VapC21 Toxin Contributes to Drug-Tolerance and Interacts With Non-cognate VapB32 Antitoxin in Mycobacterium tuberculosis

Genome-wide analysis of the common bean (Phaseolus vulgaris) laccase gene family and its functions in response to abiotic stress

BACKGROUND

Laccase (LAC) gene family plays a pivotal role in plant lignin biosynthesis and adaptation to various stresses. Limited research has been conducted on laccase genes in common beans.

RESULTS

29 LAC gene family members were identified within the common bean genome, distributed unevenly in 9 chromosomes. These members were divided into 6 distinct subclades by phylogenetic analysis. Further phylogenetic analyses and synteny analyses indicated that considerable gene duplication and loss presented throughout the evolution of the laccase gene family. Purified selection was shown to be the major evolutionary force through Ka / Ks. Transcriptional changes of PvLAC genes under low temperature and salt stress were observed, emphasizing the regulatory function of these genes in such conditions. Regulation by abscisic acid and gibberellins appears to be the case for PvLAC3, PvLAC4, PvLAC7, PvLAC13, PvLAC14, PvLAC18, PvLAC23, and PvLAC26, as indicated by hormone induction experiments. Additionally, the regulation of PvLAC3, PvLAC4, PvLAC7, and PvLAC14 in response to nicosulfuron and low-temperature stress were identified by virus-induced gene silence, which demonstrated inhibition on growth and development in common beans.

CONCLUSIONS

The research provides valuable genetic resources for improving the resistance of common beans to abiotic stresses and enhance the understanding of the functional roles of the LAC gene family.

Mycobacterium tuberculosis strain with deletions in menT3 and menT4 is attenuated and confers protection in mice and guinea pigs

The genome of Mycobacterium tuberculosis encodes for a large repertoire of toxin-antitoxin systems. In the present study, MenT3 and MenT4 toxins belonging to MenAT subfamily of TA systems have been functionally characterized. We demonstrate that ectopic expression of these toxins inhibits bacterial growth and this is rescued upon co-expression of their cognate antitoxins. Here, we show that simultaneous deletion of menT3 and menT4 results in enhanced susceptibility of M. tuberculosis upon exposure to oxidative stress and attenuated growth in guinea pigs and mice. We observed reduced expression of transcripts encoding for proteins that are essential or required for intracellular growth in mid-log phase cultures of ΔmenT4ΔT3 compared to parental strain. Further, the transcript levels of proteins involved in efficient bacterial clearance were increased in lung tissues of ΔmenT4ΔT3 infected mice relative to parental strain infected mice. We show that immunization of mice and guinea pigs with ΔmenT4ΔT3 confers significant protection against M. tuberculosis infection. Remarkably, immunization of mice with ΔmenT4ΔT3 results in increased antigen-specific TH1 bias and activated memory T cell response. We conclude that MenT3 and MenT4 are important for M. tuberculosis pathogenicity and strains lacking menT3 and menT4 have the potential to be explored further as vaccine candidates.

Phosphorylation of VapB antitoxins affects intermolecular interactions to regulate VapC toxin activity in Mycobacterium tuberculosis

Toxin-antitoxin modules are present in many bacterial pathogens. The VapBC family is particularly abundant in members of the Mycobacterium tuberculosis complex, with 50 modules present in the M. tuberculosis genome. In type IIA modules the VapB antitoxin protein binds to and inhibits the activity of the co-expressed cognate VapC toxin protein. VapB proteins also bind to promoter region sequences and repress expression of the vapB-vapC operon. Though VapB-VapC interactions can control the amount of free VapC toxin in the bacterial cell, the mechanisms that affect this interaction are poorly understood. Based on our recent finding of Ser/Thr phosphorylation of VapB proteins in M. tuberculosis, we substituted phosphomimetic or phosphoablative amino acids at the phosphorylation sites of two VapB proteins. We found that phosphomimetic substitution of VapB27 and VapB46 resulted in decreased interaction with their respective cognate VapC proteins, whereas phosphoablative substitution did not alter binding. Similarly, we determined that phosphomimetic substitution interfered with VapB binding to promoter region DNA sequences. Both decreased VapB-VapC interaction and decreased VapB repression of vapB-vapC operon transcription would result in increased free VapC in the M. tuberculosis cell. M. tuberculosis strains expressing vapB46-vapC46 constructs containing a phosphoablative vapB mutation resulted in lower toxicity compared to a strain expressing native vapB46, whereas similar or greater toxicity was observed in the strain expressing the phosphomimetic vapB mutation. These results identify a novel mechanism by which VapC toxicity activity can be regulated by VapB phosphorylation, potentially in response to extracytoplasmic as well as intracellular signals.

Analysis of whiB7 in Mycobacterium tuberculosis reveals novel AT-hook deletion mutations

Mutations in whiB7 have been associated with both hypersusceptibility and resistance to various antibiotics in Mycobacterium tuberculosis (Mtb). Unlocking the secrets of antibiotic resistance in the bacterium, we examined mutations in the coding sequences of whiB7 of over 40,000 diverse Mtb isolates. Our results unveil the dominant c.191delG (Gly64delG) mutation, present in all members of the lineage L1.2.2 and its impact on WhiB7's conserved GVWGG-motif, causing conformational changes and deletion of the C-terminal AT-hook. Excitingly, we discovered six unique mutations associated with partial or total deletion of the AT-hook, specific to certain sublineages. Our findings suggest the selective pressures driving these mutations, underlining the potential of genomics to advance our understanding of Mtb's antibiotic resistance. As tuberculosis remains a global health threat, our study offers valuable insights into the diverse nature and functional consequences of whiB7 mutations, paving the way for the development of novel therapeutic interventions.

Impact of Genomic Deletion RD16 on the Expression of the Mycobacterium bovis BCG Moreau VapBC47 Toxin-Antitoxin System

Mycobacterium bovis BCG is the only vaccine against tuberculosis. The variable forms of cultivation throughout the years, before seed-lots were developed, allowed in vitro evolution of the original strain, generating a family of vaccines with different phenotypic and genotypic characteristics. Molecular studies revealed regions of difference (RDs) in the genomes of the various BCG strains. This work aims to characterize the gene pair rv3407-rv3408 (vapB47-vapC47), coding for a toxin-antitoxin system of the VapBC family, and to evaluate possible transcriptional effects due to the adjacent BCG Moreau-specific genomic deletion RD16. We show that these genes are co-transcribed in BCG strains Moreau and Pasteur, and that the inactivation of an upstream transcriptional repressor (Rv3405c) due to RD16 has a polar effect, leading to increased vapBC47 expression. Furthermore, we detect VapB47 DNA binding in vitro, dependent on a 5' vapB47 sequence that contributes to a palindrome, spanning the promoter and coding region. Our data shed light on the regulation of VapBC systems and on the impact of the BCG Moreau RD16 deletion in the expression of adjacent genes, contributing to a better understanding of BCG Moreau physiology.

Phosphorylation-mediated regulation of the Bacillus anthracis phosphoglycerate mutase by the Ser/Thr protein kinase PrkC

Bacillus anthracis Ser/Thr protein kinase PrkC is necessary for phenotypic memory and spore germination, and the loss of PrkC-dependent phosphorylation events affect the spore development. During sporulation, Bacillus sp. can store 3-Phosphoglycerate (3-PGA) that will be required at the onset of germination when ATP will be necessary. The Phosphoglycerate mutase (Pgm) catalyzes the isomerization of 2-PGA and 3-PGA and is important for spore germination as a key metabolic enzyme that maintains 3-PGA pool at later events. Therefore, regulation of Pgm is important for an efficient spore germination process and metabolic switching. While the increased expression of Pgm in B. anthracis decreases spore germination efficiency, it remains unexplored if PrkC could directly influence Pgm activity. Here, we report the phosphorylation and regulation of Pgm by PrkC and its impact on Pgm stability and catalytic activity. Mass spectrometry revealed Pgm phosphorylation on seven threonine residues. In silico mutational analysis highlighted the role of Thr⁴⁵⁹ residue towards metal and substrate binding. Altogether, we demonstrated that PrkC-mediated Pgm phosphorylation negatively regulates its activity that is essential to maintain Pgm in its apo-like isoform before germination. This study advances the role of Pgm regulation that represents an important switch for B. anthracis resumption of metabolism and spore germination.

Functional characterization of toxin-antitoxin system in Mycobacterium tuberculosis

Toxin-Antitoxin (TA) system is abundant in the microbial genome, especially in bacteria and archaea. Its genetic elements and addiction modules with the role of bacterial persistence and virulence. The TA system consists of a toxin and most unstable antitoxin that could be a protein or non-encoded RNA, TA loci are chromosomally determined and their cellular functions are mostly unknown. Approximately 93 TA systems were demonstrated and more functionally available in M. tuberculosis (Mtb), the organism responsible for tuberculosis (TB). It is an airborne disease, which is causing ill-health to humans. M. tuberculosis possesses higher TA loci than other microbes and non-tubercle bacilli, the following TA types have been identified such as VapBC, MazEF, HigBA, RelBE, ParDE, DarTG, PemIK, MbcTA, and one tripartite type II TAC-Chaperone system. Toxin-antitoxin Database (TADB) brings a detailed update on Toxin-Antitoxin classification in the different pathogens such as staphylococcus aureus, streptococcus pneumonia, Vibrio cholerae, Salmonella typhimurium, Shigella flexneri, and helicobacter pylori, etc. So, this Toxin-Antitoxin system is a master regulator for bacterial growth, and an essential factor in analyzing the properties and function of disease persistence, biofilm formation, and pathogenicity. The TA system is an advanced tool to develop a new therapeutic agent against M. tuberculosis.

Structural and Functional Characterization of Rv0792c from Mycobacterium tuberculosis: Identifying Small Molecule Inhibitor against HutC Protein

In order to adapt in host tissues, microbial pathogens regulate their gene expression through a variety of transcription factors. Here, we have functionally characterized Rv0792c, a HutC homolog from Mycobacterium tuberculosis. In comparison to the parental strain, a strain of M. tuberculosis with a Rv0792c mutant was compromised for survival upon exposure to oxidative stress and infection in guinea pigs. RNA sequencing analysis revealed that Rv0792c regulates the expression of genes involved in stress adaptation and virulence of M. tuberculosis. Solution small-angle X-ray scattering (SAXS) data-steered model building confirmed that the C-terminal region plays a pivotal role in dimer formation. Systematic evolution of ligands by exponential enrichment (SELEX) resulted in the identification of single-strand DNA (ssDNA) aptamers that can be used as a tool to identify small-molecule inhibitors targeting Rv0792c. Using SELEX and SAXS data-based modeling, we identified residues essential for Rv0792c's aptamer binding activity. In this study, we also identified I-OMe-Tyrphostin as an inhibitor of Rv0792c's aptamer and DNA binding activity. The identified small molecule reduced the growth of intracellular M. tuberculosis in macrophages. The present study thus provides a detailed shape-function characterization of a HutC family of transcription factor from M. tuberculosis. IMPORTANCE Prokaryotes encode a large number of GntR family transcription factors that are involved in various fundamental biological processes, including stress adaptation and pathogenesis. Here, we investigated the structural and functional role of Rv0792c, a HutC homolog from M. tuberculosis. We demonstrated that Rv0792c is essential for M. tuberculosis to adapt to oxidative stress and establish disease in guinea pigs. Using a systematic evolution of ligands by exponential enrichment (SELEX) approach, we identified ssDNA aptamers from a random ssDNA library that bound to Rv0792c protein. These aptamers were thoroughly characterized using biochemical and biophysical assays. Using SAXS, we determined the structural model of Rv0792c in both the presence and absence of the aptamers. Further, using a combination of SELEX and SAXS methodologies, we identified I-OMe-Tyrphostin as a potential inhibitor of Rv0792c. Here we provide a detailed functional characterization of a transcription factor belonging to the HutC family from M. tuberculosis.

Neutralizing Efficacy of Encapsulin Nanoparticles against SARS-CoV2 Variants of Concern

Rapid emergence of the SARS-CoV-2 variants has dampened the protective efficacy of existing authorized vaccines. Nanoparticle platforms offer a means to improve vaccine immunogenicity by presenting multiple copies of desired antigens in a repetitive manner which closely mimics natural infection. We have applied nanoparticle display combined with the SpyTag-SpyCatcher system to design encapsulin-mRBD, a nanoparticle vaccine displaying 180 copies of the monomeric SARS-CoV-2 spike receptor-binding domain (RBD). Here we show that encapsulin-mRBD is strongly antigenic and thermotolerant for long durations. After two immunizations, squalene-in-water emulsion (SWE)-adjuvanted encapsulin-mRBD in mice induces potent and comparable neutralizing antibody titers of 105 against wild-type (B.1), alpha, beta, and delta variants of concern. Sera also neutralizes the recent Omicron with appreciable neutralization titers, and significant neutralization is observed even after a single immunization.

Ter-Seq: A high-throughput method to stabilize transient ternary complexes and measure associated kinetics

Regulation of biological processes by proteins often involves the formation of transient, multimeric complexes whose characterization is mechanistically important but challenging. The bacterial toxin CcdB binds and poisons DNA Gyrase. The corresponding antitoxin CcdA extracts CcdB from its complex with Gyrase through the formation of a transient ternary complex, thus rejuvenating Gyrase. We describe a high throughput methodology called Ter-Seq to stabilize probable ternary complexes and measure associated kinetics using the CcdA-CcdB-GyrA14 ternary complex as a model system. The method involves screening a yeast surface display (YSD) saturation mutagenesis library of one partner (CcdB) for mutants that show enhanced ternary complex formation. We also isolated CcdB mutants that were either resistant or sensitive to rejuvenation, and used surface plasmon resonance (SPR) with purified proteins to validate the kinetics measured using the surface display. Positions, where CcdB mutations lead to slower rejuvenation rates, are largely involved in CcdA-binding, though there were several notable exceptions suggesting allostery. Mutations at these positions reduce the affinity towards CcdA, thereby slowing down the rejuvenation process. Mutations at GyrA14-interacting positions significantly enhanced rejuvenation rates, either due to reduced affinity or complete loss of CcdB binding to GyrA14. We examined the effect of different parameters (CcdA affinity, GyrA14 affinity, surface accessibilities, evolutionary conservation) on the rate of rejuvenation. Finally, we further validated the Ter-Seq results by monitoring the kinetics of ternary complex formation for individual CcdB mutants in solution by fluorescence resonance energy transfer (FRET) studies.

Bacterial survivors: evaluating the mechanisms of antibiotic persistence

Bacteria withstand antibiotic onslaughts by employing a variety of strategies, one of which is persistence. Persistence occurs in a bacterial population where a subpopulation of cells (persisters) survives antibiotic treatment and can regrow in a drug-free environment. Persisters may cause the recalcitrance of infectious diseases and can be a stepping stone to antibiotic resistance, so understanding persistence mechanisms is critical for therapeutic applications. However, current understanding of persistence is pervaded by paradoxes that stymie research progress, and many aspects of this cellular state remain elusive. In this review, we summarize the putative persister mechanisms, including toxin-antitoxin modules, quorum sensing, indole signalling and epigenetics, as well as the reasons behind the inconsistent body of evidence. We highlight present limitations in the field and underscore a clinical context that is frequently neglected, in the hope of supporting future researchers in examining clinically important persister mechanisms.

Disruption of MenT2 toxin impairs the growth of Mycobacterium tuberculosis in guinea pigs

Toxin-antitoxin (TA) systems are abundantly present in the genomes of various bacterial pathogens. TA systems have been implicated in either plasmid maintenance or protection against phage infection, stress adaptation or disease pathogenesis. The genome of Mycobacterium tuberculosis encodes for more than 90 TA systems and 4 of these belong to the type IV subfamily (MenAT family). The toxins and antitoxins belonging to type IV TA systems share sequence homology with the AbiEii family of nucleotidyl transferases and the AbiEi family of putative transcriptional regulators, respectively. Here, we have performed experiments to understand the role of MenT2, a toxin from the type IV TA system, in mycobacterial physiology and disease pathogenesis. The ectopic expression of MenT2 using inducible vectors does not inhibit bacterial growth in liquid cultures. Bioinformatic and molecular modelling analysis suggested that the M. tuberculosis genome has an alternative start site upstream of the annotated menT2 gene. The overexpression of the reannotated MenT2 resulted in moderate growth inhibition of Mycobacterium smegmatis. We show that both menT2 and menA2 transcript levels are increased when M. tuberculosis is exposed to nitrosative stress, in vitro. When compared to the survival of the wild-type and the complemented strain, the ΔmenT2 mutant strain of M. tuberculosis was more resistant to being killed by nitrosative stress. However, the survival of both the ΔmenT2 mutant and the wild-type strain was similar in macrophages and when exposed to other stress conditions. Here, we show that MenT2 is required for the establishment of disease in guinea pigs. Gross pathology and histopathology analysis of lung tissues from guinea pigs infected with the ∆menT2 strain revealed significantly reduced tissue damage and inflammation. In summary, these results provide new insights into the role of MenT2 in mycobacterial pathogenesis.

Combining cysteine scanning with chemical labeling to map protein-protein interactions and infer bound structure in an intrinsically disordered region

The Mycobacterium tuberculosis genome harbours nine toxin-antitoxin (TA) systems of the mazEF family. These consist of two proteins, a toxin and an antitoxin, encoded in an operon. While the toxin has a conserved fold, the antitoxins are structurally diverse and the toxin binding region is typically intrinsically disordered before binding. We describe high throughput methodology for accurate mapping of interfacial residues and apply it to three MazEF complexes. The method involves screening one partner protein against a panel of chemically masked single cysteine mutants of its interacting partner, displayed on the surface of yeast cells. Such libraries have much lower diversity than those generated by saturation mutagenesis, simplifying library generation and data analysis. Further, because of the steric bulk of the masking reagent, labeling of virtually all exposed epitope residues should result in loss of binding, and buried residues are inaccessible to the labeling reagent. The binding residues are deciphered by probing the loss of binding to the labeled cognate partner by flow cytometry. Using this methodology, we have identified the interfacial residues for MazEF3, MazEF6 and MazEF9 TA systems of M. tuberculosis. In the case of MazEF9, where a crystal structure was available, there was excellent agreement between our predictions and the crystal structure, superior to those with AlphaFold2. We also report detailed biophysical characterization of the MazEF3 and MazEF9 TA systems and measured the relative affinities between cognate and non-cognate toxin–antitoxin partners in order to probe possible cross-talk between these systems.

A tRNA-Acetylating Toxin and Detoxifying Enzyme in Mycobacterium tuberculosis

Toxin-antitoxin (TA) systems allow bacteria to adapt to changing environments without altering gene expression. Despite being overrepresented in Mycobacterium tuberculosis, their physiological roles remain elusive. We describe a TA system in M. tuberculosis which we named TacAT due to its homology to previously discovered systems in Salmonella. The toxin, TacT, blocks growth by acetylating glycyl-tRNAs and inhibiting translation. Its effects are reversed by the enzyme peptidyl tRNA hydrolase (Pth), which also cleaves peptidyl tRNAs that are prematurely released from stalled ribosomes. Pth is essential in most bacteria and thereby has been proposed as a promising drug target for complex pathogens like M. tuberculosis. Transposon sequencing data suggest that the tacAT operon is nonessential for M. tuberculosis growth in vitro, and premature stop mutations in this TA system present in some clinical isolates suggest that it is also dispensable in vivo. We assessed whether TacT modulates pth essentiality in M. tuberculosis because drugs targeting Pth might prompt resistance if TacAT is disrupted. We show that pth essentiality is unaffected by the absence of tacAT. These results highlight a fundamental aspect of mycobacterial biology and indicate that Pth's essential role hinges on its peptidyl-tRNA hydrolase activity. Our work underscores Pth's potential as a viable target for new antibiotics. IMPORTANCE The global rise in antibiotic-resistant tuberculosis has prompted an urgent search for new drugs. Toxin-antitoxin (TA) systems allow bacteria to adapt rapidly to environmental changes, and Mycobacterium tuberculosis encodes more TA systems than any known pathogen. We have characterized a new TA system in M. tuberculosis: the toxin, TacT, acetylates charged tRNA to block protein synthesis. TacT's effects are reversed by the essential bacterial enzyme peptidyl tRNA hydrolase (Pth), which is currently being explored as an antibiotic target. Pth also cleaves peptidyl tRNAs that are prematurely released from stalled ribosomes. We assessed whether TacT modulates pth essentiality in M. tuberculosis because drugs targeting Pth might prompt resistance if TacT is disrupted. We show that pth essentiality is unaffected by the absence of this TA system, indicating that Pth's essential role hinges on its peptidyl-tRNA hydrolase activity. Our work underscores Pth's potential as a viable target for new antibiotics.

Comparative Immunogenicity of Bacterially Expressed Soluble Trimers and Nanoparticle Displayed Influenza Hemagglutinin Stem Immunogens

Current influenza vaccines need to be updated annually due to mutations in the globular head of the viral surface protein, hemagglutinin (HA). To address this, vaccine candidates have been designed based on the relatively conserved HA stem domain and have shown protective efficacy in animal models. Oligomerization of the antigens either by fusion to oligomerization motifs or display on self-assembling nanoparticle scaffolds, can induce more potent immune responses compared to the corresponding monomeric antigen due to multivalent engagement of B-cells. Since nanoparticle display can increase manufacturing complexity, and often involves one or more mammalian cell expressed components, it is important to characterize and compare various display and oligomerization scaffolds. Using a structure guided approach, we successfully displayed multiple copies of a previously designed soluble, trimeric influenza stem domain immunogen, pH1HA10, on the ferritin like protein, MsDps2 (12 copies), Ferritin (24 copies) and Encapsulin (180 copies). All proteins were expressed in Escherichia coli. The nanoparticle fusion immunogens were found to be well folded and bound to the influenza stem directed broadly neutralizing antibodies with high affinity. An 8.5 Å Cryo-EM map of Msdps2-pH1HA10 confirmed the successful design of the nanoparticle fusion immunogen. Mice immunization studies with the soluble trimeric stem and nanoparticle fusion constructs revealed that all of them were immunogenic, and protected mice against homologous (A/Belgium/145-MA/2009) and heterologous (A/Puerto Rico/8/1934) challenge with 10MLD50 mouse adapted virus. Although nanoparticle display conferred a small but statistically significant improvement in protection relative to the soluble trimer in a homologous challenge, heterologous protection was similar in both nanoparticle-stem immunized and trimeric stem immunized groups. Such rapidly producible, bacterially expressed antigens and nanoparticle scaffolds are useful modalities to tackle future influenza pandemics.

Functional and Biochemical Characterization of the MazEF6 Toxin-Antitoxin System of Mycobacterium tuberculosis

The Mycobacterium tuberculosis genome harbors nine toxin-antitoxin (TA) systems that are members of the mazEF family, unlike other prokaryotes, which have only one or two. Although the overall tertiary folds of MazF toxins are predicted to be similar, it is unclear how they recognize structurally different RNAs and antitoxins with divergent sequence specificity. Here, we have expressed and purified the individual components and complex of the MazEF6 TA system from M. tuberculosis. Size exclusion chromatography-multiangle light scattering (SEC-MALS) was performed to determine the oligomerization status of the toxin, antitoxin, and the complex in different stoichiometric ratios. The relative stabilities of the proteins were determined by nano-differential scanning fluorimetry (nano-DSF). Microscale thermophoresis (MST) and yeast surface display (YSD) were performed to measure the relative affinities between the cognate toxin-antitoxin partners. The interaction between MazEF6 complexes and cognate promoter DNA was also studied using MST. Analysis of paired-end RNA sequencing data revealed that the overexpression of MazF6 resulted in differential expression of 323 transcripts in M. tuberculosis. Network analysis was performed to identify the nodes from the top-response network. The analysis of mRNA protection ratios resulted in identification of putative MazF6 cleavage site in its native host, M. tuberculosis. IMPORTANCE M. tuberculosis harbors a large number of type II toxin-antitoxin (TA) systems, the exact roles for most of which are unclear. Prior studies have reported that overexpression of several of these type II toxins inhibits bacterial growth and contributes to the formation of drug-tolerant populations in vitro. To obtain insights into M. tuberculosis MazEF6 type II TA system function, we determined stability, oligomeric states, and binding affinities of cognate partners with each other and with their promoter operator DNA. Using RNA-seq data obtained from M. tuberculosis overexpression strains, we have identified putative MazF6 cleavage sites and targets in its native, cellular context.

HigB1 Toxin in Mycobacterium tuberculosis Is Upregulated During Stress and Required to Establish Infection in Guinea Pigs

The extraordinary expansion of Toxin Antitoxin (TA) modules in the genome of Mycobacterium tuberculosis has received significant attention over the last few decades. The cumulative evidence suggests that TA systems are activated in response to stress conditions and are essential for M. tuberculosis pathogenesis. In M. tuberculosis, Rv1955-Rv1956-Rv1957 constitutes the only tripartite TAC (Toxin Antitoxin Chaperone) module. In this locus, Rv1955 (HigB1) encodes for the toxin and Rv1956 (HigA1) encodes for antitoxin. Rv1957 encodes for a SecB-like chaperone that regulates HigBA1 toxin antitoxin system by preventing HigA1 degradation. Here, we have investigated the physiological role of HigB1 toxin in stress adaptation and pathogenesis of Mycobacterium tuberculosis. qPCR studies revealed that higBA1 is upregulated in nutrient limiting conditions and upon exposure to levofloxacin. We also show that the promoter activity of higBA1 locus in M. tuberculosis is (p)ppGpp dependent. We observed that HigB1 locus is non-essential for M. tuberculosis growth under different stress conditions in vitro. However, guinea pigs infected with higB1 deletion strain exhibited significantly reduced bacterial loads and pathological damage in comparison to the animals infected with the parental strain. Transcriptome analysis suggested that deletion of higB1 reduced the expression of genes involved in virulence, detoxification and adaptation. The present study describes the role of higB1 toxin in M. tuberculosis physiology and highlights the importance of higBA1 locus during infection in host tissues.

Rapid Identification of Secondary Structure and Binding Site Residues in an Intrinsically Disordered Protein Segment

Mycobacterium tuberculosis harbours nine toxin-antitoxin (TA) systems of the MazEF family. MazEF TA modules are of immense importance due to the perceived role of the MazF toxin in M. tuberculosis persistence and disease. The MazE antitoxin has a disordered C-terminal domain that binds the toxin, MazF and neutralizes its endoribonuclease activity. However, the structure of most MazEF TA complexes remains unsolved till date, obscuring structural and functional information about the antitoxins. We present a facile method to identify toxin binding residues on the disordered antitoxin. Charged residue scanning mutagenesis was used to screen a yeast surface displayed MazE6 antitoxin library against its purified cognate partner, the MazF6 toxin. Binding residues were deciphered by probing the relative reduction in binding to the ligand by flow cytometry. We have used this to identify putative antitoxin interface residues and local structure attained by the antitoxin upon interaction in the MazEF6 TA system and the same methodology is readily applicable to other intrinsically disordered protein regions.

Identification and characterization of VapBC toxin-antitoxin system in Bosea sp. PAMC 26642 isolated from Arctic lichens

Toxin-antitoxin (TA) systems are genetic modules composed of a toxin interfering with cellular processes and its cognate antitoxin, which counteracts the activity of the toxin. TA modules are widespread in bacterial and archaeal genomes. It has been suggested that TA modules participate in the adaptation of prokaryotes to unfavorable conditions. The Bosea sp. PAMC 26642 used in this study was isolated from the Arctic lichen Stereocaulon sp. There are 12 putative type II TA loci in the genome of Bosea sp. PAMC 26642. Of these, nine functional TA systems have been shown to be toxic in Escherichia coli The toxin inhibits growth, but this inhibition is reversed when the cognate antitoxin genes are coexpressed, indicating that these putative TA loci were bona fide TA modules. Only the BoVapC1 (AXW83_01405) toxin, a homolog of VapC, showed growth inhibition specific to low temperatures, which was recovered by the coexpression of BoVapB1 (AXW83_01400). Microscopic observation and growth monitoring revealed that the BoVapC1 toxin had bacteriostatic effects on the growth of E. coli and induced morphological changes. Quantitative real time polymerase chain reaction and northern blotting analyses showed that the BoVapC1 toxin had a ribonuclease activity on the initiator tRNA^fMet, implying that degradation of tRNA^fMet might trigger growth arrest in E. coli Furthermore, the BoVapBC1 system was found to contribute to survival against prolonged exposure at 4°C. This is the first study to identify the function of TA systems in cold adaptation.

Understanding the Reciprocal Interplay Between Antibiotics and Host Immune System: How Can We Improve the Anti-Mycobacterial Activity of Current Drugs to Better Control Tuberculosis?

Tuberculosis (TB), caused by Mycobacterium tuberculosis (Mtb) infection, remains a global health threat despite recent advances and insights into host-pathogen interactions and the identification of diverse pathways that may be novel therapeutic targets for TB treatment. In addition, the emergence and spread of multidrug-resistant Mtb strains led to a low success rate of TB treatments. Thus, novel strategies involving the host immune system that boost the effectiveness of existing antibiotics have been recently suggested to better control TB. However, the lack of comprehensive understanding of the immunomodulatory effects of anti-TB drugs, including first-line drugs and newly introduced antibiotics, on bystander and effector immune cells curtailed the development of effective therapeutic strategies to combat Mtb infection. In this review, we focus on the influence of host immune-mediated stresses, such as lysosomal activation, metabolic changes, oxidative stress, mitochondrial damage, and immune mediators, on the activities of anti-TB drugs. In addition, we discuss how anti-TB drugs facilitate the generation of Mtb populations that are resistant to host immune response or disrupt host immunity. Thus, further understanding the interplay between anti-TB drugs and host immune responses may enhance effective host antimicrobial activities and prevent Mtb tolerance to antibiotic and immune attacks. Finally, this review highlights novel adjunctive therapeutic approaches against Mtb infection for better disease outcomes, shorter treatment duration, and improved treatment efficacy based on reciprocal interactions between current TB antibiotics and host immune cells.