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1.  The Crystal Structures of Apo and cAMP-Bound GlxR from Corynebacterium glutamicum Reveal Structural and Dynamic Changes upon cAMP Binding in CRP/FNR Family Transcription Factors 
PLoS ONE  2014;9(12):e113265.
The cyclic AMP-dependent transcriptional regulator GlxR from Corynebacterium glutamicum is a member of the super-family of CRP/FNR (cyclic AMP receptor protein/fumarate and nitrate reduction regulator) transcriptional regulators that play central roles in bacterial metabolic regulatory networks. In C. glutamicum, which is widely used for the industrial production of amino acids and serves as a non-pathogenic model organism for members of the Corynebacteriales including Mycobacterium tuberculosis, the GlxR homodimer controls the transcription of a large number of genes involved in carbon metabolism. GlxR therefore represents a key target for understanding the regulation and coordination of C. glutamicum metabolism. Here we investigate cylic AMP and DNA binding of GlxR from C. glutamicum and describe the crystal structures of apo GlxR determined at a resolution of 2.5 Å, and two crystal forms of holo GlxR at resolutions of 2.38 and 1.82 Å, respectively. The detailed structural analysis and comparison of GlxR with CRP reveals that the protein undergoes a distinctive conformational change upon cyclic AMP binding leading to a dimer structure more compatible to DNA-binding. As the two binding sites in the GlxR homodimer are structurally identical dynamic changes upon binding of the first ligand are responsible for the allosteric behavior. The results presented here show how dynamic and structural changes in GlxR lead to optimization of orientation and distance of its two DNA-binding helices for optimal DNA recognition.
doi:10.1371/journal.pone.0113265
PMCID: PMC4254451  PMID: 25469635
2.  Functional Dissection of Intersubunit Interactions in the EspR Virulence Regulator of Mycobacterium tuberculosis 
Journal of Bacteriology  2014;196(10):1889-1900.
The nucleoid-associated protein EspR, a chromosome organizer, has pleiotropic effects on expression of genes associated with cell wall function and pathogenesis in Mycobacterium tuberculosis. In particular, EspR binds to several sites upstream of the espACD locus to promote its expression, thereby ensuring full function of the ESX-1 secretion system, a major virulence determinant. The N terminus of EspR contains the helix-turn-helix DNA-binding domain, whereas the C-terminal dimerization domain harbors residues involved in intersubunit interactions. While direct binding to DNA appears to be mediated by an EspR dimer-of-dimers, where two helix-turn-helix motifs remain free for long-range interactions, the mechanism of EspR higher-order organization and its impact on chromosome structure and gene expression are not understood. To investigate these processes, we identified seven amino acid residues using molecular dynamics and replaced them with Ala in order to probe interactions at either the dimer or the dimer-of-dimers interfaces. Arg70, Lys72, and Arg101 were important for protein stability and optimal DNA-binding activity. Moreover, the Arg70 mutant showed decreased dimerization in a mycobacterial two-hybrid system. To correlate these defects with higher-order organization and transcriptional activity, we used atomic force microscopy to observe different EspR mutant proteins in complex with the espACD promoter region. In addition, complementation of an M. tuberculosis espR knockout mutant was performed to measure their impact on EspA expression. Our results pinpoint key residues required for EspR function at the dimer (Arg70) and the dimer-of-dimers (Lys72) interface and demonstrate that EspR dimerization and higher-order oligomerization modulate espACD transcriptional activity and hence pathogenesis.
doi:10.1128/JB.00039-14
PMCID: PMC4010998  PMID: 24633871
3.  Phenotypic Profiling of Mycobacterium tuberculosis EspA Point Mutants Reveals that Blockage of ESAT-6 and CFP-10 Secretion In Vitro Does Not Always Correlate with Attenuation of Virulence 
Journal of Bacteriology  2013;195(24):5421-5430.
The EspA protein of Mycobacterium tuberculosis is essential for the type VII ESX-1 protein secretion apparatus, which delivers the principal virulence factors ESAT-6 and CFP-10. In this study, site-directed mutagenesis of EspA was performed to elucidate its influence on the ESX-1 system. Replacing Trp55 (W55) or Gly57 (G57) residues in the putative W-X-G motif of EspA with arginines impaired ESAT-6 and CFP-10 secretion in vitro and attenuated M. tuberculosis. Replacing the Phe50 (F50) and Lys62 (K62) residues, which flank the W-X-G motif, with arginine and alanine, respectively, destabilized EspA, abolished ESAT-6 and CFP-10 secretion in vitro, and attenuated M. tuberculosis. Likewise, replacing the Phe5 (F5) and Lys41 (K41) residues with arginine and alanine, respectively, also destabilized EspA and blocked ESAT-6 and CFP-10 secretion in vitro. However, these two particular mutations did not attenuate M. tuberculosis in cellular models of infection or during acute infection in mice. We have thus identified amino acid residues in EspA that are important for facilitating ESAT-6 and CFP-10 secretion and virulence. However, our data also indicate for the first time that blockage of M. tuberculosis ESAT-6 and CFP-10 secretion in vitro and attenuation are mutually exclusive.
doi:10.1128/JB.00967-13
PMCID: PMC3889621  PMID: 24078612
4.  Towards a new combination therapy for tuberculosis with next generation benzothiazinones 
EMBO Molecular Medicine  2014;6(3):372-383.
The benzothiazinone lead compound, BTZ043, kills Mycobacterium tuberculosis by inhibiting the essential flavo-enzyme DprE1, decaprenylphosphoryl-beta-D-ribose 2-epimerase. Here, we synthesized a new series of piperazine-containing benzothiazinones (PBTZ) and show that, like BTZ043, the preclinical candidate PBTZ169 binds covalently to DprE1. The crystal structure of the DprE1-PBTZ169 complex reveals formation of a semimercaptal adduct with Cys387 in the active site and explains the irreversible inactivation of the enzyme. Compared to BTZ043, PBTZ169 has improved potency, safety and efficacy in zebrafish and mouse models of tuberculosis (TB). When combined with other TB drugs, PBTZ169 showed additive activity against M. tuberculosis in vitro except with bedaquiline (BDQ) where synergy was observed. A new regimen comprising PBTZ169, BDQ and pyrazinamide was found to be more efficacious than the standard three drug treatment in a murine model of chronic disease. PBTZ169 is thus an attractive drug candidate to treat TB in humans.
Subject Categories Microbiology, Virology & Host Pathogen Interaction; Pharmacology & Drug Discovery
doi:10.1002/emmm.201303575
PMCID: PMC3958311  PMID: 24500695
benzothiazinones; combination regimens; DprE1; tuberculosis
5.  Evolution of the Chalcone Isomerase Fold from Fatty Acid-Binding to Stereospecific Enzyme 
Nature  2012;485(7399):10.1038/nature11009.
Specialized metabolic enzymes biosynthesize chemicals of ecological importance, often sharing a pedigree with primary metabolic enzymes1. However, the lineage of the enzyme chalcone isomerase (CHI) remained a quandary. In vascular plants, CHI-catalyzed conversion of chalcones to chiral (S)-flavanones is a committed step in the production of plant flavonoids, compounds that contribute to attraction, defense2, and development3. CHI operates near the diffusion limit with stereospecific control4,5. While associated primarily with plants, the CHI-fold occurs in several other eukaryotic lineages and in some bacteria. Here we report crystal structures, ligand-binding properties, and in vivo functional characterization of a non-catalytic CHI-fold family from plants. A. thaliana contains five actively transcribed CHI-fold genes, three of which additionally encode amino-terminal chloroplast-transit sequences (cTP). These three CHI-fold proteins localize to plastids, the site of de novo fatty acid (FA) biosynthesis in plant cells. Furthermore, their expression profiles correlate with those of core FA biosynthetic enzymes, with maximal expression occurring in seeds and coinciding with increased FA storage in the developing embryo. In vitro, these proteins are Fatty Acid-binding Proteins (FAP). FAP knockout A. thaliana plants exhibit elevated alpha-linolenic acid levels and marked reproductive defects, including aberrant seed formation. Notably, the FAP discovery defines the adaptive evolution of a stereospecific and catalytically ‘perfected’ enzyme6 from a non-enzymatic ancestor over a defined period of plant evolution.
doi:10.1038/nature11009
PMCID: PMC3880581  PMID: 22622584
6.  Structural basis for benzothiazinone-mediated killing of Mycobacterium tuberculosis 
Science translational medicine  2012;4(150):150ra121.
BTZ043, a tuberculosis drug candidate with nanomolar whole-cell activity, targets the DprE1 enzyme of the essential decaprenylphosphoryl-β-D-ribofuranose-2′-epimerase thus blocking biosynthesis of arabinans, vital cell-wall components of mycobacteria. Crystal structures of DprE1, in its native form and in complex with BTZ043, unambiguously reveal formation of a semimercaptal adduct between the drug and an active-site cysteine, as well as contacts to a neighbouring catalytic lysine residue. Kinetic studies confirm BTZ043 as a mechanism-based, covalent inhibitor. This explains the exquisite potency of BTZ043, which, when fluorescently labelled, localizes DprE1 at the poles of growing bacteria. Menaquinone can reoxidize the FAD cofactor in DprE1 and may be the natural electron acceptor for this reaction in the cell. Our structural and kinetic analysis provides both insight into a critical epimerization reaction and a platform for structure-based design of improved inhibitors. Surprisingly, given the colossal tuberculosis burden globally, BTZ043 is the only new drug candidate to have been co-crystallized with its target.
doi:10.1126/scitranslmed.3004395
PMCID: PMC3659392  PMID: 22956199
7.  Towards a new tuberculosis drug: pyridomycin – nature's isoniazid 
EMBO Molecular Medicine  2012;4(10):1032-1042.
Tuberculosis, a global threat to public health, is becoming untreatable due to widespread drug resistance to frontline drugs such as the InhA-inhibitor isoniazid. Historically, by inhibiting highly vulnerable targets, natural products have been an important source of antibiotics including potent anti-tuberculosis agents. Here, we describe pyridomycin, a compound produced by Dactylosporangium fulvum with specific cidal activity against mycobacteria. By selecting pyridomycin-resistant mutants of Mycobacterium tuberculosis, whole-genome sequencing and genetic validation, we identified the NADH-dependent enoyl- (Acyl-Carrier-Protein) reductase InhA as the principal target and demonstrate that pyridomycin inhibits mycolic acid synthesis in M. tuberculosis. Furthermore, biochemical and structural studies show that pyridomycin inhibits InhA directly as a competitive inhibitor of the NADH-binding site, thereby identifying a new, druggable pocket in InhA. Importantly, the most frequently encountered isoniazid-resistant clinical isolates remain fully susceptible to pyridomycin, thus opening new avenues for drug development.
doi:10.1002/emmm.201201689
PMCID: PMC3491834  PMID: 22987724
drug discovery; InhA; isoniazid; pyridomycin; tuberculosis
8.  Characterization of Molecular Determinants of the Conformational Stability of Macrophage Migration Inhibitory Factor: Leucine 46 Hydrophobic Pocket 
PLoS ONE  2012;7(9):e45024.
Macrophage Migration Inhibitory Factor (MIF) is a key mediator of inflammatory responses and innate immunity and has been implicated in the pathogenesis of several inflammatory and autoimmune diseases. The oligomerization of MIF, more specifically trimer formation, is essential for its keto-enol tautomerase activity and probably mediates several of its interactions and biological activities, including its binding to its receptor CD74 and activation of certain signaling pathways. Therefore, understanding the molecular factors governing the oligomerization of MIF and the role of quaternary structure in modulating its structural stability and multifunctional properties is crucial for understanding the function of MIF in health and disease. Herein, we describe highly conserved intersubunit interactions involving the hydrophobic packing of the side chain of Leu46 onto the β-strand β3 of one monomer within a hydrophobic pocket from the adjacent monomer constituted by residues Arg11, Val14, Phe18, Leu19, Val39, His40, Val41, Val42, and Pro43. To elucidate the structural significance of these intersubunit interactions and their relative contribution to MIF’s trimerization, structural stability and catalytic activity, we generated three point mutations where Leu46 was replaced by glycine (L46G), alanine (L46A) and phenylalanine (L46F), and their structural properties, stability, oligomerization state, and catalytic activity were characterized using a battery of biophysical methods and X-ray crystallography. Our findings provide new insights into the role of the Leu46 hydrophobic pocket in stabilizing the conformational state of MIF in solution. Disrupting the Leu46 hydrophobic interaction perturbs the secondary and tertiary structure of the protein but has no effect on its oligomerization state.
doi:10.1371/journal.pone.0045024
PMCID: PMC3448610  PMID: 23028743
9.  EspD Is Critical for the Virulence-Mediating ESX-1 Secretion System in Mycobacterium tuberculosis 
Journal of Bacteriology  2012;194(4):884-893.
ESAT-6 system 1 (ESX-1)-mediated secretion in Mycobacterium tuberculosis is dependent on proteins encoded by the cotranscribed espA-espC-espD gene cluster. While the roles of EspA and EspC with respect to the ESX-1 secretion system have been actively investigated, the function of EspD remains unknown. We show that EspD is secreted by M. tuberculosis, but unlike EspA and EsxA, its export does not exclusively require the ESX-1 system. Evidence for stabilization of cellular levels of EspA and EspC by EspD is presented, and depletion of EspD results in loss of EsxA secretion. Site-directed mutagenesis of EspD reveals that its role in the maintenance of cellular levels of EspA in M. tuberculosis is distinct from its facilitation of EsxA secretion. The same mutagenesis experiments have also shown that secretion of EspD is not required for the secretion of EsxA. Our findings highlight a critical and complex role for EspD in modulating the ESX-1 secretion system in M. tuberculosis.
doi:10.1128/JB.06417-11
PMCID: PMC3272943  PMID: 22155774
10.  Virulence Regulator EspR of Mycobacterium tuberculosis Is a Nucleoid-Associated Protein 
PLoS Pathogens  2012;8(3):e1002621.
The principal virulence determinant of Mycobacterium tuberculosis (Mtb), the ESX-1 protein secretion system, is positively controlled at the transcriptional level by EspR. Depletion of EspR reportedly affects a small number of genes, both positively or negatively, including a key ESX-1 component, the espACD operon. EspR is also thought to be an ESX-1 substrate. Using EspR-specific antibodies in ChIP-Seq experiments (chromatin immunoprecipitation followed by ultra-high throughput DNA sequencing) we show that EspR binds to at least 165 loci on the Mtb genome. Included in the EspR regulon are genes encoding not only EspA, but also EspR itself, the ESX-2 and ESX-5 systems, a host of diverse cell wall functions, such as production of the complex lipid PDIM (phenolthiocerol dimycocerosate) and the PE/PPE cell-surface proteins. EspR binding sites are not restricted to promoter regions and can be clustered. This suggests that rather than functioning as a classical regulatory protein EspR acts globally as a nucleoid-associated protein capable of long-range interactions consistent with a recently established structural model. EspR expression was shown to be growth phase-dependent, peaking in the stationary phase. Overexpression in Mtb strain H37Rv revealed that EspR influences target gene expression both positively or negatively leading to growth arrest. At no stage was EspR secreted into the culture filtrate. Thus, rather than serving as a specific activator of a virulence locus, EspR is a novel nucleoid-associated protein, with both architectural and regulatory roles, that impacts cell wall functions and pathogenesis through multiple genes.
Author Summary
A major infection mechanism employed by the causative agent of tuberculosis, Mycobacterium tuberculosis (Mtb), is the ESX-1 secretion system. It has been postulated that the DNA-binding protein EspR controls the virulence of Mtb by specifically regulating expression of the exported EspA protein, which is required for ESX-1 to function. Previous structural studies indicated that EspR forms dimers capable of multimerizing on DNA and forming loop structures, thus bringing together otherwise distant chromosomal regions. Such characteristics are reminiscent of nucleoid-associated proteins (NAPs), the histone equivalent in bacteria. Here we use ChIP-Seq technology to map EspR binding sites on the Mtb chromosome in living bacterial cells. Genome-wide analysis of EspR identified hundreds of binding-sites, with almost equal inter- and intra-genic distribution, and mostly found in proximity to genes associated with cell wall function. We validated a subset of EspR-binding sites experimentally and identified a consensus motif required for optimal binding affinity. Moreover, our study reveals that EspR expression varies with bacterial growth and that intracellular levels are not linked to EspR secretion. These findings corroborate the NAP nature of EspR and its dual roles, architectural and regulatory, that impact the Mtb chromosome and pathogenesis globally rather than the ESX-1 loci specifically.
doi:10.1371/journal.ppat.1002621
PMCID: PMC3315491  PMID: 22479184
11.  Structure and function of the transketolase from Mycobacterium tuberculosis and comparison with the human enzyme 
Open Biology  2012;2(1):110026.
The transketolase (TKT) enzyme in Mycobacterium tuberculosis represents a novel drug target for tuberculosis treatment and has low homology with the orthologous human enzyme. Here, we report on the structural and kinetic characterization of the transketolase from M. tuberculosis (TBTKT), a homodimer whose monomers each comprise 700 amino acids. We show that TBTKT catalyses the oxidation of donor sugars xylulose-5-phosphate and fructose-6-phosphate as well as the reduction of the acceptor sugar ribose-5-phosphate. An invariant residue of the TKT consensus sequence required for thiamine cofactor binding is mutated in TBTKT; yet its catalytic activities are unaffected, and the 2.5 Å resolution structure of full-length TBTKT provides an explanation for this. Key structural differences between the human and mycobacterial TKT enzymes that impact both substrate and cofactor recognition and binding were uncovered. These changes explain the kinetic differences between TBTKT and its human counterpart, and their differential inhibition by small molecules. The availability of a detailed structural model of TBTKT will enable differences between human and M. tuberculosis TKT structures to be exploited to design selective inhibitors with potential antitubercular activity.
doi:10.1098/rsob.110026
PMCID: PMC3352088  PMID: 22645655
transketolase; Mycobacterium tuberculosis; X-ray crystallography; pentose pathway; enzyme kinetics
12.  Sigma Factor F Does Not Prevent Rifampin Inhibition of RNA Polymerase or Cause Rifampin Tolerance in Mycobacterium tuberculosis▿  
Journal of Bacteriology  2010;192(20):5472-5479.
The tolerance of Mycobacterium tuberculosis to antituberculosis drugs is a major reason for the lengthy therapy needed to treat a tuberculosis infection. Rifampin is a potent inhibitor of RNA polymerase (RNAP) in vivo but has been shown to be less effective against stationary-phase bacteria. Sigma factor F is associated with bacteria entering stationary phase and has been proposed to impact rifampin activity. Here we investigate whether RNAP containing SigF is more resistant to rifampin inhibition in vitro and whether overexpression of sigF renders M. tuberculosis more tolerant to rifampin. Real-time and radiometric in vitro transcription assays revealed that rifampin equally inhibits transcription by RNAP containing sigma factors SigA and SigF, therefore ruling out the hypothesis that SigF may be responsible for increased resistance of the enzyme to rifampin in vitro. In addition, overexpression or deletion of sigF did not alter rifampin susceptibility in axenic cultures of M. tuberculosis, indicating that SigF does not affect rifampin tolerance in vivo.
doi:10.1128/JB.00687-10
PMCID: PMC2950495  PMID: 20729364
13.  Discovery and characterization of a marine bacterial SAM-dependent chlorinase 
Nature chemical biology  2007;4(1):69-74.
Halogen atom incorporation into a scaffold of bioactive compounds often amplifies biological activity, as is the case for the anticancer agent salinosporamide A (1), a chlorinated natural product from the marine bacterium Salinispora tropica. Significant effort in understanding enzymatic chlorination shows that oxidative routes predominate to form reactive electrophilic or radical chlorine species. Here we report the genetic, biochemical and structural characterization of the chlorinase SalL, which halogenates S-adenosyl-l-methionine (2) with chloride to generate 5′-chloro-5′-deoxyadenosine (3) and l-methionine (4) in a rarely observed nucleophilic substitution strategy analogous to that of Streptomyces cattleya fluorinase. Further metabolic tailoring produces a halogenated polyketide synthase substrate specific for salinosporamide A biosynthesis. SalL also accepts bromide and iodide as substrates, but not fluoride. High-resolution crystal structures of SalL and active site mutants complexed with substrates and products support the SN2 nucleophilic substitution mechanism and further illuminate halide specificity in this newly discovered halogenase family.
doi:10.1038/nchembio.2007.56
PMCID: PMC2762381  PMID: 18059261
14.  Rapid synthesis of auxin via a new tryptophan-dependent pathway is required for shade avoidance in plants 
Cell  2008;133(1):164-176.
SUMMARY
Plants grown at high densities perceive a decrease in the red to far-red (R:FR) ratio of incoming light, resulting from absorption of red light by canopy leaves and reflection of far-red light from neighboring plants. These changes in light quality trigger a series of responses known collectively as the shade avoidance syndrome. During shade avoidance, stems elongate at the expense of leaf and storage organ expansion, branching is inhibited, and flowering is accelerated. We identified several loci in Arabidopsis, mutations in which lead to plants defective in multiple shade avoidance outputs. Here we describe SAV3, an aminotransferase, and show that SAV3 catalyzes the formation of indole-3-pyruvic acid (IPA) from L-tryptophan (L-Trp), the first step in a previously proposed, but uncharacterized, auxin biosynthetic pathway. This pathway is rapidly deployed to biosynthesize auxin at the high levels required to initiate the multiple changes in body plan associated with shade avoidance.
doi:10.1016/j.cell.2008.01.049
PMCID: PMC2442466  PMID: 18394996
15.  Absence of Substrate Channeling between Active Sites in the Agrobacterium tumefaciens IspDF and IspE Enzymes of the Methyl Erythritol Phosphate Pathway† 
Biochemistry  2006;45(11):3548-3553.
The conversion of 2C-methyl-d-erythritol 4-phosphate (MEP) to 2C-methyl-d-erythritol 2,4-cyclodiphosphate (cMEDP) in the MEP entry into the isoprenoid biosynthetic pathway occurs in three consecutive steps catalyzed by the IspD, IspE, and IspF enzymes, respectively. In Agrobacterium tumefaciens the ispD and ispF genes are fused to encode a bifunctional enzyme that catalyzes the first (synthesis of 4-diphosphocytidyl-2-C-methyl d-erythritol) and third (synthesis of 2C-methyl-d-erythritol 2,4-cyclodiphosphate) steps. Sedimentation velocity experiments indicate that the bifunctional IspDF enzyme and the IspE protein associate in solution raising the possibility of substrate channeling among the active sites in these two proteins. Kinetic evidence for substrate channeling was sought by measuring the time courses for product formation during incubations of MEP, CTP, and ATP with the IspDF and IspE proteins with and without an excess of the inactive IspE (D152A) mutant in presence or absence of 30% (v/v) glycerol. The time dependencies indicate that the enzyme-generated intermediates are not transferred from the IspD active site in IspDF to the active site of IspE or from the active site in IspE to the active site in the IspF module of IspDF.
doi:10.1021/bi0520075
PMCID: PMC2516919  PMID: 16533036
bifunctional; IspDF; IspE; non-channeling
16.  Crystallization and preliminary X-ray analysis of the aromatic prenyltransferase CloQ from the clorobiocin biosynthetic cluster of Streptomyces roseochromogenes  
An aromatic prenyltransferase (CloQ) from S. roseochromogenes that is implicated in clorobiocin biosynthesis has been crystallized in space group I4122. X-ray data to 2.2 Å resolution were collected in-house.
Crystals of recombinant CloQ (subunit MW = 35 626 Da; 324 amino acids), an aromatic prenyltransferase from Streptomyces roseochromogenes, were grown by vapour diffusion. The protein crystallizes in space group I4122, with unit-cell parameters a = b = 135.19, c = 98.13 Å. Native data from a single crystal were recorded to a resolution of 2.2 Å in-house. Preliminary analysis of these data indicated that the asymmetric unit corresponds to a monomer, giving an estimated solvent content of 60.6%. CloQ is involved in the biosynthesis of the aminocoumarin antibiotic clorobiocin, which targets the essential bacterial enzyme DNA gyrase.
doi:10.1107/S1744309106042527
PMCID: PMC2225205  PMID: 17077503
CloQ; prenyltransferases; Streptomyces; clorobiocin; antibiotic biosynthesis

Results 1-16 (16)