σ(E) -dependent activation of RbpA controls transcription of the furA-katG operon in response to oxidative stress in mycobacteria

Phage protein Gp11 blocks Staphylococcus aureus cell division by inhibiting peptidoglycan biosynthesis

Phages and bacteria have a long history of co-evolution. However, these dynamics of phage-host interactions are still largely unknown; identification of phage inhibitors that remodel host metabolism will provide valuable information for target development for antimicrobials. Here, we perform a comprehensive screen for early-gene products of ΦNM1 that inhibit cell growth in Staphylococcus aureus. A small membrane protein, Gp11, with inhibitory effects on S. aureus cell division was identified. A bacterial two-hybrid library containing 345 essential S. aureus genes was constructed to screen for targets of Gp11, and Gp11 was found to interact with MurG and DivIC. Defects in cell growth and division caused by Gp11 were dependent on MurG and DivIC, which was further confirmed using CRISPRi hypersensitivity assay. Gp11 interacts with MurG, the protein essential for cell wall formation, by inhibiting the production of lipid II to regulate peptidoglycan (PG) biosynthesis on the cell membrane. Gp11 also interacts with cell division protein DivIC, an essential part of the division machinery necessary for septal cell wall assembly, to disrupt the recruitment of division protein FtsW. Mutations in Gp11 result in loss of its ability to cause growth defects, whereas infection with phage in which the gp11 gene has been deleted showed a significant increase in lipid II production in S. aureus. Together, our findings reveal that a phage early-gene product interacts with essential host proteins to disrupt PG biosynthesis and block S. aureus cell division, suggesting a potential pathway for the development of therapeutic approaches to treat pathogenic bacterial infections.

IMPORTANCE

Understanding the interplay between phages and their hosts is important for the development of novel therapies against pathogenic bacteria. Although phages have been used to control methicillin-resistant Staphylococcus aureus infections, our knowledge related to the processes in the early stages of phage infection is still limited. Owing to the fact that most of the phage early proteins have been classified as hypothetical proteins with uncertain functions, we screened phage early-gene products that inhibit cell growth in S. aureus, and one protein, Gp11, selectively targets essential host genes to block the synthesis of the peptidoglycan component lipid II, ultimately leading to cell growth arrest in S. aureus. Our study provides a novel insight into the strategy by which Gp11 blocks essential host cellular metabolism to influence phage-host interaction. Importantly, dissecting the interactions between phages and host cells will contribute to the development of new and effective therapies to treat bacterial infections.

Structural basis for transcription activation by the nitrate-responsive regulator NarL

Transcription activation is a crucial step of regulation during transcription initiation and a classic check point in response to different stimuli and stress factors. The Escherichia coli NarL is a nitrate-responsive global transcription factor that controls the expression of nearly 100 genes. However, the molecular mechanism of NarL-mediated transcription activation is not well defined. Here we present a cryo-EM structure of NarL-dependent transcription activation complex (TAC) assembled on the yeaR promoter at 3.2 Å resolution. Our structure shows that the NarL dimer binds at the -43.5 site of the promoter DNA with its C-terminal domain (CTD) not only binding to the DNA but also making interactions with RNA polymerase subunit alpha CTD (αCTD). The key role of these NarL-mediated interactions in transcription activation was further confirmed by in vivo and in vitro transcription assays. Additionally, the NarL dimer binds DNA in a different plane from that observed in the structure of class II TACs. Unlike the canonical class II activation mechanism, NarL does not interact with σ4, while RNAP αCTD is bound to DNA on the opposite side of NarL. Our findings provide a structural basis for detailed mechanistic understanding of NarL-dependent transcription activation on yeaR promoter and reveal a potentially novel mechanism of transcription activation.

SigE: A master regulator of Mycobacterium tuberculosis

The Extracellular function (ECF) sigma factor SigE is one of the best characterized out of the 13 sigma factors encoded in the Mycobacterium tuberculosis chromosome. SigE is required for blocking phagosome maturation and full virulence in both mice and guinea pigs. Moreover, it is involved in the response to several environmental stresses as surface stress, oxidative stress, acidic pH, and phosphate starvation. Underscoring its importance in M. tuberculosis physiology, SigE is subjected to a very complex regulatory system: depending on the environmental conditions, its expression is regulated by three different sigma factors (SigA, SigE, and SigH) and a two-component system (MprAB). SigE is also regulated at the post-translational level by an anti-sigma factor (RseA) which is regulated by the intracellular redox potential and by proteolysis following phosphorylation from PknB upon surface stress. The set of genes under its direct control includes other regulators, as SigB, ClgR, and MprAB, and genes involved in surface remodeling and stabilization. Recently SigE has been shown to interact with PhoP to activate a subset of genes in conditions of acidic pH. The complex structure of its regulatory network has been suggested to result in a bistable switch leading to the development of heterogeneous bacterial populations. This hypothesis has been recently reinforced by the finding of its involvement in the development of persister cells able to survive to the killing activity of several drugs.

A Feedback Regulatory Loop Containing McdR and WhiB2 Controls Cell Division and DNA Repair in Mycobacteria

Cell division must be coordinated with DNA repair, which is strictly regulated in response to different drugs and environmental stresses in bacteria. However, the mechanisms by which mycobacteria orchestrate these two processes remain largely uncharacterized. Here, we report a regulatory loop between two essential mycobacterial regulators, McdR (Rv1830) and WhiB2, in coordinating the processes of cell division and DNA repair. McdR inhibits cell division-associated whiB2 expression by binding to the AATnACAnnnnTGTnATT motif in the promoter region. Furthermore, McdR overexpression simultaneously activates imuAB and dnaE2 expression to promote error-prone DNA repair, which facilitates genetic adaptation to stress conditions. Through a feedback mechanism, WhiB2 activates mcdR expression by binding to the cGACACGc motif in the promoter region. Importantly, analyses of mutations in clinical Mycobacterium tuberculosis strains indicate that disruption of this McdR-WhiB2 feedback regulatory loop influences expression of both cell growth- and DNA repair-associated genes, which further supports the contribution of McdR-WhiB2 regulatory loop in regulating mycobacterial cell growth and drug resistance. This highly conserved feedback regulatory loop provides fresh insight into the link between mycobacterial cell growth control and stress responses. IMPORTANCE Drug-resistant M. tuberculosis poses a threat to the control and prevention of tuberculosis (TB) worldwide. Thus, there is a need to identify the mechanisms enabling M. tuberculosis to adapt and grow under drug-induced stress. Rv1830 has been shown to be associated with drug resistance in M. tuberculosis, but its mechanisms have not yet been elucidated. Here, we reveal a regulatory role of Rv1830, which coordinates cell division and DNA repair in mycobacteria, and rename it McdR (mycobacterial cell division regulator). An increase in McdR levels represses the expression of cell division-associated whiB2 but activates the DNA repair-associated, error-prone enzymes ImuA/B and DnaE2, which in turn facilitates adaptation to stress responses and drug resistance. Furthermore, WhiB2 activates the transcription of mcdR to form a conserved regulatory loop. These data provide new insights into the mechanisms controlling mycobacterial cell growth and stress responses.

Structural basis of copper-efflux-regulator-dependent transcription activation

The copper efflux regulator (CueR), a representative member of mercury resistance regulator (MerR) family metalloregulators, controls expression of copper homeostasis-regulating genes in bacteria. The mechanism of transcription activation by CueR and other MerR family regulators is bending the spacer domain of promoter DNA. Here, we report the cryo-EM structures of the intact CueR-dependent transcription activation complexes. The structures show that CueR dimer bends the 19-bp promoter spacer to realign the -35 and -10 elements for recognition by σ70-RNA polymerase holoenzyme and reveal a previously unreported interaction between the DNA-binding domain (DBD) from one CueR subunit and the σ70 nonconserved region (σNCR). Functional studies have shown that the CueR-σNCR interaction plays an auxiliary role in CueR-dependent transcription, assisting the activation mechanism of bending promoter DNA by CueR dimer. Because DBDs are highly conserved in sequence and structure, this transcription-activating mechanism could be generally used by MerR family regulators.

Structural visualization of transcription activated by a multidrug-sensing MerR family regulator

Bacterial RNA polymerase (RNAP) holoenzyme initiates transcription by recognizing the conserved -35 and -10 promoter elements that are optimally separated by a 17-bp spacer. The MerR family of transcriptional regulators activate suboptimal 19-20 bp spacer promoters in response to myriad cellular signals, ranging from heavy metals to drug-like compounds. The regulation of transcription by MerR family regulators is not fully understood. Here we report one crystal structure of a multidrug-sensing MerR family regulator EcmrR and nine cryo-electron microscopy structures that capture the EcmrR-dependent transcription process from promoter opening to initial transcription to RNA elongation. These structures reveal that EcmrR is a dual ligand-binding factor that reshapes the suboptimal 19-bp spacer DNA to enable optimal promoter recognition, sustains promoter remodeling to stabilize initial transcribing complexes, and finally dissociates from the promoter to reverse DNA remodeling and facilitate the transition to elongation. Our findings yield a comprehensive model for transcription regulation by MerR family factors and provide insights into the transition from transcription initiation to elongation.

Genome-scale analyses of transcriptional start sites in Mycobacterium marinum under normoxic and hypoxic conditions

Background

Hypoxic stress plays a critical role in the persistence of Mycobacterium tuberculosis (Mtb) infection, but the mechanisms underlying this adaptive response remain ill defined.

Material and methods

In this study, using M. marinum as a surrogate, we analyzed hypoxic responses at the transcriptional level by Cappable-seq and regular RNA-seq analyses.

Results

A total of 6808 transcriptional start sites (TSSs) were identified under normoxic and hypoxic conditions. Among these TSSs, 1112 were upregulated and 1265 were downregulated in response to hypoxic stress. Using SigE-recognized consensus sequence, we identified 59 SigE-dependent promoters and all were upregulated under hypoxic stress, suggesting an important role for SigE in this process. We also compared the performance of Cappable-seq and regular RNA-seq using the same RNA samples collected from normoxic and hypoxic conditions, and confirmed that Cappable-seq is a valuable approach for global transcriptional regulation analyses.

Conclusions

Our results provide insights and information for further characterization of responses to hypoxia in mycobacteria, and prove that Cappable-seq is a valuable approach for global transcriptional studies in mycobacteria.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07572-8.

DNA mapping and kinetic modeling of the HrdB regulon in Streptomyces coelicolor

HrdB in streptomycetes is a principal sigma factor whose deletion is lethal. This is also the reason why its regulon has not been investigated so far. To overcome experimental obstacles, for investigating the HrdB regulon, we constructed a strain whose HrdB protein was tagged by an HA epitope. ChIP-seq experiment, done in 3 repeats, identified 2137 protein-coding genes organized in 337 operons, 75 small RNAs, 62 tRNAs, 6 rRNAs and 3 miscellaneous RNAs. Subsequent kinetic modeling of regulation of protein-coding genes with HrdB alone and with a complex of HrdB and a transcriptional cofactor RbpA, using gene expression time series, identified 1694 genes that were under their direct control. When using the HrdB-RbpA complex in the model, an increase of the model fidelity was found for 322 genes. Functional analysis revealed that HrdB controls the majority of gene groups essential for the primary metabolism and the vegetative growth. Particularly, almost all ribosomal protein-coding genes were found in the HrdB regulon. Analysis of promoter binding sites revealed binding motif at the -10 region and suggested the possible role of mono- or di-nucleotides upstream of the -10 element.

Roles of three FurA paralogs in the regulation of genes pertaining to peroxide defense in Mycobacterium smegmatis mc2 155

Mycobacterium smegmatis mc² 155 has three genes (MSMEG_6383, furA1; MSMEG_3460, furA2; MSMEG_6253, furA3) encoding FurA (ferric-uptake regulator A) paralogs. Three FurA paralogs in M. smegmatis are functionally redundant and negatively regulate expression of a subset of genes involved in peroxide detoxification such as ahpC, katG1 and katG2, as well as their own genes. The FurA paralogs sense H₂ O₂ via metal-catalyzed His oxidation (MCHO) in the same way as PerR. The propensity of FurA2 and FurA3 for MCHO is greater than that of FurA1. The three furA genes are transcribed into leaderless mRNAs lacking the Shine-Dalgarno (SD) sequence. FurA1 and FurA3 have the quaternary structure of homodimers like most Fur homologs, whereas FurA2 occurs as a monomer. The monomeric structure of FurA2 is determined by the C-terminal region of its dimerization domain. FurA2 monomers appear to cooperatively bind to the FurA-binding site with an inverted repeat configuration and have a broader binding specificity for the target DNA than dimeric FurA1 and FurA3. Comparative transcriptomic analysis revealed that the FurA paralogs do not regulate genes related to iron homeostasis in M. smegmatis, and that expression of SigF-regulated genes is significantly decreased in a furA triple mutant relative to the wild-type strain of M. smegmatis.

RbpA and σB association regulates polyphosphate levels to modulate mycobacterial isoniazid-tolerance

To facilitate survival under drug stresses, a small population of Mycobacterium tuberculosis can tolerate bactericidal concentrations of drugs without genetic mutations. These drug-tolerant mycobacteria can be induced by environmental stresses and contribute to recalcitrant infections. However, mechanisms underlying the development of drug-tolerant mycobacteria remain obscure. Herein, we characterized a regulatory pathway which is important for the tolerance to isoniazid (INH) in Mycobacterium smegmatis. We found that the RNA polymerase binding protein RbpA associates with the stress response sigma factor σ^B , to activate the transcription of ppk1, the gene encoding polyphosphate kinase. Subsequently, intracellular levels of inorganic polyphosphate increase to promote INH-tolerant mycobacteria. Interestingly, σ^B and ppk1 expression varied proportionately in mycobacterial populations and positively correlated with tolerance to INH in individual mycobacteria. Moreover, sigB and ppk1 transcription are both induced upon nutrient depletion, a condition that stimulates the formation of INH-tolerant mycobacteria. Over-expression of ppk1 in rbpA knockdown or sigB deleted strains successfully restored the number of INH-tolerant mycobacteria under both normal growth and nutrient starved conditions. These data suggest that RbpA and σ^B regulate ppk1 expression to control drug tolerance both during the logarithmic growth phase and under the nutrition starved conditions.

PhoPR Positively Regulates whiB3 Expression in Response to Low pH in Pathogenic Mycobacteria

During infection, Mycobacterium tuberculosis colonizes macrophages or necrotic granulomas, in which low pH is one of the major challenges. The PhoPR two-component regulatory system and the cytosolic redox sensor WhiB3 both play important roles in the response to low pH by M. tuberculosis However, whether close association exists between PhoPR and WhiB3 remains unclear. In this study, the positive regulation of whiB3 by PhoPR in mycobacteria was characterized. We observed that the expression patterns of the whiB3 gene under acidic conditions are different among mycobacterial species, suggesting that the regulation of whiB3 differs among mycobacteria. A sequence analysis of the whiB3 promoters (whiB3p) from M. tuberculosis and two closely related species, namely, M. marinum and M. smegmatis, showed that the whiB3p regions from M. tuberculosis and M. marinum contain a new type of PhoP box that is absent in the M. smegmatiswhiB3p Direct binding of PhoP to whiB3p from M. tuberculosis and M. marinum but not that from M. smegmatis was validated by in vitro protein-DNA binding assays. The direct activation of whiB3 by PhoPR under acidic conditions was further verified by reverse transcription-quantitative PCR (qRT-PCR) analysis in M. marinum Moreover, mutating the residues important for the phosphorylation pathway of PhoPR in M. marinum abolished the activation of whiB3 expression by PhoPR under acidic conditions, suggesting that low pH triggers the phosphorylation of PhoPR, which in turn activates the transcription of whiB3 Since the PhoP box was only identified in whiB3p of pathogenic mycobacteria, we suggest that the PhoPR-whiB3 regulatory pathway may have evolved to facilitate mycobacterial infection.IMPORTANCE The low pH in macrophages is an important barrier for infection by microbes. The PhoPR two-component regulatory system is required for the response to low pH and plays a role in redox homeostasis in Mycobacterium tuberculosis WhiB3, a cytosolic redox-sensing transcriptional regulator, is also involved in these processes. However, there is no direct evidence to demonstrate the regulation of WhiB3 by PhoPR. In this study, we found that PhoPR directly activates whiB3 expression in response to low pH. An atypical PhoP box in the whiB3 promoters has been identified and is only found in pathogenic mycobacteria, which suggests that the PhoPR-whiB3 regulatory pathway may facilitate mycobacterial infection. This study provides novel information for further characterization of the PhoPR regulon.

Sigma factors mediated signaling in Mycobacterium tuberculosis

Activation of signaling cascades is critical for Mycobacterium tuberculosis (Mtb) to adapt the macrophage lifestyle. Parallel to several signal systems, sigma factor systems, especially the extra-cytoplasmic function sigma factors, are crucial for Mtb signaling. Most sigma factors lack a signal sensory domain and often are activated by various proteins that perceive the environmental cues and relay the signals through variegated post-translational modifications via the activity of antisigma factor, protein kinase and related transcriptional regulators. Antisigma factors are further controlled by multiple mechanisms. SigK senses the environmental redox state directly. Phosphorylation and lysine acetylation added another dimension to the regulatory hierarchy. This review will provide insights into Mtb pathogenesis, and lay the foundation for the discovery of novel approaches for therapeutic interventions.

Regulation of Three Virulence Strategies of Mycobacterium tuberculosis: A Success Story

Tuberculosis remains one of the deadliest diseases. Emergence of drug-resistant and multidrug-resistant M. tuberculosis strains makes treating tuberculosis increasingly challenging. In order to develop novel intervention strategies, detailed understanding of the molecular mechanisms behind the success of this pathogen is required. Here, we review recent literature to provide a systems level overview of the molecular and cellular components involved in divalent metal homeostasis and their role in regulating the three main virulence strategies of M. tuberculosis: immune modulation, dormancy and phagosomal rupture. We provide a visual and modular overview of these components and their regulation. Our analysis identified a single regulatory cascade for these three virulence strategies that respond to limited availability of divalent metals in the phagosome.

Promoter mutagenesis for fine-tuning expression of essential genes in Mycobacterium tuberculosis

A range of regulated gene expression systems has been developed for mycobacteria in the last few years to facilitate the study of essential genes, validate novel drug targets and evaluate their vulnerability. Among these, the TetR/Pip-OFF repressible promoter system was successfully used in several mycobacterial species both in vitro and in vivo. In the first version of the system, the repressible promoter was P_ptr , a strong Pip-repressible promoter of Streptomyces pristinaespiralis, which might hamper effective downregulation of genes with a low basal expression level. Here, we report an enhanced system that allows more effective control of genes expressed at low level. To this end, we subjected P_ptr to targeted mutagenesis and produced 16 different promoters with different strength. Three of them, weaker than the wild-type promoter, were selected and characterized showing that they can indeed improve the performances of TetR/Pip-OFF repressible system both in vitro and in vivo increasing its stringency. Finally, we used these promoters to construct a series of bacterial biosensors with different sensitivity to DprE1 inhibitors and developed a whole-cell screening assay to identify inhibitors of this enzyme.

Characterization of a Minimal Type of Promoter Containing the -10 Element and a Guanine at the -14 or -13 Position in Mycobacteria

Three key promoter elements, i.e., -10, -35, and T_-15G_-14N, are recognized by the σ subunit of RNA polymerase. Among them, promoters with the -10 element and either -35 or T_-15G_-14N are known to initiate transcription efficiently, but recent systematic analyses have identified a large group of promoters in Mycobacterium tuberculosis that contain only a -10 consensus. How these promoters initiate transcription remains poorly understood. Here, we show that promoters containing the -10 element and an upstream G located at the -14 or -13 position can successfully initiate transcription in mycobacteria. Importantly, this new type of promoter is active in the absence of other promoter consensuses, suggesting that it is a minimal promoter type. Mutation of the upstream G in promoters decreased the efficiencies of their binding with RNA polymerase and their abilities to initiate transcription in both in vitro and in vivo analyses. A glutamic acid in σ region 3.0 is essential for recognizing G_-14 and G_-13 and is conserved in both principal and principal-like σ factors in mycobacteria, indicating that recognition of this minimal type of promoter might be a common mechanism for transcription initiation. Consistently, more than 70% of the identified promoters in M. tuberculosis contained G_-14 or G_-13 upstream of the conserved -10 element, and thousands of promoters in representative mycobacterial species have been predicted using the -10 consensus and G_-14 or G_-13 Altogether, our study presents a universal mechanism for transcription initiation from a minimal promoter in mycobacteria, which might also be applicable to other bacteria.IMPORTANCE In contrast to the detailed information for recognizing classic promoters in the model organism Escherichia coli, very little is known about how transcription is initiated in the human pathogen Mycobacterium tuberculosis In this study, we characterized a new type of promoter in mycobacteria that requires only a -10 consensus and an upstream G_-14 or G_-13 Residues important for recognizing the -10 element and the upstream G are conserved in σ^A and σ^B from mycobacterial species. According to such features, thousands of promoters in mycobacteria can be predicted using the -10 consensus and G_-14 or G_-13, which suggests that transcription from this new type of promoter might be widespread. Our findings provide insightful information for characterizing promoters in mycobacteria.