Genetic and chemical validation identifies Mycobacterium tuberculosis topoisomerase I as an attractive anti-tubercular target

Variation of Structure and Cellular Functions of Type IA Topoisomerases across the Tree of Life

Topoisomerases regulate the topological state of cellular genomes to prevent impediments to vital cellular processes, including replication and transcription from suboptimal supercoiling of double-stranded DNA, and to untangle topological barriers generated as replication or recombination intermediates. The subfamily of type IA topoisomerases are the only topoisomerases that can alter the interlinking of both DNA and RNA. In this article, we provide a review of the mechanisms by which four highly conserved N-terminal protein domains fold into a toroidal structure, enabling cleavage and religation of a single strand of DNA or RNA. We also explore how these conserved domains can be combined with numerous non-conserved protein sequences located in the C-terminal domains to form a diverse range of type IA topoisomerases in Archaea, Bacteria, and Eukarya. There is at least one type IA topoisomerase present in nearly every free-living organism. The variation in C-terminal domain sequences and interacting partners such as helicases enable type IA topoisomerases to conduct important cellular functions that require the passage of nucleic acids through the break of a single-strand DNA or RNA that is held by the conserved N-terminal toroidal domains. In addition, this review will exam a range of human genetic disorders that have been linked to the malfunction of type IA topoisomerase.

Macromolecular crowding potently stimulates DNA supercoiling activity of Mycobacterium tuberculosis DNA gyrase

Macromolecular crowding, manifested by high concentrations of proteins and nucleic acids in living cells, significantly influences biological processes such as enzymatic reactions. Studying these reactions in vitro, using agents such as polyetthylene glycols (PEGs) and polyvinyl alcohols (PVAs) to mimic intracellular crowding conditions, is essential due to the notable differences from enzyme behaviors observed in diluted aqueous solutions. In this article, we studied Mycobacterium tuberculosis (Mtb) DNA gyrase under macromolecular crowding conditions by incorporating PEGs and PVAs into the DNA supercoiling reactions. We discovered that high concentrations of potassium glutamate, glycine betaine, PEGs, and PVA substantially stimulated the DNA supercoiling activity of Mtb DNA gyrase. Steady-state kinetic studies showed that glycine betaine and PEG400 significantly reduced the KM of Mtb DNA gyrase and simultaneously increased the Vmax or kcat of Mtb DNA gyrase for ATP and the plasmid DNA molecule. Molecular dynamics simulation studies demonstrated that PEG molecules kept the ATP lid of DNA gyrase subunit B in a closed or semiclosed conformation, which prevented ATP molecules from leaving the ATP-binding pocket of DNA gyrase subunit B. The stimulation of the DNA supercoiling activity of Mtb DNA gyrase by these molecular crowding agents likely results from a decrease in water activity and an increase in excluded volume.

Hp0521 inhibited the virulence of H. pylori 26,695 strain via regulating CagA expression

Hp0521 is the number of cag pathogenicity island (cagPAI) family in Helicobacter pylori (H. pylori, Hp), which encoded Cag2 protein. The aim of this study was to investigate the role of hp0521 on the H. pylori 26,695 strain. We constructed the recombinant prokaryotic expression plasmid pET-32a-hp0521 and pET-32a-hpc0521. Then, we co-cultured the H. pylori wild strain 26,695 and Δhp0521 strain with GES-1 cells to detect CagA protein transport and IL-8 secretion. We found that Δhp0521 mutation increased the expression of cagA, rpoB and promoted the transportation of CagA protein in GES-1 cells. In addition, we also observed that Δhp0521 mutation had no effect on other cagPAI protein stability and the expression of IL-8. Our findings suggested that hp0521 may down-regulated the expression of cagA, rpoB and inhibited the transportation of CagA protein in GES-1 cells and had no effect on growth.

Implementation of a mycobacterial CRISPRi platform in Mycobacterium abscessus and demonstration of the essentiality of ftsZMab

Mycobacterium abscessus (Mab) is a highly drug-resistant non-tuberculous mycobacterial species that causes debilitating TB-like pulmonary infections. The lack of genetic tools has hampered characterization of its extensive repertoire of virulence factors, antimicrobial resistance mechanisms, and drug targets. In this study, we evaluated the performance of a mycobacterial single plasmid CRISPRi-dCas9 system optimized for M. tuberculosis and M. smegmatis for inducible gene silencing in Mab. The efficacy of CRISPRi-mediated repression of two antibiotic resistance genes (bla_Mab, whiB7_Mab) and two putative essential genes (ftsZ_Mab,topA_Mab) was determined by measuring mRNA transcript levels and phenotypic outcomes. While our results support the utility of this mycobacterial CRISPRi dCas9_Sth1 single-plasmid platform for inducible silencing of specific target genes in Mab, they also highlighted several caveats and nuances that may warrant species-specific optimization for Mab. We observed overall lower levels of gene repression in Mab including variable silencing of different target genes despite use of PAMs of similar predicted strength. In addition, leaky gene repression in the absence of inducer was noted for some genes but not others. Nonetheless, using CRISPRi we demonstrated the silencing of multiple target genes and validated ftsZ_Mab as an essential gene and promising drug target for the first time.

Localization of Mycobacterium tuberculosis topoisomerase I C-terminal sequence motif required for inhibition by endogenous toxin MazF4

Only about half the multi-drug resistant tuberculosis (MDR-TB) cases are successfully cured. Thus, there is an urgent need of new TB treatment against a novel target. Mycobacterium tuberculosis (Mtb) topoisomerase I (TopA) is the only type IA topoisomerase in this organism and has been validated as an essential target for TB drug discovery. Toxin-antitoxin (TA) systems participate as gene regulators within bacteria. The TA systems contribute to the long-term dormancy of Mtb within the host-cell environment. Mtb's toxin MazF4 (Rv1495) that is part of the MazEF4 TA system has been shown to have dual activities as endoribonuclease and topoisomerase I inhibitor. We have developed a complementary assay using an Escherichia coli strain with temperature-sensitive topA mutation to provide new insights into the MazF4 action. The assay showed that E. coli is not sensitive to the endoribonuclease activity of Mtb MazF4 but became vulnerable to MazF4 growth inhibition when recombinant Mtb TopA relaxation activity is required for growth. Results from the complementation by Mtb TopA mutants with C-terminal deletions showed that the lysine-rich C-terminal tail is required for interaction with MazF4. Site-directed mutagenesis is utilized to identify two lysine residues within a conserved motif in this C-terminal tail that are critical for MazF4 inhibition. We performed molecular dynamics simulations to predict the Mtb TopA-MazF4 complex. Our simulation results show that the complex is stabilized by hydrogen bonds and electrostatic interactions established by residues in the TopA C-terminal tail including the two conserved lysines. The mechanism of Mtb TopA inhibition by MazF4 could be useful for the discovery of novel inhibitors against a new antibacterial target in pathogenic mycobacteria for treatment of both TB and diseases caused by the non-tuberculosis mycobacteria (NTM).

Advances in Key Drug Target Identification and New Drug Development for Tuberculosis

Tuberculosis (TB) is a severe infectious disease worldwide. The increasing emergence of drug-resistant Mycobacterium tuberculosis (Mtb) has markedly hampered TB control. Therefore, there is an urgent need to develop new anti-TB drugs to treat drug-resistant TB and shorten the standard therapy. The discovery of targets of drug action will lay a theoretical foundation for new drug development. With the development of molecular biology and the success of Mtb genome sequencing, great progress has been made in the discovery of new targets and their relevant inhibitors. In this review, we summarized 45 important drug targets and 15 new drugs that are currently being tested in clinical stages and several prospective molecules that are still at the level of preclinical studies. A comprehensive understanding of the drug targets of Mtb can provide extensive insights into the development of safer and more efficient drugs and may contribute new ideas for TB control and treatment.

Computational modeling and bioinformatic analyses of functional mutations in drug target genes in Mycobacterium tuberculosis

•

MycoTRAP-DB, a database of mutations and their impact on normal functionality of protein in M.tb genes.

•

Several secondary mutations were identified with significant impact on protein structure and function.

•

Comprehensive information gives insight for screening of suspected hotspots in advance to combat drug resistant TB.

Tuberculosis (TB) continues to be the leading cause of deaths due to its persistent drug resistance and the consequent ineffectiveness of anti-TB treatment. Recent years witnessed huge amount of sequencing data, revealing mutations responsible for drug resistance. However, the lack of an up-to-date repository remains a barrier towards utilization of these data and identifying major mutations-associated with resistance. Amongst all mutations, non-synonymous mutations alter the amino acid sequence of a protein and have a much greater effect on pathogenicity. Hence, this type of gene mutation is of prime interest of the present study. The purpose of this study is to develop an updated database comprising almost all reported substitutions within the Mycobacterium tuberculosis (M.tb) drug target genes rpoB, inhA, katG, pncA, gyrA and gyrB. Various bioinformatics prediction tools were used to assess the structural and biophysical impacts of the resistance causing non-synonymous single nucleotide polymorphisms (nsSNPs) at the molecular level. This was followed by evaluating the impact of these mutations on binding affinity of the drugs to target proteins. We have developed a comprehensive online resource named MycoTRAP-DB (Mycobacterium tuberculosis Resistance Associated Polymorphisms Database) that connects mutations in genes with their structural, functional and pathogenic implications on protein. This database is accessible at http://139.59.12.92. This integrated platform would enable comprehensive analysis and prioritization of SNPs for the development of improved diagnostics and antimycobacterial medications. Moreover, our study puts forward secondary mutations that can be important for prognostic assessments of drug-resistance mechanism and actionable anti-TB drugs.

Type IA Topoisomerases as Targets for Infectious Disease Treatments

Infectious diseases are one of the main causes of death all over the world, with antimicrobial resistance presenting a great challenge. New antibiotics need to be developed to provide therapeutic treatment options, requiring novel drug targets to be identified and pursued. DNA topoisomerases control the topology of DNA via DNA cleavage-rejoining coupled to DNA strand passage. The change in DNA topological features must be controlled in vital processes including DNA replication, transcription, and DNA repair. Type IIA topoisomerases are well established targets for antibiotics. In this review, type IA topoisomerases in bacteria are discussed as potential targets for new antibiotics. In certain bacterial pathogens, topoisomerase I is the only type IA topoisomerase present, which makes it a valuable antibiotic target. This review will summarize recent attempts that have been made to identify inhibitors of bacterial topoisomerase I as potential leads for antibiotics and use of these inhibitors as molecular probes in cellular studies. Crystal structures of inhibitor-enzyme complexes and more in-depth knowledge of their mechanisms of actions will help to establish the structure-activity relationship of potential drug leads and develop potent and selective therapeutics that can aid in combating the drug resistant bacterial infections that threaten public health.

Rapid, direct detection of bacterial topoisomerase 1-DNA adducts by RADAR/ELISA

Topoisomerases are proven drug targets, but antibiotics that poison bacterial Topoisomerase 1 (Top1) have yet to be discovered. We have developed a rapid and direct assay for quantification of Top1-DNA adducts that is suitable for high throughput assays. Adducts are recovered by "RADAR fractionation", a quick, convenient approach in which cells are lysed in chaotropic salts and detergent and nucleic acids and covalently bound adducts then precipitated with alcohol. Here we show that RADAR fractionation followed by ELISA immunodetection can quantify adducts formed by wild-type and mutant Top1 derivatives encoded by two different bacterial pathogens, Y. pestis and M. tuberculosis, expressed in E. coli or M. smegmatis, respectively. For both enzymes, quantification of adducts by RADAR/ELISA produces results comparable to the more cumbersome classical approach of CsCl density gradient fractionation. The experiments reported here establish that RADAR/ELISA assay offers a simple way to characterize Top1 mutants and analyze kinetics of adduct formation and repair. They also provide a foundation for discovery and optimization of drugs that poison bacterial Top1 using standard high-throughput approaches.

Mechanistic insights from structure of Mycobacterium smegmatis topoisomerase I with ssDNA bound to both N- and C-terminal domains

Type IA topoisomerases interact with G-strand and T-strand ssDNA to regulate DNA topology. However, simultaneous binding of two ssDNA segments to a type IA topoisomerase has not been observed previously. We report here the crystal structure of a type IA topoisomerase with ssDNA segments bound in opposite polarity to the N- and C-terminal domains. Titration of small ssDNA oligonucleotides to Mycobacterium smegmatis topoisomerase I with progressive C-terminal deletions showed that the C-terminal region has higher affinity for ssDNA than the N-terminal active site. This allows the C-terminal domains to capture one strand of underwound negatively supercoiled DNA substrate first and position the N-terminal domains to bind and cleave the opposite strand in the relaxation reaction. Efficiency of negative supercoiling relaxation increases with the number of domains that bind ssDNA primarily with conserved aromatic residues and possibly with assistance from polar/basic residues. A comparison of bacterial topoisomerase I structures showed that a conserved transesterification unit (N-terminal toroid structure) for cutting and rejoining of a ssDNA strand can be combined with two different types of C-terminal ssDNA binding domains to form diverse bacterial topoisomerase I enzymes that are highly efficient in their physiological role of preventing excess negative supercoiling in the genome.

Covalent Complex of DNA and Bacterial Topoisomerase: Implications in Antibacterial Drug Development

A topoisomerase-DNA transient covalent complex can be a druggable target for novel topoisomerase poison inhibitors that represent a new class of antibacterial or anticancer drugs. Herein, we have investigated molecular features of the functionally important Escherichia coli topoisomerase I (EctopoI)-DNA covalent complex (EctopoIcc) for molecular simulations, which is very useful in the development of new antibacterial drugs. To demonstrate the usefulness of our approach, we used a model small molecule (SM), NSC76027, obtained from virtual screening. We examined the direct binding of NSC76027 to EctopoI as well as inhibition of EctopoI relaxation activity of this SM via experimental techniques. We then performed molecular dynamics (MD) simulations to investigate the dynamics and stability of EctopoIcc and EctopoI-NSC76027-DNA ternary complex. Our simulation results show that NSC76027 forms a stable ternary complex with EctopoIcc. EctopoI investigated here also serves as a model system for investigating a complex of topoisomerase and DNA in which DNA is covalently attached to the protein.

Latent Tuberculosis Infection (LTBI) and Its Potential Targets: An Investigation into Dormant Phase Pathogens

One-third of the world's population harbours the latent tuberculosis infection (LTBI) with a lifetime risk of reactivation. Although, the treatment of LTBI relies significantly on the first-line therapy, identification of novel drug targets and therapies are the emerging focus for researchers across the globe. The current review provides an insight into the infection, diagnostic methods and epigrammatic explanations of potential molecular targets of dormant phase bacilli. This study also includes current preclinical and clinical aspects of tubercular infections and new approaches in antitubercular drug discovery.

Kinetic Study of DNA Topoisomerases by Supercoiling-Dependent Fluorescence Quenching

DNA topoisomerases are essential enzymes for all living organisms and important targets for anticancer drugs and antibiotics. Although DNA topoisomerases have been studied extensively, steady-state kinetics has not been systematically investigated because of the lack of an appropriate assay. Previously, we demonstrated that newly synthesized, fluorescently labeled plasmids pAB1_FL905 and pAB1_FL924 can be used to study DNA topoisomerase-catalyzed reactions by fluorescence resonance energy transfer (FRET) or supercoiling-dependent fluorescence quenching (SDFQ). With the FRET or SDFQ method, we performed steady-state kinetic studies for six different DNA topoisomerases including two type IA enzymes (Escherichia coli and Mycobacterium smegmatis DNA topoisomerase I), two type IB enzymes (human and variola DNA topoisomerase I), and two type IIA enzymes (E. coli DNA gyrase and human DNA topoisomerase IIα). Our results show that all DNA topoisomerases follow the classical Michaelis-Menten kinetics and have unique steady-state kinetic parameters, K _M, V _max, and k _cat. We found that k _cat for all topoisomerases are rather low and that such low values may stem from the tight binding of topoisomerases to DNA. Additionally, we confirmed that novobiocin is a competitive inhibitor for adenosine 5'-triphosphate binding to E. coli DNA gyrase, demonstrating the utility of our assay for studying topoisomerase inhibitors.

Investigating mycobacterial topoisomerase I mechanism from the analysis of metal and DNA substrate interactions at the active site

We have obtained new crystal structures of Mycobacterium tuberculosis topoisomerase I, including structures with ssDNA substrate bound to the active site, with and without Mg²⁺ ion present. Significant enzyme conformational changes upon DNA binding place the catalytic tyrosine in a pre-transition state position for cleavage of a specific phosphodiester linkage. Meanwhile, the enzyme/DNA complex with bound Mg²⁺ ion may represent the post-transition state for religation in the enzyme's multiple-step DNA relaxation catalytic cycle. The first observation of Mg²⁺ ion coordinated with the TOPRIM residues and DNA phosphate in a type IA topoisomerase active site allows assignment of likely catalytic role for the metal and draws a comparison to the proposed mechanism for type IIA topoisomerases. The critical function of a strictly conserved glutamic acid in the DNA cleavage step was assessed through site-directed mutagenesis. The functions assigned to the observed Mg²⁺ ion can account for the metal requirement for DNA rejoining but not DNA cleavage by type IA topoisomerases. This work provides new structural insights into a more stringent requirement for DNA rejoining versus cleavage in the catalytic cycle of this essential enzyme, and further establishes the potential for selective interference of DNA rejoining by this validated TB drug target.

Mycobacterium tuberculosis topoisomerases and EthR as the targets for new anti-TB drugs development

The significant increase in the detection of drug-resistant strains of Mycobacterium tuberculosis caused an urgent need for the discovery new antituberculosis drugs. Development of bioinformatics and computational sciences enabled the progress of new strategies leading to design, discovery and identification of a series of interesting drug candidates. In this short review, we would like to present recently discovered compounds targeting important mycobacterial proteins: DNA topoisomerases and the transcriptional repressor of EthA monooxygenase – EthR.

Mechanism and resistance for antimycobacterial activity of a fluoroquinophenoxazine compound

We have previously reported the inhibition of bacterial topoisomerase I activity by a fluoroquinophenoxazine compound (FP-11g) with a 6-bipiperidinyl lipophilic side chain that exhibited promising antituberculosis activity (MIC = 2.5 μM against Mycobacterium tuberculosis, SI = 9.8). Here, we found that the compound is bactericidal towards Mycobacterium smegmatis, resulting in greater than 5 Log10 reduction in colony-forming units [cfu]/mL following a 10 h incubation at 1.25 μM (4X MIC) concentration. Growth inhibition (MIC = 50 μM) and reduction in cfu could also be observed against a clinical isolate of Mycobacterium abscessus. Stepwise isolation of resistant mutants of M. smegmatis was conducted to explore the mechanism of resistance. Mutations in the resistant isolates were identified by direct comparison of whole-genome sequencing data from mutant and wild-type isolates. These include mutations in genes likely to affect the entry and retention of the compound. FP-11g inhibits Mtb topoisomerase I and Mtb gyrase with IC50 of 0.24 and 27 μM, respectively. Biophysical analysis showed that FP-11g binds DNA as an intercalator but the IC50 for inhibition of Mtb topoisomerase I activity is >10 fold lower than the compound concentrations required for producing negatively supercoiled DNA during ligation of nicked circular DNA. Thus, the DNA-binding property of FP-11g may contribute to its antimycobacterial mechanism, but that alone cannot account for the observed inhibition of Mtb topoisomerase I.

Direct observation of topoisomerase IA gate dynamics

Type IA topoisomerases cleave single-stranded DNA and relieve negative supercoils in discrete steps corresponding to the passage of the intact DNA strand through the cleaved strand. Although type IA topoisomerases are assumed to accomplish this strand passage via a protein-mediated DNA gate, opening of this gate has never been observed. We developed a single-molecule assay to directly measure gate opening of the Escherichia coli type IA topoisomerases I and III. We found that after cleavage of single-stranded DNA, the protein gate opens by as much as 6.6 nm and can close against forces in excess of 16 pN. Key differences in the cleavage, ligation, and gate dynamics of these two enzymes provide insights into their different cellular functions. The single-molecule results are broadly consistent with conformational changes obtained from molecular dynamics simulations. These results allowed us to develop a mechanistic model of interactions between type IA topoisomerases and single-stranded DNA.

Characterization of new, efficient Mycobacterium tuberculosis topoisomerase-I inhibitors and their interaction with human ABC multidrug transporters

Drug resistant tuberculosis (TB) is a major worldwide health problem. In addition to the bacterial mechanisms, human drug transporters limiting the cellular accumulation and the pharmacological disposition of drugs also influence the efficacy of treatment. Mycobacterium tuberculosis topoisomerase-I (MtTopo-I) is a promising target for antimicrobial treatment. In our previous work we have identified several hit compounds targeting the MtTopo-I by in silico docking. Here we expand the scope of the compounds around three scaffolds associated with potent MtTopo-I inhibition. In addition to measuring the effect of newly generated compounds on MtTopo-I activity, we characterized the compounds’ antimicrobial activity, toxicity in human cells, and interactions with human multidrug transporters. Some of the newly developed MtTopo-I inhibitors have strong antimicrobial activity and do not harm mammalian cells. Moreover, our studies revealed significant human ABC drug transporter interactions for several MtTopo-I compounds that may modify their ADME-Tox parameters and cellular effects. Promising new drug candidates may be selected based on these studies for further anti-TB drug development.

Evidence for Inhibition of Topoisomerase 1A by Gold(III) Macrocycles and Chelates Targeting Mycobacterium tuberculosis and Mycobacterium abscessus

Mycobacterium tuberculosis and the fast-growing species Mycobacterium abscessus are two important human pathogens causing persistent pulmonary infections that are difficult to cure and require long treatment times. The emergence of drug-resistant M. tuberculosis strains and the high level of intrinsic resistance of M. abscessus call for novel drug scaffolds that effectively target both pathogens. In this study, we evaluated the activity of bis(pyrrolide-imine) gold(III) macrocycles and chelates, originally designed as DNA intercalators capable of targeting human topoisomerase types I and II (Topo1 and Topo2), against M. abscessus and M. tuberculosis We identified a total of 5 noncytotoxic compounds active against both mycobacterial pathogens under replicating in vitro conditions. We chose one of these hits, compound 14, for detailed analysis due to its potent bactericidal mode of inhibition and scalable synthesis. The clinical relevance of this compound was demonstrated by its ability to inhibit a panel of diverse M. tuberculosis and M. abscessus clinical isolates. Prompted by previous data suggesting that compound 14 may target topoisomerase/gyrase enzymes, we demonstrated that it lacked cross-resistance with fluoroquinolones, which target the M. tuberculosis gyrase. In vitro enzyme assays confirmed the potent activity of compound 14 against bacterial topoisomerase 1A (Topo1) enzymes but not gyrase. Novel scaffolds like compound 14 with potent, selective bactericidal activity against M. tuberculosis and M. abscessus that act on validated but underexploited targets like Topo1 represent a promising starting point for the development of novel therapeutics for infections by pathogenic mycobacteria.

Discovery of novel bacterial topoisomerase I inhibitors by use of in silico docking and in vitro assays

Topoisomerases are important targets for antibacterial and anticancer therapies. Bacterial topoisomerase I remains to be exploited for antibiotics that can be used in the clinic. Inhibitors of bacterial topoisomerase I may provide leads for novel antibacterial drugs against pathogens resistant to current antibiotics. TB is the leading infectious cause of death worldwide, and new TB drugs against an alternative target are urgently needed to overcome multi-drug resistance. Mycobacterium tuberculosis topoisomerase I (MtbTopI) has been validated genetically and chemically as a TB drug target. Here we conducted in silico screening targeting an active site pocket of MtbTopI. The top hits were assayed for inhibition of MtbTopI activity. The shared structural motif found in the active hits was utilized in a second round of in silico screening and in vitro assays, yielding selective inhibitors of MtbTopI with IC₅₀s as low as 2 µM. Growth inhibition of Mycobacterium smegmatis by these compounds in combination with an efflux pump inhibitor was diminished by the overexpression of recombinant MtbTopI. This work demonstrates that in silico screening can be utilized to discover new bacterial topoisomerase I inhibitors, and identifies a novel structural motif which could be explored further for finding selective bacterial topoisomerase I inhibitors.

Targeting DNA Replication and Repair for the Development of Novel Therapeutics against Tuberculosis

Mycobacterium tuberculosis is the etiological agent of tuberculosis (TB), an infectious disease which results in approximately 10 million incident cases and 1.4 million deaths globally each year, making it the leading cause of mortality from infection. An effective frontline combination chemotherapy exists for TB; however, this regimen requires the administration of four drugs in a 2 month long intensive phase followed by a continuation phase of a further 4 months with two of the original drugs, and is only effective for the treatment of drug-sensitive TB. The emergence and global spread of multidrug-resistant (MDR) as well as extensively drug-resistant (XDR) strains of M. tuberculosis, and the complications posed by co-infection with the human immunodeficiency virus (HIV) and other co-morbidities such as diabetes, have prompted urgent efforts to develop shorter regimens comprising new compounds with novel mechanisms of action. This demands that researchers re-visit cellular pathways and functions that are essential to M. tuberculosis survival and replication in the host but which are inadequately represented amongst the targets of current anti-mycobacterial agents. Here, we consider the DNA replication and repair machinery as a source of new targets for anti-TB drug development. Like most bacteria, M. tuberculosis encodes a complex array of proteins which ensure faithful and accurate replication and repair of the chromosomal DNA. Many of these are essential; so, too, are enzymes in the ancillary pathways of nucleotide biosynthesis, salvage, and re-cycling, suggesting the potential to inhibit replication and repair functions at multiple stages. To this end, we provide an update on the state of chemotherapeutic inhibition of DNA synthesis and related pathways in M. tuberculosis. Given the established links between genotoxicity and mutagenesis, we also consider the potential implications of targeting DNA metabolic pathways implicated in the development of drug resistance in M. tuberculosis, an organism which is unusual in relying exclusively on de novo mutations and chromosomal rearrangements for evolution, including the acquisition of drug resistance. In that context, we conclude by discussing the feasibility of targeting mutagenic pathways in an ancillary, “anti-evolution” strategy aimed at protecting existing and future TB drugs.

Distinct Mechanism Evolved for Mycobacterial RNA Polymerase and Topoisomerase I Protein-Protein Interaction

We report here a distinct mechanism of interaction between topoisomerase I and RNA polymerase in Mycobacterium tuberculosis and Mycobacterium smegmatis that has evolved independently from the previously characterized interaction between bacterial topoisomerase I and RNA polymerase. Bacterial DNA topoisomerase I is responsible for preventing the hyper-negative supercoiling of genomic DNA. The association of topoisomerase I with RNA polymerase during transcription elongation could efficiently relieve transcription-driven negative supercoiling. Our results demonstrate a direct physical interaction between the C-terminal domains of topoisomerase I (TopoI-CTDs) and the β' subunit of RNA polymerase of M. smegmatis in the absence of DNA. The TopoI-CTDs in mycobacteria are evolutionarily unrelated in amino acid sequence and three-dimensional structure to the TopoI-CTD found in the majority of bacterial species outside Actinobacteria, including Escherichia coli. The functional interaction between topoisomerase I and RNA polymerase has evolved independently in mycobacteria and E. coli, with distinctively different structural elements of TopoI-CTD utilized for this protein-protein interaction. Zinc ribbon motifs in E. coli TopoI-CTD are involved in the interaction with RNA polymerase. For M. smegmatis TopoI-CTD, a 27-amino-acid tail that is rich in basic residues at the C-terminal end is responsible for the interaction with RNA polymerase. Overexpression of recombinant TopoI-CTD in M. smegmatis competed with the endogenous topoisomerase I for protein-protein interactions with RNA polymerase. The TopoI-CTD overexpression resulted in decreased survival following treatment with antibiotics and hydrogen peroxide, supporting the importance of the protein-protein interaction between topoisomerase I and RNA polymerase during stress response of mycobacteria.

A novel proposal of a simplified bacterial gene set and the neo-construction of a general minimized metabolic network

A minimal gene set (MGS) is critical for the assembly of a minimal artificial cell. We have developed a proposal of simplifying bacterial gene set to approximate a bacterial MGS by the following procedure. First, we base our simplified bacterial gene set (SBGS) on experimentally determined essential genes to ensure that the genes included in the SBGS are critical. Second, we introduced a half-retaining strategy to extract persistent essential genes to ensure stability. Third, we constructed a viable metabolic network to supplement SBGS. The proposed SBGS includes 327 genes and required 431 reactions. This report describes an SBGS that preserves both self-replication and self-maintenance systems. In the minimized metabolic network, we identified five novel hub metabolites and confirmed 20 known hubs. Highly essential genes were found to distribute the connecting metabolites into more reactions. Based on our SBGS, we expanded the pool of targets for designing broad-spectrum antibacterial drugs to reduce pathogen resistance. We also suggested a rough semi-de novo strategy to synthesize an artificial cell, with potential applications in industry.

Synthesis, evaluation, and CoMFA study of fluoroquinophenoxazine derivatives as bacterial topoisomerase IA inhibitors

New antibacterial agents with novel target and mechanism of action are urgently needed to combat problematic bacterial infections and mounting antibiotic resistances. Topoisomerase IA represents an attractive and underexplored antibacterial target, as such, there is a growing interest in developing selective and potent topoisomerase I inhibitors for antibacterial therapy. Based on our initial biological screening, fluoroquinophenoxazine 1 was discovered as a low micromolar inhibitor against E. coli topoisomerase IA. In the literature, fluoroquinophenoxazine analogs have been investigated as antibacterial and anticancer agents, however, their topoisomerase I inhibition was relatively underexplored and there is little structure-activity relationship (SAR) available. The good topoisomerase I inhibitory activity of 1 and the lack of SAR prompted us to design and synthesize a series of fluoroquinophenoxazine analogs to systematically evaluate the SAR and to probe the structural elements of the fluoroquinophenoxazine core toward topoisomerase I enzyme target recognition. In this study, a series of fluoroquinophenoxazine analogs was designed, synthesized, and evaluated as topoisomerase I inhibitors and antibacterial agents. Target-based assays revealed that the fluoroquinophenoxazine derivatives with 9-NH₂ and/or 6-substituted amine functionalities generally exhibited good to excellent inhibitory activities against topoisomerase I with IC₅₀s ranging from 0.24 to 3.9 μM. Notably, 11a bearing the 6-methylpiperazinyl and 9-amino motifs was identified as one of the most potent topoisomerase I inhibitors (IC₅₀ = 0.48 μM), and showed broad spectrum antibacterial activity (MICs = 0.78-7.6 μM) against all the bacteria strains tested. Compound 11g with the 6-bipiperidinyl lipophilic side chain exhibited the most potent antituberculosis activity (MIC = 2.5 μM, SI = 9.8). In addition, CoMFA analysis was performed to investigate the 3D-QSAR of this class of fluoroquinophenoxazine derivatives. The constructed CoMFA model produced reasonable statistics (q² = 0.688 and r² = 0.806). The predictive power of the developed model was obtained using a test set of 7 compounds, giving a predictive correlation coefficient r²_pred of 0.767. Collectively, these promising data demonstrated that fluoroquinophenoxazine derivatives have the potential to be developed as a new chemotype of potent topoisomerase IA inhibitors with antibacterial therapeutic potential.

Small-Molecule Inhibitors Targeting Topoisomerase I as Novel Antituberculosis Agents

Bacterial topoisomerase functions are required for regulation of DNA supercoiling and overcoming the DNA topological barriers that are encountered during many vital cellular processes. DNA gyrase and topoisomerase IV of the type IIA bacterial topoisomerase family are important clinical targets for antibacterial therapy. Topoisomerase I, belonging to the type IA topoisomerase family, has recently been validated as a potential antitubercular target. The topoisomerase I activity has been shown to be essential for bacterial viability and infection in a murine model of tuberculosis. Mixture-based combinatorial libraries were screened in this study to identify novel bacterial topoisomerase I inhibitors. Using positional-scanning deconvolution, selective small-molecule inhibitors of bacterial topoisomerase I were identified starting from a polyamine scaffold. Antibacterial assays demonstrated that four of these small-molecule inhibitors of bacterial topoisomerase I are bactericidal against Mycobacterium smegmatis and Mycobacterium tuberculosis The MICs for growth inhibition of M. smegmatis increased with overexpression of recombinant M. tuberculosis topoisomerase I, consistent with inhibition of intracellular topoisomerase I activity being involved in the antimycobacterial mode of action.

Targeting bacterial topoisomerases: how to counter mechanisms of resistance

DNA gyrase and topoisomerase IV are type IIA bacterial topoisomerases that are targeted by highly effective antibiotics. However, resistance via multiple mechanisms arises to limit the efficacies of these drugs. Continued research on type IIA bacterial topoisomerases has provided novel approaches to counter the most common resistance mechanism for utilization of these proven targets in antibacterial therapy. Bacterial topoisomerase I is being explored as an alternative target that is not expected to show cross-resistance. Dual targeting or combination therapy could be strategies for circumventing the development of resistance to topoisomerase-targeting antibiotics. Bacterial topoisomerases are high-value bactericidal targets that could continue to be exploited for antibacterial therapy, if new tactics to counter resistance can be adopted.

Essentiality Assessment of Cysteinyl and Lysyl-tRNA Synthetases of Mycobacterium smegmatis

Discovery of mupirocin, an antibiotic that targets isoleucyl-tRNA synthetase, established aminoacyl-tRNA synthetase as an attractive target for the discovery of novel antibacterial agents. Despite a high degree of similarity between the bacterial and human aminoacyl-tRNA synthetases, the selectivity observed with mupirocin triggered the possibility of targeting other aminoacyl-tRNA synthetases as potential drug targets. These enzymes catalyse the condensation of a specific amino acid to its cognate tRNA in an energy-dependent reaction. Therefore, each organism is expected to encode at least twenty aminoacyl-tRNA synthetases, one for each amino acid. However, a bioinformatics search for genes encoding aminoacyl-tRNA synthetases from Mycobacterium smegmatis returned multiple genes for glutamyl (GluRS), cysteinyl (CysRS), prolyl (ProRS) and lysyl (LysRS) tRNA synthetases. The pathogenic mycobacteria, namely, Mycobacterium tuberculosis and Mycobacterium leprae, were also found to possess two genes each for CysRS and LysRS. A similar search indicated the presence of additional genes for LysRS in gram negative bacteria as well. Herein, we describe sequence and structural analysis of the additional aminoacyl-tRNA synthetase genes found in M. smegmatis. Characterization of conditional expression strains of Cysteinyl and Lysyl-tRNA synthetases generated in M. smegmatis revealed that the canonical aminoacyl-tRNA synthetase are essential, while the additional ones are not essential for the growth of M. smegmatis.

Insights from the Structure of Mycobacterium tuberculosis Topoisomerase I with a Novel Protein Fold

The DNA topoisomerase I enzyme of Mycobacterium tuberculosis (MtTOP1) is essential for the viability of the organism and survival in a murine model. This topoisomerase is being pursued as a novel target for the discovery of new therapeutic agents for the treatment of drug-resistant tuberculosis. In this study, we succeeded in obtaining a structure of MtTOP1 by first predicting that the C-terminal region of MtTOP1 contains four repeated domains that do not involve the Zn-binding tetracysteine motifs seen in the C-terminal domains of Escherichia coli topoisomerase I. A construct (amino acids A2-T704), MtTOP1-704t, that includes the N-terminal domains (D1-D4) and the first predicted C-terminal domain (D5) of MtTOP1 was expressed and found to retain DNA cleavage-religation activity and catalyze single-stranded DNA catenation. MtTOP1-704t was crystallized, and a structure of 2.52Å resolution limit was obtained. The structure of the MtTOP1 N-terminal domains has features that have not been observed in other previously available bacterial topoisomerase I crystal structures. The first C-terminal domain D5 forms a novel protein fold of a four-stranded antiparallel β-sheet stabilized by a crossing-over α-helix. Since there is only one type IA topoisomerase present in Mycobacteriaceae and related Actinobacteria, this subfamily of type IA topoisomerase may be required for multiple functions in DNA replication, transcription, recombination, and repair. The unique structural features observed for MtTOP1 may allow these topoisomerase I enzymes to carry out physiological functions associated with topoisomerase III enzyme in other bacteria.

An IPTG Inducible Conditional Expression System for Mycobacteria

Conditional expression strains serve as a valuable tool to study the essentiality and to establish the vulnerability of a target under investigation in a drug discovery program. While essentiality implies an absolute requirement of a target function, vulnerability provides valuable information on the extent to which a target function needs to be depleted to achieve bacterial growth inhibition followed by cell death. The critical feature of an ideal conditional expression system is its ability to tightly regulate gene expression to achieve the full spectrum spanning from a high level of expression in order to support growth and near zero level of expression to mimic conditions of gene knockout. A number of bacterial conditional expression systems have been reported for use in mycobacteria. The utility of an isopropylthiogalactoside (IPTG) inducible system in mycobacteria has been reported for protein overexpression and anti-sense gene expression from a replicating multi-copy plasmid. Herein, we report the development of a versatile set of non-replicating IPTG inducible vectors for mycobacteria which can be used for generation of conditional expression strains through homologous recombination. The role of a single lac operator versus a double lac operator to regulate gene expression was evaluated by monitoring the expression levels of β-galactosidase in Mycobacterium smegmatis. These studies indicated a significant level of leaky expression from the vector with a single lac operator but none from the vector with double lac operator. The significance of the double lac operator vector for target validation was established by monitoring the growth kinetics of an inhA, a rpoB and a ftsZ conditional expression strain grown in the presence of different concentrations of IPTG. The utility of this inducible system in identifying target specific inhibitors was established by screening a focussed library of small molecules using an inhA and a rpoB conditional expression strain.