Rv1460, a SufR homologue, is a repressor of the suf operon in Mycobacterium tuberculosis

Multiple factors regulate the expression of sufCDSUB in Streptococcus mutans

Introduction

The sufCDSUB gene cluster, encoding the sole iron-sulfur (Fe-S) cluster assembly system in S. mutans, was recently shown to be up-regulated in response to oxidative stressors and Fe limitation.

Methods

In this study, luciferase reporter fusion assays, electrophoretic gel mobility shift assays (EMSA) and in vitro transcription assays (IVT) were used to dissect the cis- and trans-acting factors that regulate the expression of sufCDSUB.

Results and discussion

Results showed deletion of perR, for the only Fur-family transcriptional regulator in S. mutans, resulted in >5-fold increases in luciferase activity under the control of the sufCDSUB promoter (P<0.01), as compared to the parent strain, UA159 when the reporter strains were grown in medium with no supplemental iron. Site-directed mutagenesis of a PerR-box in the promoter region led to elevation of the reporter activity by >1.6-fold (P<0.01). In an EMSA, recombinant PerR (rPerR) was shown to bind to the cognate sufCDSUB promoter leading to mobility retardation. On the other hand, the reporter activity was increased by >84-fold (P<0.001) in response to the addition of cysteine at 4 mM to the culture medium. Deletion of cysR, for a LysR-type of transcriptional regulator, led to reduction of the reporter activity by >11.6-fold (P<0.001). Addition of recombinant CysR (rCysR) to an EMSA caused mobility shift of the sufCDSUB promoter probe, indicative of rCysR-promoter interaction, and rCysR was shown to enhance sufC transcription under the direction of sufCDSUB promoter in vitro. These results suggest that multiple factors are involved in the regulation of sufCDSUB expression in response to environmental cues, including cysteine and Fe availability, consistent with the important role of sufCDSUB in S. mutans physiology.

Mycobacterium marinum MMAR_0267-regulated copper utilization facilitates bacterial escape from phagolysosome

The host limits Mycobacterium tuberculosis (Mtb) by enriching copper in high concentrations. This research investigates how Mtb escapes copper stress. The membrane protein encoded by Mtb Rv0102, when its homolog in M. smegmatis (MSMEG_4702) was knocked out, resulted in a fourfold decrease in intracellular copper levels and enhanced tolerance to elevated extracellular copper concentrations. Similarly, knockout mutants of its homolog in M. marinum (MMAR_0267) showed increased virulence in zebrafish and higher bacterial load within macrophages. In THP-1 cells infected with MMAR_0267 deletion mutants, the intracellular survival of these mutants increased, along with reduced THP-1 cell apoptosis. Deficiency in copper down-regulated the transcriptional level of the virulence factor CFP-10 in M. marinum, suppressed cytosolic signaling via the macrophage STING pathway, leading to decreased production of IFN-β and reduced cell apoptosis. In conclusion, these findings highlight the significant impact of copper on the survival and reproduction of mycobacteria, underscoring the importance of studying mycobacterial adaptation mechanisms in copper-rich environments.

Intracellular peroxynitrite perturbs redox balance, bioenergetics, and Fe-S cluster homeostasis in Mycobacterium tuberculosis

The ability of Mycobacterium tuberculosis (Mtb) to tolerate nitric oxide (•NO) and superoxide (O2•-) produced by phagocytes contributes to its success as a human pathogen. Recombination of •NO and O2•- generates peroxynitrite (ONOO-), a potent oxidant produced inside activated macrophages causing lethality in diverse organisms. While the response of Mtb toward •NO and O2•- is well established, how Mtb responds to ONOO- remains unclear. Filling this knowledge gap is important to understand the persistence mechanisms of Mtb during infection. We synthesized a series of compounds that generate both •NO and O2•-, which should combine to produce ONOO-. From this library, we identified CJ067 that permeates Mtb to reliably enhance intracellular ONOO- levels. CJ067-exposed Mtb strains, including multidrug-resistant (MDR) and extensively drug-resistant (XDR) clinical isolates, exhibited dose-dependent, long-lasting oxidative stress and growth inhibition. In contrast, Mycobacterium smegmatis (Msm), a fast-growing, non-pathogenic mycobacterial species, maintained redox balance and growth in response to intracellular ONOO-. RNA-sequencing with Mtb revealed that CJ067 induces antioxidant machinery, sulphur metabolism, metal homeostasis, and a 4Fe-4S cluster repair pathway (suf operon). CJ067 impaired the activity of the 4Fe-4S cluster-containing TCA cycle enzyme, aconitase, and diminished bioenergetics of Mtb. Work with Mtb strains defective in SUF and IscS involved in Fe-S cluster biogenesis pathways showed that both systems cooperatively protect Mtb from intracellular ONOO- in vitro and inducible nitric oxide synthase (iNOS)-dependent growth inhibition during macrophage infection. Thus, Mtb is uniquely sensitive to intracellular ONOO- and targeting Fe-S cluster homeostasis is expected to promote iNOS-dependent host immunity against tuberculosis (TB).

Genetic dissection of the bacterial Fe-S protein biogenesis machineries

Iron‑sulfur (Fe-S) clusters are one of the most ancient and versatile inorganic cofactors present in the three domains of life. Fe-S clusters are essential cofactors for the activity of a large variety of metalloproteins that play crucial physiological roles. Fe-S protein biogenesis is a complex process that starts with the acquisition of the elements (iron and sulfur atoms) and their assembly into an Fe-S cluster that is subsequently inserted into the target proteins. The Fe-S protein biogenesis is ensured by multiproteic systems conserved across all domains of life. Here, we provide an overview on how bacterial genetics approaches have permitted to reveal and dissect the Fe-S protein biogenesis process in vivo.

Genome mining of Mycobacterium tuberculosis: targeting SufD as a novel drug candidate through in silico characterization and inhibitor screening

Tuberculosis (TB) stands as the second most fatal infectious disease globally, causing 1.3 million deaths in 2022. The resurgence of TB and the alarming rise of antibiotic resistance demand urgent call to develop novel antituberculosis drugs. Despite concerted efforts to control TB, the disease persists and spreads rapidly on a global scale. Targeting stress response pathways in Mycobacterium tuberculosis (Mtb) has become imperative to achieve complete eradication. This study employs subtractive genomics to identify and prioritize potential drug targets among the hypothetical proteins of Mtb, focusing on indispensable pathways. Amongst 177 essential hypothetical proteins, 152 were nonhomologous to human. These proteins participated in 34 pathways, and a 20-fold enrichment of SUF pathway genes led to its selection as a target pathway. Fe-S clusters are fundamental, widely distributed protein cofactors involved in vital cellular processes. The survival of Mtb in a hypoxic environment relies on the iron-sulfur (Fe-S) cluster biogenesis pathway for the repair of damaged Fe-S clusters. It also protects pathogen against drugs, ensuring controlled iron utilization and contributing to drug resistance. In Mtb, six proteins of Fe-S cluster assembly pathway are encoded by the suf operon. The present study was focused on SufD because of its role in iron acquisition and prevention of Fenton reaction. The research further delves into the in silico characterization of SufD, utilizing bioinformatics tools for sequence and structure based analysis. The protein's structural features, including the identification of conserved regions, motifs, and 3D structure prediction enhanced functional annotation. Target based virtual screening of compounds from the ChEMBL database resulted in 12 inhibitors with best binding affinities. Drug likeness and ADMET profiling of potential inhibitors identified promising compounds with favorable drug-like properties. The study also involved cloning in SUMO-pRSF-Duet1 expression vector, overexpression, and purification of recombinant SufD from E. coli BL21 (DE3) cells. Optimization of expression conditions resulted in soluble production, and subsequent purification highlighting the efficacy of the SUMO fusion system for challenging Mtb proteins in E. coli. These findings provide valuable insights into pharmacological targets for future experimental studies, holding promise for the development of targeted therapy against Mtb.

Investigating Mycobacterium tuberculosis sufR (rv1460) in vitro and ex vivo expression and immunogenicity

Iron is vital metal for Mycobacterium tuberculosis infection, survival, and persistence within its human host. The mobilization of sulphur (SUF) operon encodes the primary iron-sulphur (Fe-S) biogenesis system in M. tuberculosis and is induced during iron limitation and intracellular growth of M. tuberculosis, pointing to its importance during infection. To study sufR expression at single cell level during intracellular growth of M. tuberculosis a fluorescent reporter was generated by cloning a 123 bp sufR promoter region upstream of a promotorless mcherry gene in an integrating vector. Expression analysis and fluorescence measurements during in vitro culture revealed that the reporter was useful for measuring induction of the promoter but was unable to detect subsequent repression due to the stability of mCherry. During intracellular growth in THP-1 macrophages, increased fluorescence was observed in the strain harbouring the reporter relative to the control strain, however this induction was only observed in a small sub-set of the population. Since SufR levels are predicted to be elevated during infection we hypothesize that it is immunogenic and may induce an immune response in M. tuberculosis infected individuals. The immune response elicited by SufR for both whole blood assay (WBA, a short term 12-hr stimulation to characterise the production of cytokines/growth factors suggestive of an effector response) and lymphocyte proliferation assay (LPA, a longer term 7-day stimulation to see if SufR induces a memory type immune response) were low and did not show a strong immune response for the selected Luminex analytes (MCP-1, RANTES, IL-1b, IL-8, MIP-1b, IFN-g, IL-6 and MMP-9) measured in three clinical groups, namely active TB, QuantiFERON positive (QFN pos) and QFN negative (QFN neg) individuals.

The Mycobacterium smegmatis HesB Protein, MSMEG_4272, Is Required for In Vitro Growth and Iron Homeostasis

A-type carrier (ATC) proteins are proposed to function in the biogenesis of Fe-S clusters, although their exact role remains controversial. The genome of Mycobacterium smegmatis encodes a single ATC protein, MSMEG_4272, which belongs to the HesB/YadR/YfhF family of proteins. Attempts to generate an MSMEG_4272 deletion mutant by two-step allelic exchange were unsuccessful, suggesting that the gene is essential for in vitro growth. CRISPRi-mediated transcriptional knock-down of MSMEG_4272 resulted in a growth defect under standard culture conditions, which was exacerbated in mineral-defined media. The knockdown strain displayed reduced intracellular iron levels under iron-replete conditions and increased susceptibility to clofazimine, 2,3-dimethoxy-1,4-naphthoquinone (DMNQ), and isoniazid, while the activity of the Fe-S containing enzymes, succinate dehydrogenase, and aconitase were not affected. This study suggests that MSMEG_4272 plays a role in the regulation of intracellular iron levels and is required for in vitro growth of M. smegmatis, particularly during exponential growth.

The impact of genotype on the phenotype of Mycobacterium tuberculosis ΔsufR mutants

Iron-sulphur (FeS) cluster biogenesis is a tightly regulated process in vivo. In Mycobacterium tuberculosis (Mtb), SufR functions as a transcriptional repressor of the operon encoding the primary FeS cluster biogenesis system. Previously, three independently isolated mutants (ΔRv1460stop_1.19, ΔRv1460stop _5.19 and ΔRv1460stop _5.20) harbouring the same deletion in sufR, displayed different growth kinetics in OADC supplemented 7H9 media. To investigate this discrepancy, we performed whole genome sequencing of the 3 mutants and the wild-type progenitor. Single nucleotide polymorphisms (SNPs) were identified in 3 genes in the ΔRv1460stop_1.19 mutant and one gene in the ΔRv1460stop_5.20 mutant. Phenotyping of the ΔRv1460stop_5.19 mutant, which had no additional SNPs, revealed increased susceptibility to clofazimine, DMNQ and menadione, while uptake and survival in THP-1 cells were not significantly different from the wild-type strain. Given that these results differ from those reported for other sufR deletion mutants (ΔSufR_MTB and MtbΔSufR), they suggest that the position of the sufR deletion and the genotype of the progenitor strain impact the resulting phenotype.

Structural and Biochemical Characterization of Mycobacterium tuberculosis Zinc SufU-SufS Complex

Iron-sulfur (Fe-S) clusters are inorganic prosthetic groups in proteins composed exclusively of iron and inorganic sulfide. These cofactors are required in a wide range of critical cellular pathways. Iron-sulfur clusters do not form spontaneously in vivo; several proteins are required to mobilize sulfur and iron, assemble and traffic-nascent clusters. Bacteria have developed several Fe-S assembly systems, such as the ISC, NIF, and SUF systems. Interestingly, in Mycobacterium tuberculosis (Mtb), the causative agent of tuberculosis (TB), the SUF machinery is the primary Fe-S biogenesis system. This operon is essential for the viability of Mtb under normal growth conditions, and the genes it contains are known to be vulnerable, revealing the Mtb SUF system as an interesting target in the fight against tuberculosis. In the present study, two proteins of the Mtb SUF system were characterized for the first time: Rv1464(sufS) and Rv1465(sufU). The results presented reveal how these two proteins work together and thus provide insights into Fe-S biogenesis/metabolism by this pathogen. Combining biochemistry and structural approaches, we showed that Rv1464 is a type II cysteine-desulfurase enzyme and that Rv1465 is a zinc-dependent protein interacting with Rv1464. Endowed with a sulfurtransferase activity, Rv1465 significantly enhances the cysteine-desulfurase activity of Rv1464 by transferring the sulfur atom from persulfide on Rv1464 to its conserved Cys40 residue. The zinc ion is important for the sulfur transfer reaction between SufS and SufU, and His354 in SufS plays an essential role in this reaction. Finally, we showed that Mtb SufS-SufU is more resistant to oxidative stress than E. coli SufS-SufE and that the presence of zinc in SufU is likely responsible for this improved resistance. This study on Rv1464 and Rv1465 will help guide the design of future anti-tuberculosis agents.

A Diverged Transcriptional Network for Usage of Two Fe-S Cluster Biogenesis Machineries in the Delta-Proteobacterium Myxococcus xanthus

Myxococcus xanthus possesses two Fe-S cluster biogenesis machineries, ISC (iron-sulfur cluster) and SUF (sulfur mobilization). Here, we show that in comparison to the phylogenetically distant Enterobacteria, which also have both machineries, M. xanthus evolved an independent transcriptional scheme to coordinately regulate the expression of these machineries. This transcriptional response is directed by RisR, which we show to belong to a phylogenetically distant and biochemically distinct subgroup of the Rrf2 transcription factor family, in comparison to IscR that regulates the isc and suf operons in Enterobacteria. We report that RisR harbors an Fe-S cluster and that holo-RisR acts as a repressor of both the isc and suf operons, in contrast to Escherichia coli, where holo-IscR represses the isc operon whereas apo-IscR activates the suf operon. In addition, we establish that the nature of the cluster and the DNA binding sites of RisR, in the isc and suf operons, diverge from those of IscR. We further show that in M. xanthus, the two machineries appear to be fully interchangeable in maintaining housekeeping levels of Fe-S cluster biogenesis and in synthesizing the Fe-S cluster for their common regulator, RisR. We also demonstrate that in response to oxidative stress and iron limitation, transcriptional upregulation of the M. xanthus isc and suf operons was mediated solely by RisR and that the contribution of the SUF machinery was greater than the ISC machinery. Altogether, these findings shed light on the diversity of homeostatic mechanisms exploited by bacteria to coordinately use two Fe-S cluster biogenesis machineries. IMPORTANCE Fe-S proteins are ubiquitous and control a wide variety of key biological processes; therefore, maintaining Fe-S cluster homeostasis is an essential task for all organisms. Here, we provide the first example of how a bacterium from the Deltaproteobacteria branch coordinates expression of two Fe-S cluster biogenesis machineries. The results revealed a new model of coordination, highlighting the unique and common features that have independently emerged in phylogenetically distant bacteria to maintain Fe-S cluster homeostasis in response to environmental changes. Regulation is orchestrated by a previously uncharacterized transcriptional regulator, RisR, belonging to the Rrf2 superfamily, whose members are known to sense diverse environmental stresses frequently encountered by bacteria. Understanding how M. xanthus maintains Fe-S cluster homeostasis via RisR regulation revealed a strategy reflective of the aerobic lifestyle of this organsim. This new knowledge also paves the way to improve production of Fe-S-dependent secondary metabolites using M. xanthus as a chassis.

Iron–Sulfur Clusters toward Stresses: Implication for Understanding and Fighting Tuberculosis

Tuberculosis (TB) remains the leading cause of death due to a single pathogen, accounting for 1.5 million deaths annually on the global level. Mycobacterium tuberculosis, the causative agent of TB, is persistently exposed to stresses such as reactive oxygen species (ROS), reactive nitrogen species (RNS), acidic conditions, starvation, and hypoxic conditions, all contributing toward inhibiting bacterial proliferation and survival. Iron–sulfur (Fe-S) clusters, which are among the most ancient protein prosthetic groups, are good targets for ROS and RNS, and are susceptible to Fe starvation. Mtb holds Fe-S containing proteins involved in essential biological process for Mtb. Fe-S cluster assembly is achieved via complex protein machineries. Many organisms contain several Fe-S assembly systems, while the SUF system is the only one in some pathogens such as Mtb. The essentiality of the SUF machinery and its functionality under the stress conditions encountered by Mtb underlines how it constitutes an attractive target for the development of novel anti-TB.

Mycobacterium tuberculosis requires SufT for Fe-S cluster maturation, metabolism, and survival in vivo

Iron-sulfur (Fe-S) cluster proteins carry out essential cellular functions in diverse organisms, including the human pathogen Mycobacterium tuberculosis (Mtb). The mechanisms underlying Fe-S cluster biogenesis are poorly defined in Mtb. Here, we show that Mtb SufT (Rv1466), a DUF59 domain-containing essential protein, is required for the Fe-S cluster maturation. Mtb SufT homodimerizes and interacts with Fe-S cluster biogenesis proteins; SufS and SufU. SufT also interacts with the 4Fe-4S cluster containing proteins; aconitase and SufR. Importantly, a hyperactive cysteine in the DUF59 domain mediates interaction of SufT with SufS, SufU, aconitase, and SufR. We efficiently repressed the expression of SufT to generate a SufT knock-down strain in Mtb (SufT-KD) using CRISPR interference. Depleting SufT reduces aconitase's enzymatic activity under standard growth conditions and in response to oxidative stress and iron limitation. The SufT-KD strain exhibited defective growth and an altered pool of tricarboxylic acid cycle intermediates, amino acids, and sulfur metabolites. Using Seahorse Extracellular Flux analyzer, we demonstrated that SufT depletion diminishes glycolytic rate and oxidative phosphorylation in Mtb. The SufT-KD strain showed defective survival upon exposure to oxidative stress and nitric oxide. Lastly, SufT depletion reduced the survival of Mtb in macrophages and attenuated the ability of Mtb to persist in mice. Altogether, SufT assists in Fe-S cluster maturation and couples this process to bioenergetics of Mtb for survival under low and high demand for Fe-S clusters.

Bacterial Approaches for Assembling Iron-Sulfur Proteins

Building iron-sulfur (Fe-S) clusters and assembling Fe-S proteins are essential actions for life on Earth. The three processes that sustain life, photosynthesis, nitrogen fixation, and respiration, require Fe-S proteins. Genes coding for Fe-S proteins can be found in nearly every sequenced genome. Fe-S proteins have a wide variety of functions, and therefore, defective assembly of Fe-S proteins results in cell death or global metabolic defects. Compared to alternative essential cellular processes, there is less known about Fe-S cluster synthesis and Fe-S protein maturation. Moreover, new factors involved in Fe-S protein assembly continue to be discovered. These facts highlight the growing need to develop a deeper biological understanding of Fe-S cluster synthesis, holo-protein maturation, and Fe-S cluster repair. Here, we outline bacterial strategies used to assemble Fe-S proteins and the genetic regulation of these processes. We focus on recent and relevant findings and discuss future directions, including the proposal of using Fe-S protein assembly as an antipathogen target.

Host and Bacterial Iron Homeostasis, an Underexplored Area in Tuberculosis Biomarker Research

Mycobacterium tuberculosis (Mtb) "a human adapted pathogen" has found multiple ways to manipulate the host immune response during infection. The human immune response to Mtb infection is a highly complex cascade of reactions, with macrophages as preferred intracellular location. Interaction with the host through infection gives rise to expression of specific gene products for survival and multiplication within the host. The signals that the pathogens encounter during infection cause them to selectively express genes in response to signals. One strategy to identify Mtb antigens with diagnostic potential is to identify genes that are specifically induced during infection or in specific disease stages. The shortcomings of current immunodiagnostics include the failure to detect progression from latent infection to active tuberculosis disease, and the inability to monitor treatment efficacy. This highlights the need for new tuberculosis biomarkers. These biomarkers should be highly sensitive and specific diagnosing TB infection, specifically distinguishing between latent infection and active disease. The regulation of iron levels by the host plays a crucial role in the susceptibility and outcome of Mtb infection. Of interest are the siderophore biosynthetic genes, encoded by the mbt-1 and mbt-2 loci and the SUF (mobilization of sulphur) operon (sufR-sufB-sufD-sufC-csd-nifU-sufT), which encodes the primary iron-sulphur cluster biogenesis system. These genes are induced during iron limitation and intracellular growth of Mtb, pointing to their importance during infection.

Mycobacterium tuberculosis SufR responds to nitric oxide via its 4Fe-4S cluster and regulates Fe-S cluster biogenesis for persistence in mice

The persistence of Mycobacterium tuberculosis (Mtb) is a major problem in managing tuberculosis (TB). Host-generated nitric oxide (NO) is perceived as one of the signals by Mtb to reprogram metabolism and respiration for persistence. However, the mechanisms involved in NO sensing and reorganizing Mtb's physiology are not fully understood. Since NO damages iron-sulfur (Fe-S) clusters of essential enzymes, the mechanism(s) involved in regulating Fe-S cluster biogenesis could help Mtb persist in host tissues. Here, we show that a transcription factor SufR (Rv1460) senses NO via its 4Fe-4S cluster and promotes persistence of Mtb by mobilizing the Fe-S cluster biogenesis system; suf operon (Rv1460-Rv1466). Analysis of anaerobically purified SufR by UV-visible spectroscopy, circular dichroism, and iron-sulfide estimation confirms the presence of a 4Fe-4S cluster. Atmospheric O₂ and H₂O₂ gradually degrade the 4Fe-4S cluster of SufR. Furthermore, electron paramagnetic resonance (EPR) analysis demonstrates that NO directly targets SufR 4Fe-4S cluster by forming a protein-bound dinitrosyl-iron-dithiol complex. DNase I footprinting, gel-shift, and in vitro transcription assays confirm that SufR directly regulates the expression of the suf operon in response to NO. Consistent with this, RNA-sequencing of MtbΔsufR demonstrates deregulation of the suf operon under NO stress. Strikingly, NO inflicted irreversible damage upon Fe-S clusters to exhaust respiratory and redox buffering capacity of MtbΔsufR. Lastly, MtbΔsufR failed to recover from a NO-induced non-growing state and displayed persistence defect inside immune-activated macrophages and murine lungs in a NO-dependent manner. Data suggest that SufR is a sensor of NO that supports persistence by reprogramming Fe-S cluster metabolism and bioenergetics.

The Multifaceted Bacterial Cysteine Desulfurases: From Metabolism to Pathogenesis

Living cells have developed a relay system to efficiently transfer sulfur (S) from cysteine to various thio-cofactors (iron-sulfur (Fe-S) clusters, thiamine, molybdopterin, lipoic acid, and biotin) and thiolated tRNA. The presence of such a transit route involves multiple protein components that allow the flux of S to be precisely regulated as a function of environmental cues to avoid the unnecessary accumulation of toxic concentrations of soluble sulfide (S2-). The first enzyme in this relay system is cysteine desulfurase (CSD). CSD catalyzes the release of sulfane S from L-cysteine by converting it to L-alanine by forming an enzyme-linked persulfide intermediate on its conserved cysteine residue. The persulfide S is then transferred to diverse acceptor proteins for its incorporation into the thio-cofactors. The thio-cofactor binding-proteins participate in essential and diverse cellular processes, including DNA repair, respiration, intermediary metabolism, gene regulation, and redox sensing. Additionally, CSD modulates pathogenesis, antibiotic susceptibility, metabolism, and survival of several pathogenic microbes within their hosts. In this review, we aim to comprehensively illustrate the impact of CSD on bacterial core metabolic processes and its requirement to combat redox stresses and antibiotics. Targeting CSD in human pathogens can be a potential therapy for better treatment outcomes.

The ATP-Binding Cassette (ABC) Transport Systems in Mycobacterium tuberculosis: Structure, Function, and Possible Targets for Therapeutics

Simple Summary

Mycobacterium tuberculosis is a bacterium of great medical importance because it causes tuberculosis, a disease that affects millions of people worldwide. Two important features are related to this bacterium: its ability to infect and survive inside the host, minimizing the immune response, and the burden of clinical isolates that are highly resistant to antibiotics treatment. These two phenomena are directly affected by cell envelope proteins, such as proteins from the ATP-Binding Cassette (ABC transporters) superfamily. In this review, we have compiled information on all the M. tuberculosis ABC transporters described so far, both from a functional and structural point of view, and show their relevance for the bacillus and the potential targets for studies aiming to control the microorganism and structural features.

Abstract

Mycobacterium tuberculosis is the etiological agent of tuberculosis (TB), a disease that affects millions of people in the world and that is associated with several human diseases. The bacillus is highly adapted to infect and survive inside the host, mainly because of its cellular envelope plasticity, which can be modulated to adapt to an unfriendly host environment; to manipulate the host immune response; and to resist therapeutic treatment, increasing in this way the drug resistance of TB. The superfamily of ATP-Binding Cassette (ABC) transporters are integral membrane proteins that include both importers and exporters. Both types share a similar structural organization, yet only importers have a periplasmic substrate-binding domain, which is essential for substrate uptake and transport. ABC transporter-type importers play an important role in the bacillus physiology through the transport of several substrates that will interfere with nutrition, pathogenesis, and virulence. Equally relevant, exporters have been involved in cell detoxification, nutrient recycling, and antibiotics and drug efflux, largely affecting the survival and development of multiple drug-resistant strains. Here, we review known ABC transporters from M. tuberculosis, with particular focus on the diversity of their structural features and relevance in infection and drug resistance.

SufT is required for growth of Mycobacterium smegmatis under iron limiting conditions

Iron-sulphur (FeS) clusters are versatile cofactors required for a range of biological processes within cells. Due to the reactive nature of the constituent molecules, assembly and delivery of these cofactors requires a multi-protein machinery in vivo. In prokaryotes, SufT homologues are proposed to function in the maturation and transfer of FeS clusters to apo-proteins. This study used targeted gene deletion to investigate the role of SufT in the physiology of mycobacteria, using Mycobacterium smegmatis as a model organism. Deletion of the sufT gene in M. smegmatis had no impact on growth under standard culture conditions and did not significantly alter activity of the FeS cluster dependent enzymes succinate dehydrogenase (SDH) and aconitase (ACN). Furthermore, the ΔsufT mutant was no more sensitive than the wild-type strain to the redox cycler 2,3-dimethoxy-1,4-naphthoquinone (DMNQ), or the anti-tuberculosis drugs isoniazid, clofazimine or rifampicin. In contrast, the ΔsufT mutant displayed a growth defect under iron limiting conditions, and an increased requirement for iron during biofilm formation. This data suggests that SufT is an accessory factor in FeS cluster biogenesis in mycobacteria which is required under conditions of iron limitation.

Targeting redox heterogeneity to counteract drug tolerance in replicating Mycobacterium tuberculosis

The capacity of Mycobacterium tuberculosis (Mtb) to tolerate multiple antibiotics represents a major problem in tuberculosis (TB) management. Heterogeneity in Mtb populations is one of the factors that drives antibiotic tolerance during infection. However, the mechanisms underpinning this variation in bacterial population remain poorly understood. Here, we show that phagosomal acidification alters the redox physiology of Mtb to generate a population of replicating bacteria that display drug tolerance during infection. RNA sequencing of this redox-altered population revealed the involvement of iron-sulfur (Fe-S) cluster biogenesis, hydrogen sulfide (H₂S) gas, and drug efflux pumps in antibiotic tolerance. The fraction of the pH- and redox-dependent tolerant population increased when Mtb infected macrophages with actively replicating HIV-1, suggesting that redox heterogeneity could contribute to high rates of TB therapy failure during HIV-TB coinfection. Pharmacological inhibition of phagosomal acidification by the antimalarial drug chloroquine (CQ) eradicated drug-tolerant Mtb, ameliorated lung pathology, and reduced postchemotherapeutic relapse in in vivo models. The pharmacological profile of CQ (C _max and AUC_last) exhibited no major drug-drug interaction when coadministered with first line anti-TB drugs in mice. Our data establish a link between phagosomal pH, redox metabolism, and drug tolerance in replicating Mtb and suggest repositioning of CQ to shorten TB therapy and achieve a relapse-free cure.

SufR, a [4Fe-4S] Cluster-Containing Transcription Factor, Represses the sufRBDCSU Operon in Streptomyces avermitilis Iron-Sulfur Cluster Assembly

Iron-sulfur (Fe-S) clusters are ubiquitous and versatile inorganic cofactors that are crucial for many fundamental bioprocesses in nearly all organisms. How cells maintain Fe-S cluster homeostasis is not well understood in Gram-positive bacteria. Genomic analysis showed that the Suf system, which is encoded by the sufRBDCSU operon, is the sole Fe-S cluster assembly system in the genus Streptomyces Streptomyces avermitilis is the industrial producer of avermectins, which are widely used as agricultural pesticides and antiparasitic agents. sufR (SAV6324) encodes a putative ArsR-family transcriptional regulator, which was characterized as a repressor of the sufRBDCSU operon in this investigation. Spectroscopy and mass spectrometry demonstrated that anaerobically isolated SufR contained an oxidation-sensitive [4Fe-4S] cluster and existed as a homodimer. Electrophoretic mobility shift assays (EMSAs) and DNase I footprinting analyses revealed that [4Fe-4S]-SufR bound specifically and tightly to a 14-bp palindromic sequence (CAAC-N6-GTTG) in the promoter region of the sufR operon, repressing expression of the sufRBDCSU operon. The presence of the [4Fe-4S] cluster is critical for the DNA-binding activity of SufR. Cys¹⁸², Cys¹⁹⁵, and Cys²²³ in the C-terminal region of SufR are essential for [4Fe-4S] cluster coordination, but Cys¹⁷⁸ is not. The fourth non-Cys ligand in coordination of the [4Fe-4S] cluster for SufR remains to be identified. The findings clarify the transcriptional control of the suf operon by [4Fe-4S] SufR to satisfy the various Fe-S cluster demands. SufR senses the intracellular Fe-S cluster status and modulates the expression of the sole Fe-S cluster assembly system via its Fe-S cluster occupancy.IMPORTANCE Fe-S clusters function as cofactors of proteins controlling diverse biological processes, such as respiration, photosynthesis, nitrogen fixation, DNA replication, and gene regulation. The mechanism of how Actinobacteria regulate the expression of the sole Fe-S cluster assembly system in response to the various Fe-S cluster demands remains to be elucidated. In this study, we showed that SufR functions as a transcriptional repressor of the sole Fe-S cluster assembly system in the avermectin producer S. avermitilis [4Fe-4S]-SufR binds to the promoter region of the suf operon and represses its expression. When Fe-S cluster levels are insufficient, SufR loses its [4Fe-4S] cluster and DNA-binding activity. Apo-SufR dissociates from the promoter region of suf operon, and the expression of the suf system is strongly increased by derepression to promote the synthesis of Fe-S clusters. The study clarifies how Streptomyces maintains its Fe-S cluster homeostasis through the activity of SufR to modulate the various Fe-S cluster demands.

Mycobacterium tuberculosis SufR Responds to Nitric oxide via its 4Fe-4S cluster and Regulates Fe-S cluster Biogenesis for Persistence in Mice.. [DOI: 10.1101/2020.08.10.245365] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

The persistence of Mycobacterium tuberculosis (Mtb) is a major problem in managing tuberculosis. Host–generated nitric oxide (NO) is perceived as one of the signals by Mtb to reprogram metabolism and respiration for persistence. However, the mechanisms involved in NO sensing and reorganizing Mtb′s physiology are not fully understood. Since NO damages iron–sulfur (Fe–S) clusters of essential enzymes, the mechanism(s) involved in regulating Fe–S cluster biogenesis could help Mtb persist in host tissues. Here, we show that a transcription factor SufR (Rv1460) senses NO via its 4Fe–4S cluster and promotes persistence of Mtb by mobilizing the Fe-S cluster biogenesis system; suf operon (Rv1460–Rv1466). Analysis of anaerobically purified SufR by UV-visible spectroscopy, circular dichroism, and iron-sulfide estimation confirms the presence of a 4Fe–4S cluster. Atmospheric O2 and H2O2 gradually degrade the 4Fe–4S cluster of SufR. Furthermore, electron paramagnetic resonance (EPR) analysis demonstrates that NO directly targets SufR 4Fe–4S cluster by forming a protein-bound dinitrosyl–iron–dithiol complex. DNase I footprinting, gel–shift, and in vitro transcription assays confirm that SufR directly regulates the expression of the suf operon in response to NO. Consistent with this, RNA–sequencing of Mtb ΔsufR demonstrates deregulation of the suf operon under NO stress. Strikingly, NO inflicted irreversible damage upon Fe–S clusters to exhaust respiratory and redox buffering capacity of MtbΔsufR. Lastly, Mtb ΔsufR failed to recover from a NO-induced non-growing state and displayed persistence defect inside immune–activated macrophages and murine lungs in a NO–dependent manner. Data suggest that SufR is a sensor of NO that supports persistence by reprogramming Fe–S cluster metabolism and bioenergetics.

A transcriptional regulator, IscR, of Burkholderia multivorans acts as both repressor and activator for transcription of iron-sulfur cluster-biosynthetic isc operon

Bacterial iron-sulfur (Fe-S) clusters are essential cofactors for many metabolic pathways, and Fe-S cluster-containing proteins (Fe-S proteins) regulate the expression of various important genes. However, biosynthesis of such clusters has remained unknown in genus Burkholderia. Here, we clarified that Burkholderia multivorans ATCC 17616 relies on the ISC system for the biosynthesis of Fe-S clusters, and that the biosynthetic genes are organized as an isc operon, whose first gene encodes IscR, a transcriptional regulatory Fe-S protein. Transcription of the isc operon was repressed and activated under iron-rich and -limiting conditions, respectively, and Fur, an iron-responsive global transcriptional regulator, was indicated to indirectly regulate the expression of isc operon. Further analysis using a ΔiscR mutant in combination with a constitutive expression system of IscR and its derivatives indicated transcriptional repression and activation of isc operon by holo- and apo-forms of IscR, respectively, through their binding to the sequences within an isc promoter-containing (Pisc) fragment. Biochemical analysis using the Pisc fragment suggested that the apo-IscR binding sequence differs from the holo-IscR binding sequence. The results obtained in this study revealed a unique regulatory system for the expression of the ATCC 17616 isc operon that has not been observed in other genera.

Discovery of a Natural Product That Binds to the Mycobacterium tuberculosis Protein Rv1466 Using Native Mass Spectrometry

Elucidation of the mechanism of action of compounds with cellular bioactivity is important for progressing compounds into future drug development. In recent years, phenotype-based drug discovery has become the dominant approach to drug discovery over target-based drug discovery, which relies on the knowledge of a specific drug target of a disease. Still, when targeting an infectious disease via a high throughput phenotypic assay it is highly advantageous to identifying the compound’s cellular activity. A fraction derived from the plant Polyalthia sp. showed activity against Mycobacterium tuberculosis at 62.5 μge/μL. A known compound, altholactone, was identified from this fraction that showed activity towards M. tuberculosis at an minimum inhibitory concentration (MIC) of 64 μM. Retrospective analysis of a target-based screen against a TB proteome panel using native mass spectrometry established that the active fraction was bound to the mycobacterial protein Rv1466 with an estimated pseudo-K_d of 42.0 ± 6.1 µM. Our findings established Rv1466 as the potential molecular target of altholactone, which is responsible for the observed in vivo toxicity towards M. tuberculosis.

Making iron-sulfur cluster: structure, regulation and evolution of the bacterial ISC system

Iron sulfur (Fe-S) clusters rank among the most ancient and conserved prosthetic groups. Fe-S clusters containing proteins are present in most, if not all, organisms. Fe-S clusters containing proteins are involved in a wide range of cellular processes, from gene regulation to central metabolism, via gene expression, RNA modification or bioenergetics. Fe-S clusters are built by biogenesis machineries conserved throughout both prokaryotes and eukaryotes. We focus mostly on bacterial ISC machinery, but not exclusively, as we refer to eukaryotic ISC system when it brings significant complementary information. Besides covering the structural and regulatory aspects of Fe-S biogenesis, this review aims to highlight Fe-S biogenesis facets remaining matters of discussion, such as the role of frataxin, or the link between fatty acid metabolism and Fe-S homeostasis. Last, we discuss recent advances on strategies used by different species to make and use Fe-S clusters in changing redox environmental conditions.

Protein kinase B controls Mycobacterium tuberculosis growth via phosphorylation of the transcriptional regulator Lsr2 at threonine 112

Mycobacterium tuberculosis (Mtb) is able to persist in the body through months of multi‐drug therapy. Mycobacteria possess a wide range of regulatory proteins, including the protein kinase B (PknB) which controls peptidoglycan biosynthesis during growth. Here, we observed that depletion of PknB resulted in specific transcriptional changes that are likely caused by reduced phosphorylation of the H‐NS‐like regulator Lsr2 at threonine 112. The activity of PknB towards this phosphosite was confirmed with purified proteins, and this site was required for adaptation of Mtb to hypoxic conditions, and growth on solid media. Like H‐NS, Lsr2 binds DNA in sequence‐dependent and non‐specific modes. PknB phosphorylation of Lsr2 reduced DNA binding, measured by fluorescence anisotropy and electrophoretic mobility shift assays, and our NMR structure of phosphomimetic T112D Lsr2 suggests that this may be due to increased dynamics of the DNA‐binding domain. Conversely, the phosphoablative T112A Lsr2 had increased binding to certain DNA sites in ChIP‐sequencing, and Mtb containing this variant showed transcriptional changes that correspond with the change in DNA binding. In summary, PknB controls Mtb growth and adaptations to the changing host environment by phosphorylating the global transcriptional regulator Lsr2.

Peroxisomes and Oxidative Stress: Their Implications in the Modulation of Cellular Immunity During Mycobacterial Infection

Host redox dependent physiological responses play crucial roles in the determination of mycobacterial infection process. Mtb explores oxygen rich lung microenvironments to initiate infection process, however, later on the bacilli adapt to oxygen depleted conditions and become non-replicative and unresponsive toward anti-TB drugs to enter in the latency stage. Mtb is equipped with various sensory mechanisms and a battery of pro- and anti-oxidant enzymes to protect themselves from the host oxidative stress mechanisms. After host cell invasion, mycobacteria induces the expression of NADPH oxidase 2 (NOX2) to generate superoxide radicals (O 2 - ), which are then converted to more toxic hydrogen peroxide (H2O2) by superoxide dismutase (SOD) and subsequently reduced to water by catalase. However, the metabolic cascades and their key regulators associated with cellular redox homeostasis are poorly understood. Phagocytosed mycobacteria en route through different subcellular organelles, where the local environment generated during infection determines the outcome of disease. For a long time, mitochondria were considered as the key player in the redox regulation, however, accumulating evidences report vital role for peroxisomes in the maintenance of cellular redox equilibrium in eukaryotic cells. Deletion of peroxisome-associated peroxin genes impaired detoxification of reactive oxygen species and peroxisome turnover post-infection, thereby leading to altered synthesis of transcription factors, various cell-signaling cascades in favor of the bacilli. This review focuses on how mycobacteria would utilize host peroxisomes to alter redox balance and metabolic regulatory mechanisms to support infection process. Here, we discuss implications of peroxisome biogenesis in the modulation of host responses against mycobacterial infection.

Iron homeostasis in Mycobacterium tuberculosis is essential for persistence

Tuberculosis, caused by the obligate intracellular pathogen Mycobacterium tuberculosis (Mtb), is responsible for 2-3 million deaths annually worldwide. Intracellular adaptability, which is critical for long-term persistence, requires the pathogen to neutralize host-mediated insults. The iron-sulphur (Fe-S) cofactor is essential for many enzymes critical for such 'adaptation'. The Mtb genome harbors only one putative iron-sulphur cluster (ISC) operon (rv1460-66) predicted to be involved in the generation of the Fe-S cofactor. Except for rv1460, all other genes in this operon are anticipated to be essential. The current study investigated the role of rv1460, an sufR homologue of Mtb (sufRTB), in maintaining intracellular Fe homeostasis and its implications on mycobacterial pathogenesis. We found that Mtb ISC locus (rv1461-66) was transcribed as a single multigene transcript. We successfully generated the sufRTB null mutant strain (ΔsufRTB) of Mtb, suggesting nonessentiality of the gene under normal growth conditions. The mutant strain demonstrated enhanced biofilm generation and failed to grow under a low-Fe condition. Growth characterization studies indicated that SufRTB-mediated intracellular Fe homeostasis is essential for Mtb to persist in the host. Targeting mycobacterial persistence by inhibiting SufRTB protein activity may be a novel intervention strategy in tuberculosis treatment.