Genome-wide mapping of transcriptional start sites defines an extensive leaderless transcriptome in Mycobacterium tuberculosis

Mycobacterium marinum as a model for understanding principles of mycobacterial pathogenesis

Mycobacterium marinum is a fish pathogen that has become a powerful and well-established model that has accelerated our understanding of the mechanisms of mycobacterial disease. M. marinum is a versatile surrogate for understanding the closely related human pathogen M. tuberculosis, which causes tuberculosis in humans. M. marinum has defined key mechanisms of pathogenesis, both shared with M. tuberculosis and unique to this species. In this review, we discuss the discovery of M. marinum as an occasional human pathogen, the shared aspects of pathogenesis with M. tuberculosis, and how M. marinum has been exploited as a model to define the molecular mechanisms of mycobacterial pathogenesis across several phases of infection.

Structural studies of ribosome from an anaerobic Bacteroidetes human pathogen Porphyromonas gingivalis

Porphyromonas gingivalis, an anaerobic pathogen in chronic periodontitis, belongs to the Bacteroidota phylum and is associated with various virulence factors. Its antibiotic-resistant strains and its propensity to form biofilms pose a challenge to effective treatment. To explore therapeutic avenues, we studied the high-resolution cryogenic electron microscope structures of ribosomes from the wild-type P. gingivalis W83 and the macrolide-resistant mutant strain ermΔporN. The structural analysis revealed unique features primarily at the ribosome periphery. Together with the distinctive distribution of ribosomal RNA modifications, these findings offer insights into the therapeutical potential, such as creation of novel therapeutic compounds inhibiting the specific cellular functions of the P. gingivalis ribosomes. Moreover, the high-resolution structure of the ermΔporN ribosome in its complex with the approved antibiotic lefamulin suggests its repurposing against P. gingivalis. Furthermore, we provide a foundation for additional effective strategies to treat periodontitis and associated systemic diseases.

Regulation of Steady State Ribosomal Transcription in Mycobacterium tuberculosis: Intersection of Sigma Subunits, Superhelicity, and Transcription Factors

The regulation of ribosomal RNA (rRNA) is closely tied to nutrient availability, growth phase, and global gene expression, serving as a key factor in bacterial adaptability and pathogenicity. Mycobacterium tuberculosis (Mtb) stands out from other species with a single ribosomal operon controlled by two promoters: rrnAP3 and rrnAP1 and a high ratio of sigma (σ) factors to genome size. While the primary σ factor σA is known to drive ribosomal transcription, the alternative σ factor σB has been proposed to contribute to the transcription of housekeeping genes, including rRNA under a range of conditions. However, σB's precise role remains unclear. Here, we quantify steady-state rates in reconstituted transcription reactions and establish that σA-mediated transcription from rrnAP3 dominates rRNA production by almost two orders of magnitude with minimal contributions from σB holoenzymes and/or rrnAP1 under all conditions tested. We measure and compare the kinetics of individual initiation steps for both holoenzymes which, taken together with the steady-state rate measurements, lead us to a model where σB holoenzymes exhibit slower DNA unwinding and slower holoenzyme recycling. Our data further demonstrate that the transcription factors CarD and RbpA reverse or buffer the stimulatory effect of negative superhelicity on σA and σB holoenzymes respectively. Lastly, we show that a major determinant of σA's increased activity is due to its N-terminal 205 amino acids. Taken together, our data reveal the intricate interplay of promoter sequence, σ factor identity, DNA superhelicity, and transcription factors in shaping transcription initiation kinetics and, by extension, the steady-state rates of rRNA production in Mtb.

An Actively Homing Insertion Element in a Phage Methylase Contains a Hidden HNH Endonuclease

Background/Objectives: The ShiLan domain was previously identified as an insertion sequence in a phage DNA methylase gene that exhibited similar evolutionary patterns to that of an active intein or self-splicing intron but could not be identified as either. It produces no internal stop codons when read in frame with its host methylase gene, leading to the thought that it may not be an intron and rather be an abnormal type of intein. However, the sequence has no detectable self-splicing domains, which are essential for intein persistence, as preventing an intein from successfully splicing is often detrimental to proper host protein function. Methods: The analysis of alternate open reading frames for the full nucleotide sequence of this insertion element revealed the insertion to be an out-of-frame histidine-asparagine-histidine (HNH) endonuclease. A GTG start codon is located 18 bp into the insertion, and a TAA stop codon within the last four bases of the insertion (TAAC). When this frame is read, an HNH endonuclease is revealed. In-depth computational analysis could not retrieve support for this element being any known type of self-splicing element, neither intein nor intron. When read in-frame with the methylase gene, this insertion is predicted to take on a looping structure that may be able to avoid interference with the DNA methylase activity. We performed searches for sequences similar in nature to the inserted out-of-frame HNH and found several in other phages and prokaryotes. We present our survey of these out-of-frame endonuclease insertion elements as well as some speculation on how these endonucleases are getting translated to facilitate their homing activity. Conclusions: These findings expand our understanding of the possible arrangements for and prevalence of unorthodox mobile genetic elements and overlapping open reading frames in phages.

The AP2/ERF Transcription Factor ERF56 Negatively Regulating Nitrate-Dependent Plant Growth in Arabidopsis

ERF56, a member of the APETALA2/ETHYLENE-RESPONSIVE FACTOR (AP2/ERF) transcription factor (TF) family, was reported to be an early nitrate-responsive TF in Arabidopsis. But the function of ERF56 in nitrate signaling remains not entirely clear. This study aimed to investigate the role of ERF56 in nitrate-dependent plant growth and nitrate signaling. We confirmed with reverse transcription quantitative PCR (RT-qPCR) that the transcription of ERF56 is quickly induced by nitrate. ERF56 overexpressors displayed decreased nitrate-dependent plant growth, while erf56 mutants exhibited increased plant growth. Confocal imaging demonstrated that ERF56 is localized into nuclei. Assays with the glucuronidase (GUS) reporter showed that ERF56 is mainly expressed at the region of maturation of roots and in anthers. The dual-luciferase assay manifested that the transcription of ERF56 is not directly regulated by NIN-LIKE PROTEIN 7 (NLP7). The transcriptome analysis identified 1038 candidate genes regulated by ERF56 directly. A gene ontology (GO) over-representation analysis showed that ERF56 is involved in the processes of water transport, inorganic molecule transmembrane transport, secondary metabolite biosynthesis, and cell wall organization. We revealed that ERF56 represses nitrate-dependent growth through regulating the processes of inorganic molecule transmembrane transport, the secondary metabolite biosynthesis, and cell wall organization.

Insights into the solution structure and transcriptional regulation of the MazE9 antitoxin in Mycobacterium tuberculosis

The present study endeavors to decode the details of the transcriptional autoregulation effected by the MazE9 antitoxin of the Mycobacterium tuberculosis MazEF9 toxin-antitoxin system. Regulation of this bicistronic operon at the level of transcription is a critical biochemical process that is key for the organism's stress adaptation and virulence. Here, we have reported the solution structure of the DNA binding domain of MazE9 and scrutinized the thermodynamic and kinetic parameters operational in its interaction with the promoter/operator region, specific to the mazEF9 operon. A HADDOCK model of MazE9 bound to its operator DNA has been calculated based on the information on interacting residues obtained from these studies. The thermodynamics and kinetics of the interaction of MazE9 with the functionally related mazEF6 operon indicate that the potential for intracellular cross-regulation is unlikely. An interesting feature of MazE9 is the cis ⇌ trans conformational isomerization of proline residues in the intrinsically disordered C-terminal domain of this antitoxin.

Functional Analysis of Promoters, mRNA Cleavage, and mRNA Secondary Structure on esxB-esxA in Mycolicibacterium smegmatis

The ESX-1 secretion system is critical for the virulence of Mycobacterium tuberculosis as well as for conjugation in the saprophytic model Mycolicibacterium smegmatis. EsxB (CFP-10) and EsxA (ESAT-6) are secreted effectors required for the function of ESX-1 systems. While some transcription factors regulating the expression of esxB and esxA have been identified, little work has addressed their promoter structures or other determinants of their expression. Here, we defined two promoters, one located two genes upstream of esxB and one located immediately upstream, that contribute substantially to the expression of esxB and esxA. We also defined an mRNA cleavage site within the esxB 5' untranslated region (UTR) and found that a single-nucleotide substitution reprogramed the position of this cleavage event without impacting esxB-esxA transcript abundance. We furthermore investigated the impact of a double stem-loop structure in the esxB 5' UTR and found that it does not confer stability on a reporter gene transcript. Consistent with this, there was no detectable correlation between mRNA half-life and secondary structure near the 5' ends of 5' UTRs on a transcriptome-wide basis. Collectively, these data shed light on the determinants of esxB-esxA expression in M. smegmatis as well as provide broader insight into the determinants of mRNA cleavage in mycobacteria and the relationship between 5' UTR secondary structure and mRNA stability.

Widespread loss-of-function mutations implicating preexisting resistance to new or repurposed anti-tuberculosis drugs

BACKGROUND

Five New or Repurposed Drugs (NRDs) were approved in the last decade for treatment of multi-drug resistant tuberculosis: bedaquiline, clofazimine, linezolid, delamanid, and pretomanid. Unfortunately, resistance to these drugs emerged faster than anticipated, potentially due to preexisting resistance in naïve strains. Previous investigations into the rapid emergence have mostly included short variants. For the first time, we utilize de novo-assembled genomes, and systematically include Structural Variations (SV) and heterogeneity to comprehensively study this rapid emergence. We show high prevalence of preexisting resistance, identify novel markers of resistance, and lay the foundation for preventing preexisting resistance in future drug development.

METHODS

First, a systematic literature review revealed 313 NRD resistance variants in 13 genes. Next, 409 globally diverse clinical isolates collected prior to the drugs' programmatic use (308 were multidrug resistant, 106 had de novo assembled genomes) were utilized to study the 13 genes comprehensively for conventional, structural, and heterogeneous variants.

FINDINGS

We identified 5 previously reported and 67 novel putative NRD resistance variants. These variants were 2 promoter mutations (in 8/409 isolates), 13 frameshifts (21/409), 6 SVs (9/409), 35 heterogeneous frameshifts (32/409) and 11 heterogeneous SVs (12/106). Delamanid and pretomanid resistance mutations were most prevalent (48/409), while linezolid resistance mutations were least prevalent (8/409).

INTERPRETATION

Preexisting mutations implicated in resistance to at least one NRD was highly prevalent (85/409, 21 %). This was mostly caused by loss-of-function mutations in genes responsible for prodrug activation and efflux pump regulation. These preexisting mutations may have emerged through a bet-hedging strategy, or through cross-resistance with non-tuberculosis drugs such as metronidazole. Future drugs that could be resisted through loss-of-function in non-essential genes may suffer from preexisting resistance. The methods used here for comprehensive preexisting resistance assessment (especially SVs and heterogeneity) may mitigate this risk during early-stage drug development.

A Novel Deinococcus Antioxidant Peptide Mitigates Oxidative Stress in Irradiated CHO-K1 Cells

Reactive oxygen species (ROS), byproducts of cellular metabolism and environmental factors, are linked to diseases like cancer and aging. Antioxidant peptides (AOPs) have emerged as effective countermeasures against ROS-induced damage. The Deinococcus genus is well known for its extraordinary resilience to ionizing radiation (IR) and possesses complex antioxidant systems designed to neutralize ROS generated by IR. In this study, we developed four peptides, each containing 9 to 11 amino acids, from the leaderless mRNA (lmRNA) sequences of D. deserti. Lacking a 5' untranslated region, lmRNAs directly initiate protein synthesis, potentially encoding small peptides such as AOPs. Of the four peptides, Ddes-P3 was found to exhibit significant antioxidant capabilities in vitro, effectively scavenging ABTS radicals. Ddes-P3 provided considerable defense against IR-induced oxidative stress in CHO-K1 cells, demonstrating a notable reduction in ROS production and lipid peroxidation. The peptide's potential was highlighted by its ability to enhance cell survival and maintain mitochondrial membrane potential under irradiative stress, suggesting its utility as a nontoxic and effective radioprotector in mitigating radiation-induced cellular damage. This study explores the potential role of lmRNA in synthesizing AOPs within Deinococcus. Identifying lmRNAs that encode AOPs could deepen our understanding of their cellular resistance to oxidative stress and pave the way for creating innovative biotechnological and therapeutic AOPs.

Identification of a novel alternate promoter element in the pheST operon of Escherichia coli

BACKGROUND

Earlier work in this laboratory revealed that fitA was same as pheS as a recombinant clone, pSRJ5R1 harboring pheS+ gene complemented transcription-defective fitA76 Ts (temperature sensitive) mutant. However, this clone lacked the native promoter (NP) of pheST operon. A putative - 10 promoter like element was suggested to act as promoter in this clone. This work investigated the veracity of this putative promoter as well as its downstream regulatory region towards driving the pheS expression.

METHODS

Plasmid clones with promoter-mutations or -deletions were constructed by PCR-based cloning and their ability to complement fitA76 Ts mutant strains was checked. Chromosomal mutations were transferred into various genetic backgrounds via P1-transductions. Relative viability assays were performed to check the extent of complementation.

RESULTS

Clones harboring point mutations (PM-pheS) or deletion (PD1-pheS) of - 10 region of the putative promoter did not abolish complementation of the fitA76 Ts phenotype. Subsequently, a novel alternate promoter (AP) was discovered by downstream deletion clone (PD2-pheS) which failed to complement. Keeping PD1-pheS intact but mutating initiation codon of pheS (ATG→TTG) failed to complement. Complementation ability of novel alternate promoter is poor in HfrC strain background unlike native promoter which complements well independent of strain background.

CONCLUSION

A novel alternate-promoter of pheST operon was identified by mutational/deletional analyses and earlier reported putative - 10 promoter was shown to be dispensable. Alternate promoter is relA dependent.

Exploring the transcription start sites and other genomic features facilitates the accurate identification and annotation of small RNAs across multiple stress conditions in Mycobacterium tuberculosis

Mycobacterium tuberculosis (MTB) is a pathogen that is known for its ability to persist in harsh environments and cause chronic infections. Understanding the regulatory networks of MTB is crucial for developing effective treatments. Small regulatory RNAs (sRNAs) play important roles in gene expression regulation in all kingdoms of life, and their classification based solely on genomic location can be imprecise due to the computational-based prediction of protein-coding genes in bacteria, which often neglects segments of mRNA such as 5'UTRs, 3'UTRs, and intercistronic regions of operons. To address this issue, our study simultaneously discovered genomic features such as TSSs, UTRs, and operons together with sRNAs in the M. tuberculosis H37Rv strain (ATCC 27294) across multiple stress conditions. Our analysis identified 1,376 sRNA candidates and 8,173 TSSs in MTB, providing valuable insights into its complex regulatory landscape. TSS mapping enabled us to classify these sRNAs into more specific categories, including promoter-associated sRNAs, 5'UTR-derived sRNAs, 3'UTR-derived sRNAs, true intergenic sRNAs, and antisense sRNAs. Three of these sRNA candidates were experimentally validated using 3'-RACE-PCR: predictedRNA_0240, predictedRNA_0325, and predictedRNA_0578. Future characterization and validation are necessary to fully elucidate the functions and roles of these sRNAs in MTB. Our study is the first to simultaneously unravel TSSs and sRNAs in MTB and demonstrate that the identification of other genomic features, such as TSSs, UTRs, and operons, allows for more accurate and specific classification of sRNAs.

Diverse intrinsic properties shape transcript stability and stabilization in Mycolicibacterium smegmatis

Mycobacteria regulate transcript degradation to facilitate adaptation to environmental stress. However, the mechanisms underlying this regulation are unknown. Here we sought to gain understanding of the mechanisms controlling mRNA stability by investigating the transcript properties associated with variance in transcript stability and stress-induced transcript stabilization. We measured mRNA half-lives transcriptome-wide in Mycolicibacterium smegmatis in log phase growth and hypoxia-induced growth arrest. The transcriptome was globally stabilized in response to hypoxia, but transcripts of essential genes were generally stabilized more than those of non-essential genes. We then developed machine learning models that enabled us to identify the non-linear collective effect of a compendium of transcript properties on transcript stability and stabilization. We identified properties that were more predictive of half-life in log phase as well as properties that were more predictive in hypoxia, and many of these varied between leadered and leaderless transcripts. In summary, we found that transcript properties are differentially associated with transcript stability depending on both the transcript type and the growth condition. Our results reveal the complex interplay between transcript features and microenvironment that shapes transcript stability in mycobacteria.

M. tuberculosis PrrA binds the dosR promoter and regulates mycobacterial adaptation to hypoxia

The PrrAB two-component system (TCS) is essential for Mycobacterium tuberculosis viability. Previously, it was demonstrated that PrrA binds DNA in the absence of PrrB-mediated transphosphorylation and that non-cognate serine/threonine-kinases phosphorylate PrrA threonine-6 (T6). Therefore, we investigated the differential binding affinity and regulatory properties of the M. tuberculosis-derived wild-type PrrA, PrrA phosphomimetic (D58E, T6E), and PrrA phosphoablative (D58A, T6A) proteins with the prrAMtb, dosRMtb, and cydAMtb genes. While we hypothesized greater DNA binding affinity and more pronounced regulation by PrrA phosphomimetic variants, recombinant, wild-type PrrAMtb bound DNA with greatest affinity. Collectively, wild-type PrrAMtb recombinant protein displayed the highest binding affinity to the dosRMtb promoter (KD 3.46 ± 2.09 nM), followed by the prrAMtb promoter (KD 9.00 ± 2.66 nM). To establish PrrAMtb regulatory activity, we constructed M. smegmatis ΔprrABMsmeg::prrAMtb strains with each of the PrrAMtb variants and extrachromosomal prrAMtb, dosRMtb, and cydAMtb promoter-mCherry reporter fusions. Our findings showed that PrrAMtb is autoregulatory and induces dosRMtb expression only during in vitro, hypoxic growth. Combined expression of prrABMtb in M. smegmatis ΔprrAB significantly induced cydAMtb promoter-mCherry expression. Our studies advanced the understanding of PrrA function and PrrAB phosphorylation-mediated regulatory mechanisms and control of mycobacterial dosR and cydA hypoxic and low-oxygen responsive genes.

Devising Isolation Forest-Based Method to Investigate the sRNAome of Mycobacterium tuberculosis Using sRNA-seq Data

Small non-coding RNAs (sRNAs) regulate the synthesis of virulence factors and other pathogenic traits, which enables the bacteria to survive and proliferate after host infection. While high-throughput sequencing data have proved useful in identifying sRNAs from the intergenic regions (IGRs) of the genome, it remains a challenge to present a complete genome-wide map of the expression of the sRNAs. Moreover, existing methodologies necessitate multiple dependencies for executing their algorithm and also lack a targeted approach for the de novo sRNA identification. We developed an Isolation Forest algorithm-based method and the tool Prediction Of sRNAs using Isolation Forest for the de novo identification of sRNAs from available bacterial sRNA-seq data (http://posif.ibab.ac.in/). Using this framework, we predicted 1120 sRNAs and 46 small proteins in Mycobacterium tuberculosis. Besides, we highlight the context-dependent expression of novel sRNAs, their probable synthesis, and their potential relevance in stress response mechanisms manifested by M. tuberculosis.

Facile metabolic reprogramming distinguishes mycobacterial adaptation to hypoxia and starvation: ketosis drives starvation-induced persistence in M. bovis BCG

Mycobacteria adapt to infection stresses by entering a reversible non-replicating persistence (NRP) with slow or no cell growth and broad antimicrobial tolerance. Hypoxia and nutrient deprivation are two well-studied stresses commonly used to model the NRP, yet little is known about the molecular differences in mycobacterial adaptation to these distinct stresses that lead to a comparable NRP phenotype. Here we performed a multisystem interrogation of the Mycobacterium bovis BCG (BCG) starvation response, which revealed a coordinated metabolic shift away from the glycolysis of nutrient-replete growth to depletion of lipid stores, lipolysis, and fatty acid ß-oxidation in NRP. This contrasts with BCG's NRP hypoxia response involving a shift to cholesterol metabolism and triglyceride storage. Our analysis reveals cryptic metabolic vulnerabilities of the starvation-induced NRP state, such as their newfound hypersensitivity to H2O2. These observations pave the way for developing precision therapeutics against these otherwise drug refractory pathogens.

Integrated sequence and -omic features reveal novel small proteome of Mycobacterium tuberculosis

Bioinformatic studies on small proteins are under-represented due to difficulties in annotation posed by their small size. However, recent discoveries emphasize the functional significance of small proteins in cellular processes including cell signaling, metabolism, and adaptation to stress. In this study, we utilized a Random Forest classifier trained on sequence features, RNA-Seq, and Ribo-Seq data to uncover small proteins (smORFs) in M. tuberculosis. Independent predictions for the exponential and starvation conditions resulted in 695 potential smORFs. We examined the functional implications of these smORFs using homology searches, LC-MS/MS, and ChIP-seq data, testing their expression in diverse growth conditions, and identifying protein domains. We provide evidence that some of these smORFs could be part of operons, or exist as upstream ORFs. This expanded data resource for the proteins of M. tuberculosis would aid in fine-tuning the existing protein and gene regulatory networks, thereby improving system-wide studies. The primary goal of this study was to uncover and characterize smORFs in M. tuberculosis through bioinformatic analysis, shedding light on their functional roles and genomic organization. Further investigation of these potential smORFs would provide valuable insights into the genome organization and functional diversity of the M. tuberculosis proteome.

Cyclic AMP binding to a universal stress protein in Mycobacterium tuberculosis is essential for viability

Mycobacterial genomes encode multiple adenylyl cyclases and cAMP effector proteins, underscoring the diverse ways these bacteria utilize cAMP. We identified universal stress proteins, Rv1636 and MSMEG_3811 in Mycobacterium tuberculosis and Mycobacterium smegmatis, respectively, as abundantly expressed, novel cAMP-binding proteins. Rv1636 is secreted via the SecA2 secretion system in M. tuberculosis but is not directly responsible for the efflux of cAMP from the cell. In slow-growing mycobacteria, intrabacterial concentrations of Rv1636 were equivalent to the concentrations of cAMP present in the cell. In contrast, levels of intrabacterial MSMEG_3811 in M. smegmatis were lower than that of cAMP and therefore, overexpression of Rv1636 increased levels of "bound" cAMP. While msmeg_3811 could be readily deleted from the genome of M. smegmatis, we found that the rv1636 gene is essential for the viability of M. tuberculosis and is dependent on the cAMP-binding ability of Rv1636. Therefore, Rv1636 may function to regulate cAMP signaling by direct sequestration of the second messenger. This is the first evidence of a "sponge" for any second messenger in bacterial signaling that would allow mycobacterial cells to regulate the available intrabacterial "free" pool of cAMP.

MLDSPP: Bacterial Promoter Prediction Tool Using DNA Structural Properties with Machine Learning and Explainable AI

Bacterial promoters play a crucial role in gene expression by serving as docking sites for the transcription initiation machinery. However, accurately identifying promoter regions in bacterial genomes remains a challenge due to their diverse architecture and variations. In this study, we propose MLDSPP (Machine Learning and Duplex Stability based Promoter prediction in Prokaryotes), a machine learning-based promoter prediction tool, to comprehensively screen bacterial promoter regions in 12 diverse genomes. We leveraged biologically relevant and informative DNA structural properties, such as DNA duplex stability and base stacking, and state-of-the-art machine learning (ML) strategies to gain insights into promoter characteristics. We evaluated several machine learning models, including Support Vector Machines, Random Forests, and XGBoost, and assessed their performance using accuracy, precision, recall, specificity, F1 score, and MCC metrics. Our findings reveal that XGBoost outperformed other models and current state-of-the-art promoter prediction tools, namely Sigma70pred and iPromoter2L, achieving F1-scores >95% in most systems. Significantly, the use of one-hot encoding for representing nucleotide sequences complements these structural features, enhancing our XGBoost model's predictive capabilities. To address the challenge of model interpretability, we incorporated explainable AI techniques using Shapley values. This enhancement allows for a better understanding and interpretation of the predictions of our model. In conclusion, our study presents MLDSPP as a novel, generic tool for predicting promoter regions in bacteria, utilizing original downstream sequences as nonpromoter controls. This tool has the potential to significantly advance the field of bacterial genomics and contribute to our understanding of gene regulation in diverse bacterial systems.

Structural and functional characterization of AfsR, an SARP family transcriptional activator of antibiotic biosynthesis in Streptomyces

Streptomyces antibiotic regulatory proteins (SARPs) are widely distributed activators of antibiotic biosynthesis. Streptomyces coelicolor AfsR is an SARP regulator with an additional nucleotide-binding oligomerization domain (NOD) and a tetratricopeptide repeat (TPR) domain. Here, we present cryo-electron microscopy (cryo-EM) structures and in vitro assays to demonstrate how the SARP domain activates transcription and how it is modulated by NOD and TPR domains. The structures of transcription initiation complexes (TICs) show that the SARP domain forms a side-by-side dimer to simultaneously engage the afs box overlapping the -35 element and the σHrdB region 4 (R4), resembling a sigma adaptation mechanism. The SARP extensively interacts with the subunits of the RNA polymerase (RNAP) core enzyme including the β-flap tip helix (FTH), the β' zinc-binding domain (ZBD), and the highly flexible C-terminal domain of the α subunit (αCTD). Transcription assays of full-length AfsR and truncated proteins reveal the inhibitory effect of NOD and TPR on SARP transcription activation, which can be eliminated by ATP binding. In vitro phosphorylation hardly affects transcription activation of AfsR, but counteracts the disinhibition of ATP binding. Overall, our results present a detailed molecular view of how AfsR serves to activate transcription.

Incomplete transcripts dominate the Mycobacterium tuberculosis transcriptome

Mycobacterium tuberculosis (Mtb) is a bacterial pathogen that causes tuberculosis (TB), an infectious disease that is responsible for major health and economic costs worldwide1. Mtb encounters diverse environments during its life cycle and responds to these changes largely by reprogramming its transcriptional output2. However, the mechanisms of Mtb transcription and how they are regulated remain poorly understood. Here we use a sequencing method that simultaneously determines both termini of individual RNA molecules in bacterial cells3 to profile the Mtb transcriptome at high resolution. Unexpectedly, we find that most Mtb transcripts are incomplete, with their 5' ends aligned at transcription start sites and 3' ends located 200-500 nucleotides downstream. We show that these short RNAs are mainly associated with paused RNA polymerases (RNAPs) rather than being products of premature termination. We further show that the high propensity of Mtb RNAP to pause early in transcription relies on the binding of the σ-factor. Finally, we show that a translating ribosome promotes transcription elongation, revealing a potential role for transcription-translation coupling in controlling Mtb gene expression. In sum, our findings depict a mycobacterial transcriptome that prominently features incomplete transcripts resulting from RNAP pausing. We propose that the pausing phase constitutes an important transcriptional checkpoint in Mtb that allows the bacterium to adapt to environmental changes and could be exploited for TB therapeutics.

How Dedicated Ribosomes Translate a Leaderless mRNA

In bacteriophage λ lysogens, the λcI repressor is encoded by the leaderless transcript (lmRNA) initiated at the λpRM promoter. Translation is enhanced in rpsB mutants deficient in ribosomal protein uS2. Although translation initiation of lmRNA is conserved in bacteria, archaea, and eukaryotes, structural insight of a lmRNA translation initiation complex is missing. Here, we use cryo-EM to solve the structures of the uS2-deficient 70S ribosome of host E. coli mutant rpsB11 and the wild-type 70S complex with λcI lmRNA and fMet-tRNAfMet. Importantly, the uS2-deficient 70S ribosome also lacks protein bS21. The anti-Shine-Dalgarno (aSD) region is structurally supported by bS21, so that the absence of the latter causes the aSD to divert from the normal mRNA exit pathway, easing the exit of lmRNA. A π-stacking interaction between the monitor base A1493 and A(+4) of lmRNA potentially acts as a recognition signal. Coulomb charge flow, along with peristalsis-like dynamics within the mRNA entrance channel due to the increased 30S head rotation caused by the absence of uS2, are likely to facilitate the propagation of lmRNA through the ribosome. These findings lay the groundwork for future research on the mechanism of translation and the co-evolution of lmRNA and mRNA that includes the emergence of a defined ribosome-binding site of the transcript.

Quantitative analysis of transcription start site selection reveals control by DNA sequence, RNA polymerase II activity and NTP levels

Transcription start site (TSS) selection is a key step in gene expression and occurs at many promoter positions over a wide range of efficiencies. Here we develop a massively parallel reporter assay to quantitatively dissect contributions of promoter sequence, nucleoside triphosphate substrate levels and RNA polymerase II (Pol II) activity to TSS selection by 'promoter scanning' in Saccharomyces cerevisiae (Pol II MAssively Systematic Transcript End Readout, 'Pol II MASTER'). Using Pol II MASTER, we measure the efficiency of Pol II initiation at 1,000,000 individual TSS sequences in a defined promoter context. Pol II MASTER confirms proposed critical qualities of S. cerevisiae TSS -8, -1 and +1 positions, quantitatively, in a controlled promoter context. Pol II MASTER extends quantitative analysis to surrounding sequences and determines that they tune initiation over a wide range of efficiencies. These results enabled the development of a predictive model for initiation efficiency based on sequence. We show that genetic perturbation of Pol II catalytic activity alters initiation efficiency mostly independently of TSS sequence, but selectively modulates preference for the initiating nucleotide. Intriguingly, we find that Pol II initiation efficiency is directly sensitive to guanosine-5'-triphosphate levels at the first five transcript positions and to cytosine-5'-triphosphate and uridine-5'-triphosphate levels at the second position genome wide. These results suggest individual nucleoside triphosphate levels can have transcript-specific effects on initiation, representing a cryptic layer of potential regulation at the level of Pol II biochemical properties. The results establish Pol II MASTER as a method for quantitative dissection of transcription initiation in eukaryotes.

Whole genome sequencing reveals candidate genes involving in PAS resistance in M. Tuberculosis isolated from patients in Thailand

Para-amino salicylic acid (PAS) was first reported by Lehmann in 1946 and used for tuberculosis treatment. However, due to its adverse effects, it is now used only as a second line anti-tuberculosis drug for treatment of multidrug resistant or extensively drug resistant M. tuberculosis. The structure of PAS is similar to para-amino benzoic acid (pABA), an intermediate metabolite in the folate synthesis pathway. The study has identified mutations in genes in folate pathway and their intergenic regions for their possibilities in responsible for PAS resistance. Genomic DNA from 120 PAS-resistant and 49 PAS-sensitive M. tuberculosis isolated from tuberculosis patients in Thailand were studied by whole genome sequencing. Twelve genes in the folate synthesis pathway were investigated for variants associated with PAS resistance. Fifty-one SNVs were found in nine genes and their intergenic regions (pabC, pabB, folC, ribD, thyX, dfrA, thyA, folK, folP). Functional correlation test confirmed mutations in RibD, ThyX, and ThyA are responsible for PAS resistance. Detection of mutation in thyA, folC, intergenic regions of thyX, ribD, and double deletion of thyA dfrA are proposed for determination of PAS resistant M. tuberculosis.

Mycobacterial RNase E cleaves with a distinct sequence preference and controls the degradation rates of most Mycolicibacterium smegmatis mRNAs

The mechanisms and regulation of RNA degradation in mycobacteria have been subject to increased interest following the identification of interplay between RNA metabolism and drug resistance. Mycobacteria encode multiple ribonucleases predicted to participate in mRNA degradation and/or processing of stable RNAs. RNase E is hypothesized to play a major role in mRNA degradation because of its essentiality in mycobacteria and its role in mRNA degradation in gram-negative bacteria. Here, we defined the impact of RNase E on mRNA degradation rates transcriptome-wide in the nonpathogenic model Mycolicibacterium smegmatis. RNase E played a rate-limiting role in degradation of the transcripts encoded by at least 89% of protein-coding genes, with leadered transcripts often being more affected by RNase E repression than leaderless transcripts. There was an apparent global slowing of transcription in response to knockdown of RNase E, suggesting that M. smegmatis regulates transcription in responses to changes in mRNA degradation. This compensation was incomplete, as the abundance of most transcripts increased upon RNase E knockdown. We assessed the sequence preferences for cleavage by RNase E transcriptome-wide in M. smegmatis and Mycobacterium tuberculosis and found a consistent bias for cleavage in C-rich regions. Purified RNase E had a clear preference for cleavage immediately upstream of cytidines, distinct from the sequence preferences of RNase E in gram-negative bacteria. We furthermore report a high-resolution map of mRNA cleavage sites in M. tuberculosis, which occur primarily within the RNase E-preferred sequence context, confirming that RNase E has a broad impact on the M. tuberculosis transcriptome.

Gene recoding by synonymous mutations creates promiscuous intragenic transcription initiation in mycobacteria

IMPORTANCE

Mycobacterium tuberculosis (Mtb) is the causative agent of tuberculosis, one of the deadliest infectious diseases worldwide. Previous studies have established that synonymous recoding to introduce rare codon pairings can attenuate viral pathogens. We hypothesized that non-optimal codon pairing could be an effective strategy for attenuating gene expression to create a live vaccine for Mtb. We instead discovered that these synonymous changes enabled the transcription of functional mRNA that initiated in the middle of the open reading frame and from which many smaller protein products were expressed. To our knowledge, this is one of the first reports that synonymous recoding of a gene in any organism can create or induce intragenic transcription start sites.

Structural insight into translation initiation of the λcl leaderless mRNA

In bacteriophage λ lysogens, the λcI repressor is encoded by the leaderless transcript (lmRNA) initiated at the λpRM promoter. Translation is enhanced in rpsB mutants deficient in ribosomal protein uS2. Although translation initiation of lmRNA is conserved in bacteria, archaea, and eukaryotes, structural insight of a lmRNA translation initiation complex is missing. Here, we use cryo-EM to solve the structures of the uS2-deficient 70S ribosome of host E. coli mutant rpsB11 and the wild-type 70S complex with λcI lmRNA and fmet-tRNAfMet. Importantly, the uS2-deficient 70S ribosome also lacks protein bS21. The anti-Shine-Dalgarno (aSD) region is structurally supported by bS21, so that the absence of the latter causes the aSD to divert from the normal mRNA exit pathway, easing the exit of lmRNA. A π-stacking interaction between the monitor base A1493 and A(+4) of lmRNA potentially acts as a recognition signal. Coulomb charge flow, along with peristalsis-like dynamics within the mRNA entry channel due to the increased 30S head rotation caused by the absence of uS2, are likely to facilitate the propagation of lmRNA through the ribosome. These findings lay the groundwork for future research on the mechanism of translation and the co-evolution of lmRNA and mRNA that includes the emergence of a defined ribosome-binding site of the transcript.

Intricate link between siderophore secretion and drug efflux in Mycobacterium tuberculosis

Drug-resistant Mycobacterium tuberculosis is a worldwide health-care problem rendering current tuberculosis (TB) drugs ineffective. Drug efflux is an important mechanism in bacterial drug resistance. The MmpL4 and MmpL5 transporters form functionally redundant complexes with their associated MmpS4 and MmpS5 proteins and constitute the inner membrane components of an essential siderophore secretion system of M. tuberculosis. Inactivating siderophore secretion is toxic for M. tuberculosis due to self-poisoning at low-iron conditions and leads to a strong virulence defect in mice. In this study, we show that M. tuberculosis mutants lacking components of the MmpS4-MmpL4 and MmpS5-MmpL5 systems are more susceptible to bedaquiline, clofazimine, and rifabutin, important drugs for treatment of drug-resistant TB. While genetic deletion experiments revealed similar functions of the MmpL4 and MmpL5 transporters in siderophore and drug secretion, complementation experiments indicated that the MmpS4-MmpL4 proteins alone are not sufficient to restore drug efflux in an M. tuberculosis mutant lacking both operons, in contrast to MmpS5-MmpL5. Importantly, an M. tuberculosis mutant lacking the recently discovered periplasmic Rv0455c protein, which is also essential for siderophore secretion, is more susceptible to the same drugs. These results reveal a promising target for the development of dual-function TB drugs, which might poison M. tuberculosis by blocking siderophore secretion and synergize with other drugs by impairing drug efflux.

Telacebec Interferes with Virulence Lipid Biosynthesis Protein Expression and Sensitizes to Other Antibiotics

Tuberculosis (TB), caused by Mycobacterium tuberculosis (Mtb), remains a public health issue, particularly due to multi-drug-resistant Mtb. The bacillus is wrapped in a waxy envelope containing lipids acting as essential virulence factors, accounting for the natural antibiotic resistance of mycobacteria. Telacebec (previously known as Q203) is a promising new anti-TB agent inhibiting the cytochrome bc1 complex of a mycobacterial electron transport chain (ETC). Here, we show that the telacebec-challenged M. bovis BCG exhibited a reduced expression of proteins involved in the synthesis of phthiocerol dimycocerosates (PDIMs)/phenolic glycolipids (PGLs), lipid virulence factors associated with cell envelope impermeability. Consistently, telacebec, at concentrations lower than its MIC, downregulated the transcription of a PDIM/PGL-synthesizing operon, suggesting a metabolic vulnerability triggered by the drug. The drug was able to synergize on BCG with rifampicin or vancomycin, the latter being a drug exerting a marginal effect on PDIM-bearing bacilli. Telacebec at a concentration higher than its MIC had no detectable effect on cell wall PDIMs, as shown by TLC analysis, a finding potentially explained by the retaining of previously synthesized PDIMs due to the inhibition of growth. The study extends the potential of telacebec, demonstrating an effect on mycobacterial virulence lipids, allowing for the development of new anti-TB strategies.

In vitro activity of new combinations of β-lactam and β-lactamase inhibitors against the Mycobacterium tuberculosis complex

As meropenem-clavulanic acid is recommended for the treatment of drug-resistant tuberculosis, the repurposing of new carbapenem combinations may provide new treatment options, including oral alternatives. Therefore, we studied the in vitro activities of meropenem-vaborbactam, meropenem-clavulanic acid, and tebipenem-clavulanic acid. One hundred nine Mycobacterium tuberculosis complex (MTBC) clinical isolates were tested, of which 69 were pan-susceptible and the remaining pyrazinamide- or multidrug-resistant. Broth microdilution MICs were determined using the EUCAST reference method. Meropenem and tebipenem were tested individually and in combination with vaborbactam 8 mg/L and clavulanic-acid 2 and 4 mg/L, respectively. Whole-genome sequencing was performed to explore resistance mechanisms. Clavulanic acid lowered the modal tebipenem MIC approximately 16-fold (from 16 to 1 mg/L). The modal meropenem MIC was reduced twofold by vaborbactam compared with an approximately eightfold decrease by clavulanic acid. The only previously described high-confidence carbapenem resistance mutation, crfA T62A, was shared by a subgroup of lineage 4.3.4.1 isolates and did not correlate with elevated MICs. The presence of a β-lactamase inhibitor reduced the MTBC MICs of tebipenem and meropenem. The resulting MIC distribution was lowest for the orally available drugs tebipenem-clavulanic acid. Whether this in vitro activity translates to similar or greater clinical efficacy of tebipenem-clavulanic acid compared with the currently WHO-endorsed meropenem-clavulanic acid requires clinical studies. IMPORTANCE Repurposing of already approved antibiotics, such as β-lactams in combination with β-lactamase inhibitors, may provide new treatment alternatives for drug-resistant tuberculosis. Meropenem-clavulanic acid was more active in vitro compared to meropenem-vaborbactam. Notably, tebipenem-clavulanic acid showed even better activity, raising the potential of an all-oral treatment option. Clinical data are needed to investigate whether the better in vitro activity of tebipenem-clavulanic acid correlates with greater clinical efficacy compared with the currently WHO-endorsed meropenem-clavulanic acid.

Transcriptome architecture of the three main lineages of agrobacteria

Agrobacteria are a diverse, polyphyletic group of prokaryotes with multipartite genomes capable of transferring DNA into the genomes of host plants, making them an essential tool in plant biotechnology. Despite their utility in plant transformation, genome-wide transcriptional regulation is not well understood across the three main lineages of agrobacteria. Transcription start sites (TSSs) are a necessary component of gene expression and regulation. In this study, we used differential RNA-seq and a TSS identification algorithm optimized on manually annotated TSS, then validated with existing TSS to identify thousands of TSS with nucleotide resolution for representatives of each lineage. We extend upon the 356 TSSs previously reported in Agrobacterium fabrum C58 by identifying 1,916 TSSs. In addition, we completed genomes and phenotyping of Rhizobium rhizogenes C16/80 and Allorhizobium vitis T60/94, identifying 2,650 and 2,432 TSSs, respectively. Parameter optimization was crucial for an accurate, high-resolution view of genome and transcriptional dynamics, highlighting the importance of algorithm optimization in genome-wide TSS identification and genomics at large. The optimized algorithm reduced the number of TSSs identified internal and antisense to the coding sequence on average by 90.5% and 91.9%, respectively. Comparison of TSS conservation between orthologs of the three lineages revealed differences in cell cycle regulation of ctrA as well as divergence of transcriptional regulation of chemotaxis-related genes when grown in conditions that simulate the plant environment. These results provide a framework to elucidate the mechanistic basis and evolution of pathology across the three main lineages of agrobacteria. IMPORTANCE Transcription start sites (TSSs) are fundamental for understanding gene expression and regulation. Agrobacteria, a group of prokaryotes with the ability to transfer DNA into the genomes of host plants, are widely used in plant biotechnology. However, the genome-wide transcriptional regulation of agrobacteria is not well understood, especially in less-studied lineages. Differential RNA-seq and an optimized algorithm enabled identification of thousands of TSSs with nucleotide resolution for representatives of each lineage. The results of this study provide a framework for elucidating the mechanistic basis and evolution of pathology across the three main lineages of agrobacteria. The optimized algorithm also highlights the importance of parameter optimization in genome-wide TSS identification and genomics at large.

Structural basis of DNA binding by the WhiB-like transcription factor WhiB3 in Mycobacterium tuberculosis

Mycobacterium tuberculosis (Mtb) WhiB3 is an iron-sulfur cluster-containing transcription factor belonging to a subclass of the WhiB-Like (Wbl) family that is widely distributed in the phylum Actinobacteria. WhiB3 plays a crucial role in the survival and pathogenesis of Mtb. It binds to the conserved region 4 of the principal sigma factor (σA4) in the RNA polymerase holoenzyme to regulate gene expression like other known Wbl proteins in Mtb. However, the structural basis of how WhiB3 coordinates with σA4 to bind DNA and regulate transcription is unclear. Here we determined crystal structures of the WhiB3:σA4 complex without and with DNA at 1.5 Å and 2.45 Å, respectively, to elucidate how WhiB3 interacts with DNA to regulate gene expression. These structures reveal that the WhiB3:σA4 complex shares a molecular interface similar to other structurally characterized Wbl proteins and also possesses a subclass-specific Arg-rich DNA-binding motif. We demonstrate that this newly defined Arg-rich motif is required for WhiB3 binding to DNA in vitro and transcriptional regulation in Mycobacterium smegmatis. Together, our study provides empirical evidence of how WhiB3 regulates gene expression in Mtb by partnering with σA4 and engaging with DNA via the subclass-specific structural motif, distinct from the modes of DNA interaction by WhiB1 and WhiB7.

Transcription regulation by CarD in mycobacteria is guided by basal promoter kinetics

Bacterial pathogens like Mycobacterium tuberculosis (Mtb) employ transcription factors to adapt their physiology to the diverse environments within their host. CarD is a conserved bacterial transcription factor that is essential for viability in Mtb. Unlike classical transcription factors that recognize promoters by binding to specific DNA sequence motifs, CarD binds directly to the RNA polymerase to stabilize the open complex intermediate (RPo) during transcription initiation. We previously showed using RNA-sequencing that CarD is capable of both activating and repressing transcription in vivo. However, it is unknown how CarD achieves promoter-specific regulatory outcomes in Mtb despite binding indiscriminate of DNA sequence. We propose a model where CarD's regulatory outcome depends on the promoter's basal RPo stability and test this model using in vitro transcription from a panel of promoters with varying levels of RPo stability. We show that CarD directly activates full-length transcript production from the Mtb ribosomal RNA promoter rrnAP3 (AP3) and that the degree of transcription activation by CarD is negatively correlated with RPo stability. Using targeted mutations in the extended -10 and discriminator region of AP3, we show that CarD directly represses transcription from promoters that form relatively stable RPo. DNA supercoiling also influenced RPo stability and affected the direction of CarD regulation, indicating that the outcome of CarD activity can be regulated by factors beyond promoter sequence. Our results provide experimental evidence for how RNA polymerase-binding transcription factors like CarD can exert specific regulatory outcomes based on the kinetic properties of a promoter.

Premature termination of transcription is shaped by Rho and translated uORFS in Mycobacterium tuberculosis

Little is known about the decisions behind transcription elongation versus termination in the human pathogen Mycobacterium tuberculosis (M.TB). By applying Term-seq to M.TB we found that the majority of transcription termination is premature and associated with translated regions, i.e., within previously annotated or newly identified open reading frames. Computational predictions and Term-seq analysis, upon depletion of termination factor Rho, suggests that Rho-dependent transcription termination dominates all transcription termination sites (TTS), including those associated with regulatory 5' leaders. Moreover, our results suggest that tightly coupled translation, in the form of overlapping stop and start codons, may suppress Rho-dependent termination. This study provides detailed insights into novel M.TB cis-regulatory elements, where Rho-dependent, conditional termination of transcription and translational coupling together play major roles in gene expression control. Our findings contribute to a deeper understanding of the fundamental regulatory mechanisms that enable M.TB adaptation to the host environment offering novel potential points of intervention.

Using a whole genome co-expression network to inform the functional characterisation of predicted genomic elements from Mycobacterium tuberculosis transcriptomic data

A whole genome co-expression network was created using Mycobacterium tuberculosis transcriptomic data from publicly available RNA-sequencing experiments covering a wide variety of experimental conditions. The network includes expressed regions with no formal annotation, including putative short RNAs and untranslated regions of expressed transcripts, along with the protein-coding genes. These unannotated expressed transcripts were among the best-connected members of the module sub-networks, making up more than half of the 'hub' elements in modules that include protein-coding genes known to be part of regulatory systems involved in stress response and host adaptation. This data set provides a valuable resource for investigating the role of non-coding RNA, and conserved hypothetical proteins, in transcriptomic remodelling. Based on their connections to genes with known functional groupings and correlations with replicated host conditions, predicted expressed transcripts can be screened as suitable candidates for further experimental validation.

Mycobacterial phosphodiesterase Rv0805 is a virulence determinant and its cyclic nucleotide hydrolytic activity is required for propionate detoxification

Cyclic AMP (cAMP) signaling is essential to Mycobacterium tuberculosis (Mtb) pathogenesis. However, the roles of phosphodiesterases (PDEs) Rv0805, and the recently identified Rv1339, in cAMP homeostasis and Mtb biology are unclear. We found that Rv0805 modulates Mtb growth within mice, macrophages and on host-associated carbon sources. Mycobacterium bovis BCG grown on a combination of propionate and glycerol as carbon sources showed high levels of cAMP and had a strict requirement for Rv0805 cNMP hydrolytic activity. Supplementation with vitamin B12 or spontaneous genetic mutations in the pta-ackA operon restored the growth of BCGΔRv0805 and eliminated propionate-associated cAMP increases. Surprisingly, reduction of total cAMP levels by ectopic expression of Rv1339 restored only 20% of growth, while Rv0805 complementation fully restored growth despite a smaller effect on total cAMP levels. Deletion of an Rv0805 localization domain also reduced BCG growth in the presence of propionate and glycerol. We propose that localized Rv0805 cAMP hydrolysis modulates activity of a specialized pathway associated with propionate metabolism, while Rv1339 has a broader role in cAMP homeostasis. Future studies will address the biological roles of Rv0805 and Rv1339, including their impacts on metabolism, cAMP signaling and Mtb pathogenesis.

The small mycobacterial ribosomal protein, bS22, modulates aminoglycoside accessibility to its 16S rRNA helix-44 binding site

Treatment of tuberculosis continues to be challenging due to the widespread latent form of the disease and the emergence of antibiotic-resistant strains of the pathogen, Mycobacterium tuberculosis. Bacterial ribosomes are a common and effective target for antibiotics. Several second line anti-tuberculosis drugs, e.g. kanamycin, amikacin, and capreomycin, target ribosomal RNA to inhibit protein synthesis. However, M. tuberculosis can acquire resistance to these drugs, emphasizing the need to identify new drug targets. Previous cryo-EM structures of the M. tuberculosis and M. smegmatis ribosomes identified two novel ribosomal proteins, bS22 and bL37, in the vicinity of two crucial drug-binding sites: the mRNA-decoding center on the small (30S), and the peptidyl-transferase center on the large (50S) ribosomal subunits, respectively. The functional significance of these two small proteins is unknown. In this study, we observe that an M. smegmatis strain lacking the bs22 gene shows enhanced susceptibility to kanamycin compared to the wild-type strain. Cryo-EM structures of the ribosomes lacking bS22 in the presence and absence of kanamycin suggest a direct role of bS22 in modulating the 16S rRNA kanamycin-binding site. Our structures suggest that amino-acid residue Lys-16 of bS22 interacts directly with the phosphate backbone of helix 44 of 16S rRNA to influence the micro-configuration of the kanamycin-binding pocket. Our analysis shows that similar interactions occur between eukaryotic homologues of bS22, and their corresponding rRNAs, pointing to a common mechanism of aminoglycoside resistance in higher organisms.

Gene recoding by synonymous mutations creates promiscuous intragenic transcription initiation in mycobacteria

Each genome encodes some codons more frequently than their synonyms (codon usage bias), but codons are also arranged more frequently into specific pairs (codon pair bias). Recoding viral genomes and yeast or bacterial genes with non-optimal codon pairs has been shown to decrease gene expression. Gene expression is thus importantly regulated not only by the use of particular codons but by their proper juxtaposition. We therefore hypothesized that non-optimal codon pairing could likewise attenuate Mtb genes. We explored the role of codon pair bias by recoding Mtb genes ( rpoB, mmpL3, ndh ) and assessing their expression in the closely related and tractable model organism M. smegmatis . To our surprise, recoding caused the expression of multiple smaller protein isoforms from all three genes. We confirmed that these smaller proteins were not due to protein degradation, but instead issued from new transcription initiation sites positioned within the open reading frame. New transcripts gave rise to intragenic translation initiation sites, which in turn led to the expression of smaller proteins. We next identified the nucleotide changes associated with these new sites of transcription and translation. Our results demonstrated that apparently benign, synonymous changes can drastically alter gene expression in mycobacteria. More generally, our work expands our understanding of the codon-level parameters that control translation and transcription initiation.

IMPORTANCE

Mycobacterium tuberculosis ( Mtb ) is the causative agent of tuberculosis, one of the deadliest infectious diseases worldwide. Previous studies have established that synonymous recoding to introduce rare codon pairings can attenuate viral pathogens. We hypothesized that non-optimal codon pairing could be an effective strategy for attenuating gene expression to create a live vaccine for Mtb . We instead discovered that these synonymous changes enabled the transcription of functional mRNA that initiated in the middle of the open reading frame and from which many smaller protein products were expressed. To our knowledge, this is the first report that synonymous recoding of a gene in any organism can create or induce intragenic transcription start sites.

Transcription regulation by CarD in mycobacteria is guided by basal promoter kinetics

Bacterial pathogens like Mycobacterium tuberculosis ( Mtb ) employ transcription factors to adapt their physiology to the diverse environments within their host. CarD is a conserved bacterial transcription factor that is essential for viability in Mtb . Unlike classical transcription factors that recognize promoters by binding to specific DNA sequence motifs, CarD binds directly to the RNA polymerase (RNAP) to stabilize the open complex intermediate (RP _o ) during transcription initiation. We previously showed using RNA-sequencing that CarD is capable of both activating and repressing transcription in vivo . However, it is unknown how CarD achieves promoter specific regulatory outcomes in Mtb despite binding indiscriminate of DNA sequence. We propose a model where CarD's regulatory outcome depends on the promoter's basal RP _o stability and test this model using in vitro transcription from a panel of promoters with varying levels of RP _o stability. We show that CarD directly activates full-length transcript production from the Mtb ribosomal RNA promoter rrnA P3 (AP3) and that the degree of transcription activation by CarD is negatively correlated with RP _o stability. Using targeted mutations in the extended -10 and discriminator region of AP3, we show that CarD directly represses transcription from promoters that form relatively stable RP _o . DNA supercoiling also influenced RP _o stability and affected the direction of CarD regulation, indicating that the outcome of CarD activity can be regulated by factors beyond promoter sequence. Our results provide experimental evidence for how RNAP-binding transcription factors like CarD can exert specific regulatory outcomes based on the kinetic properties of a promoter.

Incomplete transcripts dominate the Mycobacterium tuberculosis transcriptome

Mycobacterium tuberculosis (Mtb) is a bacterial pathogen that causes tuberculosis, an infectious disease that inflicts major health and economic costs around the world 1 . Mtb encounters a diversity of environments during its lifecycle, and responds to these changing environments by reprogramming its transcriptional output 2 . However, the transcriptomic features of Mtb remain poorly characterized. In this work, we comprehensively profile the Mtb transcriptome using the SEnd-seq method that simultaneously captures the 5' and 3' ends of RNA 3 . Surprisingly, we find that the RNA coverage for most of the Mtb transcription units display a gradual drop-off within a 200-500 nucleotide window downstream of the transcription start site, yielding a massive number of incomplete transcripts with heterogeneous 3' ends. We further show that the accumulation of these short RNAs is mainly due to the intrinsically low processivity of the Mtb transcription machinery rather than trans-acting factors such as Rho. Finally, we demonstrate that transcription-translation coupling plays a critical role in generating full-length protein-coding transcripts in Mtb. In sum, our results depict a mycobacterial transcriptome that is dominated by incomplete RNA products, suggesting a distinctive set of transcriptional regulatory mechanisms that could be exploited for new therapeutics.

Molecular dynamics simulation-based trinucleotide and tetranucleotide level structural and energy characterization of the functional units of genomic DNA

Genomes of most organisms on earth are written in a universal language of life, made up of four units - adenine (A), thymine (T), guanine (G), and cytosine (C), and understanding the way they are put together has been a great challenge to date. Multiple efforts have been made to annotate this wonderfully engineered string of DNA using different methods but they lack a universal character. In this article, we have investigated the structural and energetic profiles of both prokaryotes and eukaryotes by considering two essential genomic sites, viz., the transcription start sites (TSS) and exon-intron boundaries. We have characterized these sites by mapping the structural and energy features of DNA obtained from molecular dynamics simulations, which considers all possible trinucleotide and tetranucleotide steps. For DNA, these physicochemical properties show distinct signatures at the TSS and intron-exon boundaries. Our results firmly convey the idea that DNA uses the same dialect for prokaryotes and eukaryotes and that it is worth going beyond sequence-level analyses to physicochemical space to determine the functional destiny of DNA sequences.

Discovery of a LuxR-type regulator involved in isoniazid-dependent gene regulation in Mycobacterium smegmatis

OBJECTIVE

Most non-tuberculous mycobacteria exhibit intrinsic resistance against the anti-tuberculosis drug isoniazid (INH). We previously found that a pyrazinamidase/nicotinamidase of Mycobacterium smegmatis, named PzaA, has an enzymatic activity to hydrolyze INH, which may contribute to intrinsic resistance. Furthermore, PzaA expression is strongly induced by INH under nitrogen-depleted conditions, although the precise mechanism of this phenomenon remains unclear. Here, we aimed to reveal the mechanism underlying the INH-dependent induction of PzaA using a transcriptomic approach.

METHODS

RNA sequencing was performed to identify INH-inducible genes other than pzaA. 5' rapid amplification of cDNA ends analysis was employed to identify the transcription start sites of INH-induced transcription units. The function of a LuxR-like regulator gene (MSMEI_1050) found within the gene cluster containing pzaA was confirmed by gene deletion and complementation experiments involving INH hydrolysis assay and quantitative reverse transcription PCR.

RESULTS

RNA sequencing revealed 23 genes that INH strongly induced under conditions of nitrogen depletion, 17 of which were in a gene cluster containing pzaA. This cluster comprised at least three transcription units, including a non-INH-inducible monocistronic unit containing MSMEI_1050. Deletion of this gene deprived M. smegmatis of the ability to respond to INH, and complementation restored this ability.

CONCLUSIONS

MSMEI_1050 plays a key role in INH-dependent gene regulation. The precise mechanism of action is to be determined in future studies.

Regulation of CRISPR-Associated Genes by Rv1776c (CasR) in Mycobacterium tuberculosis

The CRISPR-Cas system is an adaptive immune system for many bacteria and archaea to defend against foreign nucleic acid invasion, and this system is conserved in the genome of M. tuberculosis (Mtb). Although the CRISPR-Cas system-mediated immune defense mechanism has been revealed in Mtb, the regulation of cas gene expression is poorly understood. In this study, we identified a transcription factor, CasR (CRISPR-associated protein repressor, encoded by Rv1776c), and it could bind to the upstream DNA sequence of the CRISPR-Cas gene cluster and regulate the expression of cas genes. EMSA and ChIP assays confirmed that CasR could interact with the upstream sequence of the csm6 promoter, both in vivo and in vitro. Furthermore, DNA footprinting assay revealed that CasR recognized a 20 bp palindromic sequence motif and negatively regulated the expression of csm6. In conclusion, our research elucidates the regulatory effect of CasR on the expression of CRISPR-associated genes in mycobacteria, thus providing insight into gene expression regulation of the CRISPR-Cas system.

Exopolysaccharide Biosynthesis in Rhizobium leguminosarum bv. trifolii Requires a Complementary Function of Two Homologous Glycosyltransferases PssG and PssI

The Pss-I region of Rhizobium leguminosarum bv. trifolii TA1 comprises more than 20 genes coding for glycosyltransferases, modifying enzymes, and polymerization/export proteins, altogether determining the biosynthesis of symbiotically relevant exopolysaccharides. In this study, the role of homologous PssG and PssI glycosyltransferases in exopolysaccharide subunit synthesis were analyzed. It was shown that the glycosyltransferase-encoding genes of the Pss-I region were part of a single large transcriptional unit with potential downstream promoters activated in specific conditions. The ΔpssG and ΔpssI mutants produced significantly lower amounts of the exopolysaccharide, while the double deletion mutant ΔpssIΔpssG produced no exopolysaccharide. Complementation of double mutation with individual genes restored exopolysaccharide synthesis, but only to the level similar to that observed for the single ΔpssI or ΔpssG mutants, indicating that PssG and PssI serve complementary functions in the process. PssG and PssI interacted with each other in vivo and in vitro. Moreover, PssI displayed an expanded in vivo interaction network comprising other GTs involved in subunit assembly and polymerization/export proteins. PssG and PssI proteins were shown to interact with the inner membrane through amphipathic helices at their C-termini, and PssG also required other proteins involved in exopolysaccharide synthesis to localize in the membrane protein fraction.

Mycobacterium smegmatis: The Vanguard of Mycobacterial Research

The genus Mycobacterium contains several slow-growing human pathogens, including Mycobacterium tuberculosis, Mycobacterium leprae, and Mycobacterium avium. Mycobacterium smegmatis is a nonpathogenic and fast growing species within this genus. In 1990, a mutant of M. smegmatis, designated mc2155, that could be transformed with episomal plasmids was isolated, elevating M. smegmatis to model status as the ideal surrogate for mycobacterial research. Classical bacterial models, such as Escherichia coli, were inadequate for mycobacteria research because they have low genetic conservation, different physiology, and lack the novel envelope structure that distinguishes the Mycobacterium genus. By contrast, M. smegmatis encodes thousands of conserved mycobacterial gene orthologs and has the same cell architecture and physiology. Dissection and characterization of conserved genes, structures, and processes in genetically tractable M. smegmatis mc2155 have since provided previously unattainable insights on these same features in its slow-growing relatives. Notably, tuberculosis (TB) drugs, including the first-line drugs isoniazid and ethambutol, are active against M. smegmatis, but not against E. coli, allowing the identification of their physiological targets. Furthermore, Bedaquiline, the first new TB drug in 40 years, was discovered through an M. smegmatis screen. M. smegmatis has become a model bacterium, not only for M. tuberculosis, but for all other Mycobacterium species and related genera. With a repertoire of bioinformatic and physical resources, including the recently established Mycobacterial Systems Resource, M. smegmatis will continue to accelerate mycobacterial research and advance the field of microbiology.

Structural and mutational analysis of MazE6-operator DNA complex provide insights into autoregulation of toxin-antitoxin systems

Of the 10 paralogs of MazEF Toxin-Antitoxin system in Mycobacterium tuberculosis, MazEF6 plays an important role in multidrug tolerance, virulence, stress adaptation and Non Replicative Persistant (NRP) state establishment. The solution structures of the DNA binding domain of MazE6 and of its complex with the cognate operator DNA show that transcriptional regulation occurs by binding of MazE6 to an 18 bp operator sequence bearing the TANNNT motif (-10 region). Kinetics and thermodynamics of association, as determined by NMR and ITC, indicate that the nMazE6-DNA complex is of high affinity. Residues in N-terminal region of MazE6 that are key for its homodimerization, DNA binding specificity, and the base pairs in the operator DNA essential for the protein-DNA interaction, have been identified. It provides a basis for design of chemotherapeutic agents that will act via disruption of TA autoregulation, leading to cell death.

The dimeric MazE6 antitoxin binds to a specific sequence in its cognate operator DNA for autoregulation, and the key residues for dimerization and DNA binding are identified.

Genome-wide transcription start site mapping in the facultative intracellular pathogen Brucella melitensis by Capping-seq

Brucella melitensis (B. melitensis) is an important facultative intracellular bacterium that causes global zoonotic diseases. Continuous intracellular survival and replication are the main obstruction responsible for the accessibility of prevention and treatment of brucellosis. Bacteria respond to complex environment by regulating gene expression. Many regulatory factors function at loci where RNA polymerase initiates messenger RNA synthesis. However, limited gene annotation is a current obstacle for the research on expression regulation in bacteria. To improve annotation and explore potential functional sites, we proposed a novel genome-wide method called Capping-seq for transcription start site (TSS) mapping in B. melitensis. This technique combines capture of capped primary transcripts with Single Molecule Real-Time (SMRT) sequencing technology. We identified 2,369 TSSs at single nucleotide resolution by Capping-seq. TSSs analysis of Brucella transcripts showed a preference of purine on the TSS positions. Our results revealed that -35 and -10 elements of promoter contained consensus sequences of TTGNNN and TATNNN, respectively. The 5' ends analysis showed that 57% genes are associated with more than one TSS and 47% genes contain long leader regions, suggested potential complex regulation at the 5' ends of genes in B. melitensis. Moreover, we identified 52 leaderless genes that are mainly involved in the metabolic processes. Overall, Capping-seq technology provides a unique solution for TSS determination in prokaryotes. Our findings develop a systematic insight into the primary transcriptome characterization of B. melitensis. This study represents a critical basis for investigating gene regulation and pathogenesis of Brucella.

Ribosome profiling enhances understanding of mycobacterial translation

A recent addition to the -omics toolkit, ribosome profiling, enables researchers to gain insight into the process and regulation of translation by mapping fragments of mRNA protected from nuclease digestion by ribosome binding. In this review, we discuss how ribosome profiling applied to mycobacteria has led to discoveries about translational regulation. Using case studies, we show that the traditional view of “canonical” translation mechanisms needs expanding to encompass features of mycobacterial translation that are more widespread than previously recognized. We also discuss the limitations of the method and potential future developments that could yield further insight into the fundamental biology of this important human pathogen.

Distribution of Common and Rare Genetic Markers of Second-Line-Injectable-Drug Resistance in Mycobacterium tuberculosis Revealed by a Genome-Wide Association Study

Point mutations in the rrs gene and the eis promoter are known to confer resistance to the second-line injectable drugs (SLIDs) amikacin (AMK), capreomycin (CAP), and kanamycin (KAN). While mutations in these canonical genes confer the majority of SLID resistance, alternative mechanisms of resistance are not uncommon and threaten effective treatment decisions when using conventional molecular diagnostics. In total, 1,184 clinical Mycobacterium tuberculosis isolates from 7 countries were studied for genomic markers associated with phenotypic resistance. The markers rrs:A1401G and rrs:G1484T were associated with resistance to all three SLIDs, and three known markers in the eis promoter (eis:G-10A, eis:C-12T, and eis:C-14T) were similarly associated with kanamycin resistance (KAN-R). Among 325, 324, and 270 AMK-R, CAP-R, and KAN-R isolates, 274 (84.3%), 250 (77.2%), and 249 (92.3%) harbored canonical mutations, respectively. Thirteen isolates harbored more than one canonical mutation. Canonical mutations did not account for 103 of the phenotypically resistant isolates. A genome-wide association study identified three genes and promoters with mutations that, on aggregate, were associated with unexplained resistance to at least one SLID. Our analysis associated whiB7 5'-untranslated-region mutations with KAN resistance, supporting clinical relevance for this previously demonstrated mechanism of KAN resistance. We also provide evidence for the novel association of CAP resistance with the promoter of the Rv2680-Rv2681 operon, which encodes an exoribonuclease that may influence the binding of CAP to the ribosome. Aggregating mutations by gene can provide additional insight and therefore is recommended for identifying rare mechanisms of resistance when individual mutations carry insufficient statistical power.

Genome-Wide TSS Distribution in Three Related Clostridia with Normalized Capp-Switch Sequencing

Transcription initiation is a tightly regulated process that is crucial for many aspects of prokaryotic physiology. High-throughput transcription start site (TSS) mapping can shed light on global and local regulation of transcription initiation, which in turn may help us understand and predict microbial behavior. In this study, we used Capp-Switch sequencing to determine the TSS positions in the genomes of three model solventogenic clostridia: Clostridium acetobutylicum ATCC 824, C. beijerinckii DSM 6423, and C. beijerinckii NCIMB 8052. We first refined the approach by implementing a normalization pipeline accounting for gene expression, yielding a total of 12,114 mapped TSSs across the species. We further compared the distributions of these sites in the three strains. Results indicated similar distribution patterns at the genome scale, but also some sharp differences, such as for the butyryl-CoA synthesis operon, particularly when comparing C. acetobutylicum to the C. beijerinckii strains. Lastly, we found that promoter structure is generally poorly conserved between C. acetobutylicum and C. beijerinckii. A few conserved promoters across species are discussed, showing interesting examples of how TSS determination and comparison can improve our understanding of gene expression regulation at the transcript level. IMPORTANCE Solventogenic clostridia have been employed in industry for more than a century, initially being used in the acetone-butanol-ethanol (ABE) fermentation process for acetone and butanol production. Interest in these bacteria has recently increased in the context of green chemistry and sustainable development. However, our current understanding of their genomes and physiology limits their optimal use as industrial solvent production platforms. The gene regulatory mechanisms of solventogenesis are still only partly understood, impeding efforts to increase rates and yields. Genome-wide mapping of transcription start sites (TSSs) for three model solventogenic Clostridium strains is an important step toward understanding mechanisms of gene regulation in these industrially important bacteria.

Pervasive translation in Mycobacterium tuberculosis

Most bacterial ORFs are identified by automated prediction algorithms. However, these algorithms often fail to identify ORFs lacking canonical features such as a length of >50 codons or the presence of an upstream Shine-Dalgarno sequence. Here, we use ribosome profiling approaches to identify actively translated ORFs in Mycobacterium tuberculosis. Most of the ORFs we identify have not been previously described, indicating that the M. tuberculosis transcriptome is pervasively translated. The newly described ORFs are predominantly short, with many encoding proteins of ≤50 amino acids. Codon usage of the newly discovered ORFs suggests that most have not been subject to purifying selection, and hence are unlikely to contribute to cell fitness. Nevertheless, we identify 90 new ORFs (median length of 52 codons) that bear the hallmarks of purifying selection. Thus, our data suggest that pervasive translation of short ORFs in Mycobacterium tuberculosis serves as a rich source for the evolution of new functional proteins.