lymph nodes and spleen. We conclude with a model, in which the RNA thermometer acts as translational roadblock in a two-layered regulatory cascade that tightly controls provision of the CNF_Y toxin during acute infection. Similar RNA structures upstream of various cnfY homologs suggest that RNA thermosensors dictate the production of secreted toxins in a wide range of pathogens.

Bacterial pathogens closely survey the ambient conditions and induce virulence genes only at appropriate conditions. Upon host contact, many pathogens secrete toxins in order to subvert host defense systems. We find that such a secreted toxin in enteropathogenic Yersinia pseudotuberculosis is produced only at host body temperature. This regulation depends on a temperature-responsive RNA structure, an RNA thermometer, in the 5’-untranslated region of the toxin mRNA, which prevents translation at low temperatures when the bacterium is outside the host. Preventing melting of the RNA structure at 37°C by nucleotide substitutions that stabilize base pairing resulted in avirulent Yersinia strains unable to infect mice. Given that similar RNA thermometer-like structures exist upstream of related toxin genes in various bacterial pathogens, we propose that RNA thermometer-mediated toxin production is an evolutionary conserved mechanism. Interfering with opening of such regulatory structures might thus be a promising strategy targeting a broad spectrum of bacterial pathogens.

Surface Plasmon Resonance for Investigating Molecular Interactions with RNA

Surface plasmon resonance (SPR)-based instruments have become gold-standard tools for investigating molecular interactions involving macromolecules. The major advantage is that the measured signal is sensitive to changes in mass. Therefore, all kinds of complexes can be analyzed including those with compounds as small as cations. SPR is mainly used to determine the dissociation equilibrium constant and the binding rates of a reaction if slow enough. SPR is well suited for analysis molecular interactions with nucleic acids because these negatively charged macromolecules do not have a tendency to stick to the sensor chip surface as some proteins can do. To illustrate the use of SPR with RNA molecules, we describe methods that we used for monitoring the interaction between the protein Rop from E. coli and two RNA-RNA loop-loop complexes. One is derived from the natural target of Rop, RNAI-RNAII. The other one is an RNA-RNA complex formed between a shortened version of the TAR element of HIV-1 and a structured RNA, TAR^* rationally designed to interact with TAR through loop-loop interactions. These methods can be easily adapted to other complexes involving RNA molecules and to other SPR instruments.

Probing excited conformational states of nucleic acids by nitrogen CEST NMR spectroscopy

Characterizing low-populated and short-lived excited conformational states has become increasingly important for understanding mechanisms of RNA function. Interconversion between RNA ground and excited conformational states often involves base pairing rearrangements that lead to changes in the hydrogen-bond network. Here, we present two 15N chemical exchange saturation transfer (CEST) NMR experiments that utilize protonated and non-protonated nitrogens, which are key hydrogen-bond donors and acceptors, for characterizing excited conformational states in RNA. We demonstrated these approaches on the B. Cereus fluoride riboswitch, where 15N CEST profiles complement 13C CEST profiles in depicting a potential pathway for ligand-dependent allosteric regulation of the excited conformational state of the fluoride riboswitch.

Architecture of the Escherichia coli nucleoid

How genomes are organized within cells and how the 3D architecture of a genome influences cellular functions are significant questions in biology. A bacterial genomic DNA resides inside cells in a highly condensed and functionally organized form called nucleoid (nucleus-like structure without a nuclear membrane). The Escherichia coli chromosome or nucleoid is composed of the genomic DNA, RNA, and protein. The nucleoid forms by condensation and functional arrangement of a single chromosomal DNA with the help of chromosomal architectural proteins and RNA molecules as well as DNA supercoiling. Although a high-resolution structure of a bacterial nucleoid is yet to come, five decades of research has established the following salient features of the E. coli nucleoid elaborated below: 1) The chromosomal DNA is on the average a negatively supercoiled molecule that is folded as plectonemic loops, which are confined into many independent topological domains due to supercoiling diffusion barriers; 2) The loops spatially organize into megabase size regions called macrodomains, which are defined by more frequent physical interactions among DNA sites within the same macrodomain than between different macrodomains; 3) The condensed and spatially organized DNA takes the form of a helical ellipsoid radially confined in the cell; and 4) The DNA in the chromosome appears to have a condition-dependent 3-D structure that is linked to gene expression so that the nucleoid architecture and gene transcription are tightly interdependent, influencing each other reciprocally. Current advents of high-resolution microscopy, single-molecule analysis and molecular structure determination of the components are expected to reveal the total structure and function of the bacterial nucleoid.

Biogenesis and functions of aminocarboxypropyluridine in tRNA

Transfer (t)RNAs contain a wide variety of post-transcriptional modifications, which play critical roles in tRNA stability and functions. 3-(3-amino-3-carboxypropyl)uridine (acp3U) is a highly conserved modification found in variable- and D-loops of tRNAs. Biogenesis and functions of acp3U have not been extensively investigated. Using a reverse-genetic approach supported by comparative genomics, we find here that the Escherichia coli yfiP gene, which we rename tapT (tRNA aminocarboxypropyltransferase), is responsible for acp3U formation in tRNA. Recombinant TapT synthesizes acp3U at position 47 of tRNAs in the presence of S-adenosylmethionine. Biochemical experiments reveal that acp3U47 confers thermal stability on tRNA. Curiously, the ΔtapT strain exhibits genome instability under continuous heat stress. We also find that the human homologs of tapT, DTWD1 and DTWD2, are responsible for acp3U formation at positions 20 and 20a of tRNAs, respectively. Double knockout cells of DTWD1 and DTWD2 exhibit growth retardation, indicating that acp3U is physiologically important in mammals.

Elucidation of a non-thermal mechanism for DNA/RNA fragmentation and protein degradation when using Lyse-It

Rapid sample preparation is one of the leading bottlenecks to low-cost and efficient sample component detection. To overcome this setback, a technology known as Lyse-It has been developed to rapidly (less than 60 seconds) lyse Gram-positive and-negative bacteria alike, while simultaneously fragmenting DNA/RNA and proteins into tunable sizes. This technology has been used with a variety of organisms, but the underlying mechanism behind how the technology actually works to fragment DNA/RNA and proteins has hitherto been studied. It is generally understood how temperature affects cellular lysing, but for DNA/RNA and protein degradation, the temperature and amount of energy introduced by microwave irradiation of the sample, cannot explain the degradation of the biomolecules to the extent that was being observed. Thus, an investigation into the microwave generation of reactive oxygen species, in particular singlet oxygen, hydroxyl radicals, and superoxide anion radicals, was undertaken. Herein, we probe one aspect, the generation of reactive oxygen species (ROS), which is thought to contribute to a non-thermal mechanism behind biomolecule fragmentation with the Lyse-It technology. By utilizing off/on (Photoinduced electron transfer) PET fluorescent-based probes highly specific for reactive oxygen species, it was found that as oxygen concentration in the sample and/or microwave irradiation power increases, more reactive oxygen species are generated and ultimately, more oxidation and biomolecule fragmentation occurs within the microwave cavity.

Effect of Vitamin D Supplementation on Faecal Microbiota: A Randomised Clinical Trial

In animal studies, vitamin D supplementation has been shown to improve gut microbiota and intestinal inflammation. However, limited evidence exists on the effect of vitamin D supplementation on the human gut microbiota. We examined the effect of vitamin D supplementation on faecal microbiota in 26 vitamin D-deficient (25-hydroxyvitamin D (25(OH)D) ≤50 nmol/L), overweight or obese (BMI ≥25 kg/m²) otherwise healthy adults. Our study was ancillary to a community based double-blind randomised clinical trial, conducted between 2014 and 2016. The participants provided stool samples at baseline and after 100,000 international units (IU) loading dose of cholecalciferol followed by 4000 IU daily or matching placebo for 16 weeks. Faecal microbiota was analysed using 16S rRNA sequencing; V6-8 region. There was no significant difference in microbiome α-diversity between vitamin D and placebo groups at baseline and follow-up (all p > 0.05). In addition, no clustering was found based on vitamin D supplementation at follow-up (p = 0.3). However, there was a significant association between community composition and vitamin D supplementation at the genus level (p = 0.04). The vitamin D group had a higher abundance of genus Lachnospira, and lower abundance of genus Blautia (linear discriminate analysis >3.0). Moreover, individuals with 25(OH)D >75 nmol/L had a higher abundance of genus Coprococcus and lower abundance of genus Ruminococcus compared to those with 25(OH)D <50 nmol/L. Our findings suggest that vitamin D supplementation has some distinct effects on faecal microbiota. Future studies need to explore whether these effects would translate into improved clinical outcomes.