Structural insights into bacterial resistance to cerulenin

Optimizing Archaeal Lipid Biosynthesis in Escherichia coli

Membrane lipid chemistry is remarkably different in archaea compared with bacteria and eukaryotes. In the evolutionary context, this is also termed the lipid divide and is reflected by distinct biosynthetic pathways. Contemporary organisms have almost without exception only one type of membrane lipid. During early membrane evolution, mixed membrane stages likely occurred, and it was hypothesized that the instability of such mixtures was the driving force for the lipid divide. To examine the compatibility between archaeal and bacterial lipids, the bacterium Escherichia coli has been engineered to contain both types of lipids with varying success. Only limited production of archaeal lipid archaetidylethanolamine was achieved. Here, we substantially increased its production in E. coli by overexpression of an archaeal phosphatidylserine synthase needed for ethanolamine headgroup attachment. Furthermore, we introduced a synthetic isoprenoid utilization pathway to increase the supply of isopentenyl-diphosphate and dimethylallyl diphosphate. This improved archaeal lipid production substantially. The archaeal phospholipids also served as a substrate for the E. coli cardiolipin synthase, resulting in archaeal and novel hybrid archaeal/bacterial cardiolipin species not seen in living organisms before. Growth of the E. coli strain with the mixed membrane shows an enhanced sensitivity to the inhibitor of fatty acid biosynthesis, cerulenin, indicating a critical dependence of the engineered E. coli strain on its native phospholipids.

An essential host dietary fatty acid promotes TcpH inhibition of TcpP proteolysis promoting virulence gene expression in Vibrio cholerae

Vibrio cholerae is a Gram-negative gastrointestinal pathogen responsible for the diarrheal disease cholera. Expression of key virulence factors, cholera toxin and toxin-coregulated pilus, is regulated directly by ToxT and indirectly by two transmembrane transcription regulators (TTRs), ToxR and TcpP, that promote the expression of toxT. TcpP abundance and activity are controlled by TcpH, a single-pass transmembrane protein, which protects TcpP from a two-step proteolytic process known as regulated intramembrane proteolysis (RIP). The mechanism of TcpH-mediated protection of TcpP represents a major gap in our understanding of V. cholerae pathogenesis. The absence of tcpH leads to unimpeded degradation of TcpP in vitro and a colonization defect in a neonate mouse model of V. cholerae colonization. Here, we show that TcpH protects TcpP from RIP via direct interaction. We also demonstrate that α-linolenic acid, a dietary fatty acid, promotes TcpH-dependent inhibition of RIP via co-association of TcpP and TcpH molecules within detergent-resistant membranes (DRMs) in a mechanism requiring the TcpH transmembrane domain. Taken together, our data support a model where V. cholerae cells use exogenous α-linolenic acid to remodel the phospholipid bilayer in vivo, leading to co-association of TcpP and TcpH within DRMs where RIP of TcpP is inhibited by TcpH, thereby promoting V. cholerae pathogenicity.

IMPORTANCE

Vibrio cholerae continues to pose a significant global burden on health and an alternative therapeutic approach is needed, due to evolving multidrug resistance strains. Transcription of toxT, stimulated by TcpP and ToxR, is essential for V. cholerae pathogenesis. Our results show that TcpP, one of the major regulators of toxT gene expression, is protected from proteolysis by TcpH, via direct interaction. Furthermore, we identified a gut metabolite, α-linolenic acid, that stimulates the co-association of TcpP and TcpH within detergent-resistant membranes (also known as lipid-ordered membrane domains), thereby supporting TcpH-dependent antagonism of TcpP proteolysis. Data presented here extend our knowledge of RIP, virulence gene regulation in V. cholerae, and, to the best of our knowledge, provides the first evidence that lipid-ordered membranes exist within V. cholerae. The model presented here also suggests that TTRs, common among bacteria and archaea, and co-component signal transduction systems present in Enterobacteria, could also be influenced similarly.

Cryptic Cerulenin Rearrangement in Ketosynthase Covalent Inhibition

The antibiotic cerulenin is a fungal natural product identified as a covalent inhibitor of ketosynthases within fatty acid and polyketide biosynthesis. Due to its selective and potent inhibitory activity, cerulenin has found significant utility in multidisciplinary biochemical, biomedical, and clinical studies. Although its covalent inhibition profile has been confirmed, cerulenin's mechanism has not been fully determined at a molecular level, frustrating the drug development of related analogues. Herein, we describe the use of stable isotopic tracking with NMR and MS methods to unravel the covalent mechanism of cerulenin against type II fatty acid ketosynthases. We detail the discovery of a unique C2-C3 retro-aldol bond cleavage and a structural rearrangement upon covalent inhibition of cerulenin at the active cysteine residue in E. coli type II fatty acid ketosynthases FabB and FabF.

An inhibitory mechanism of AasS, an exogenous fatty acid scavenger: Implications for re-sensitization of FAS II antimicrobials

Antimicrobial resistance is an ongoing "one health" challenge of global concern. The acyl-ACP synthetase (termed AasS) of the zoonotic pathogen Vibrio harveyi recycles exogenous fatty acid (eFA), bypassing the requirement of type II fatty acid synthesis (FAS II), a druggable pathway. A growing body of bacterial AasS-type isoenzymes compromises the clinical efficacy of FAS II-directed antimicrobials, like cerulenin. Very recently, an acyl adenylate mimic, C10-AMS, was proposed as a lead compound against AasS activity. However, the underlying mechanism remains poorly understood. Here we present two high-resolution cryo-EM structures of AasS liganded with C10-AMS inhibitor (2.33 Å) and C10-AMP intermediate (2.19 Å) in addition to its apo form (2.53 Å). Apart from our measurements for C10-AMS' Ki value of around 0.6 μM, structural and functional analyses explained how this inhibitor interacts with AasS enzyme. Unlike an open state of AasS, ready for C10-AMP formation, a closed conformation is trapped by the C10-AMS inhibitor. Tight binding of C10-AMS blocks fatty acyl substrate entry, and therefore inhibits AasS action. Additionally, this intermediate analog C10-AMS appears to be a mixed-type AasS inhibitor. In summary, our results provide the proof of principle that inhibiting salvage of eFA by AasS reverses the FAS II bypass. This facilitates the development of next-generation anti-bacterial therapeutics, esp. the dual therapy consisting of C10-AMS scaffold derivatives combined with certain FAS II inhibitors.

Design, quality and validation of the EU-OPENSCREEN fragment library poised to a high-throughput screening collection

The EU-OPENSCREEN (EU-OS) European Research Infrastructure Consortium (ERIC) is a multinational, not-for-profit initiative that integrates high-capacity screening platforms and chemistry groups across Europe to facilitate research in chemical biology and early drug discovery. Over the years, the EU-OS has assembled a high-throughput screening compound collection, the European Chemical Biology Library (ECBL), that contains approximately 100 000 commercially available small molecules and a growing number of thousands of academic compounds crowdsourced through our network of European and non-European chemists. As an extension of the ECBL, here we describe the computational design, quality control and use case screenings of the European Fragment Screening Library (EFSL) composed of 1056 mini and small chemical fragments selected from a substructure analysis of the ECBL. Access to the EFSL is open to researchers from both academia and industry. Using EFSL, eight fragment screening campaigns using different structural and biophysical methods have successfully identified fragment hits in the last two years. As one of the highlighted projects for antibiotics, we describe the screening by Bio-Layer Interferometry (BLI) of the EFSL, the identification of a 35 μM fragment hit targeting the beta-ketoacyl-ACP synthase 2 (FabF), its binding confirmation to the protein by X-ray crystallography (PDB 8PJ0), its subsequent rapid exploration of its surrounding chemical space through hit-picking of ECBL compounds that contain the fragment hit as a core substructure, and the final binding confirmation of two follow-up hits by X-ray crystallography (PDB 8R0I and 8R1V).

Diversity in fatty acid elongation enzymes: The FabB long-chain β-ketoacyl-ACP synthase I initiates fatty acid synthesis in Pseudomonas putida F1

The condensation of acetyl-CoA with malonyl-acyl carrier protein (ACP) by β-ketoacyl-ACP synthase III (KAS III, FabH) and decarboxylation of malonyl-ACP by malonyl-ACP decarboxylase are the two pathways that initiate bacterial fatty acid synthesis (FAS) in Escherichia coli. In addition to these two routes, we report that Pseudomonas putida F1 β-ketoacyl-ACP synthase I (FabB), in addition to playing a key role in fatty acid elongation, also initiates FAS in vivo. We report that although two P. putida F1 fabH genes (PpfabH1 and PpfabH2) both encode functional KAS III enzymes, neither is essential for growth. PpFabH1 is a canonical KAS III similar to E. coli FabH whereas PpFabH2 catalyzes condensation of malonyl-ACP with short- and medium-chain length acyl-CoAs. Since these two KAS III enzymes are not essential for FAS in P. putida F1, we sought the P. putida initiation enzyme and unexpectedly found that it was FabB, the elongation enzyme of the oxygen-independent unsaturated fatty acid pathway. P. putida FabB decarboxylates malonyl-ACP and condenses the acetyl-ACP product with malonyl-ACP for initiation of FAS. These data show that P. putida FabB, unlike the paradigm E. coli FabB, can catalyze the initiation reaction in FAS.

Masked cerulenin enables a dual-site selective protein crosslink

Protein-reactive natural products such as the fungal metabolite cerulenin are recognized for their value as therapeutic candidates, due to their ability to selectively react with catalytic residues within a protein active site or a complex of protein domains. Here, we explore the development of fatty-acid and polyketide-synthase probes by synthetically modulating cerulenin's functional moieties. Using a mechanism-based approach, we reveal unique reactivity within cerulenin and adapt it for fluorescent labeling and crosslinking of fatty-acid and iterative type-I polyketide synthases. We also describe two new classes of silylcyanohydrin and silylhemiaminal masked crosslinking probes that serve as new tools for activity and structure studies of these biosynthetic pathways.

Enzymology of standalone elongating ketosynthases

The β-ketoacyl-acyl carrier protein synthase, or ketosynthase (KS), catalyses carbon-carbon bond formation in fatty acid and polyketide biosynthesis via a decarboxylative Claisen-like condensation. In prokaryotes, standalone elongating KSs interact with the acyl carrier protein (ACP) which shuttles substrates to each partner enzyme in the elongation cycle for catalysis. Despite ongoing research for more than 50 years since KS was first identified in E. coli, the complex mechanism of KSs continues to be unravelled, including recent understanding of gating motifs, KS-ACP interactions, substrate recognition and delivery, and roles in unsaturated fatty acid biosynthesis. In this review, we summarize the latest studies, primarily conducted through structural biology and molecular probe design, that shed light on the emerging enzymology of standalone elongating KSs.

Strategies To Design Selective Histone Deacetylase Inhibitors

This review classifies drug-design strategies successfully implemented in the development of histone deacetylase (HDAC) inhibitors, which have many applications including cancer treatment. Our focus is on especially demanded selective HDAC inhibitors and their structure-activity relationships in relation to corresponding protein structures. The main part of the paper is divided into six subsections each narrating how optimization of one of six structural features can influence inhibitor selectivity. It starts with the impact of the zinc binding group on selectivity, continues with the optimization of the linker placed in the substrate binding tunnel as well as the adjustment of the cap group interacting with the surface of the protein, and ends with the addition of groups targeting class-specific sub-pockets: the side-pocket-, lower-pocket- and foot-pocket-targeting groups. The review is rounded off with a conclusion and an outlook on the future of HDAC inhibitor design.

Chemical Highlights Supporting the Role of Lipid A in Efficient Biological Adaptation of Gram-Negative Bacteria to External Stresses

The outer membrane (OM) of Gram-negative bacteria provides an efficient barrier against external noxious compounds such as antimicrobial agents. Associated with drug target modification, it contributes to the overall failure of chemotherapy. In the complex OM architecture, Lipid A plays an essential role by anchoring the lipopolysaccharide in the membrane and ensuring the spatial organization between lipids, proteins, and sugars. Currently, the targets of almost all antibiotics are intracellularly located and require translocation across membranes. We report herein an integrated view of Lipid A synthesis, membrane assembly, a structure comparison at the molecular structure level of numerous Gram-negative bacterial species, as well as its recent use as a target for original antibacterial molecules. This review paves the way for a new vision of a key membrane component that acts during bacterial adaptation to environmental stresses and for the development of new weapons against microbial resistance to usual antibiotics.

Structural Basis for Enzymatic Off-Loading of Hybrid Polyketides by Dieckmann Condensation

While several bioactive natural products that contain tetramate or pyridone heterocycles have been described, information on the enzymology underpinning these functionalities has been limited. Here we biochemically characterize an off-loading Dieckmann cyclase, NcmC, that installs the tetramate headgroup in nocamycin, a hybrid polyketide/nonribosomal peptide natural product. Crystal structures of the enzyme (1.6 Å) and its covalent complex with the epoxide cerulenin (1.6 Å) guide additional structure-based mutagenesis and product-profile analyses. Our results offer mechanistic insights into how the conserved thioesterase-like scaffold has been adapted to perform a new chemical reaction, namely, heterocyclization. Additional bioinformatics combined with docking and modeling identifies likely candidates for heterocycle formation in underexplored gene clusters and uncovers a modular basis of substrate recognition by the two subdomains of these Dieckmann cyclases.

Activity Mapping the Acyl Carrier Protein: Elongating Ketosynthase Interaction in Fatty Acid Biosynthesis

Elongating ketosynthases (KSs) catalyze carbon-carbon bond-forming reactions during the committed step for each round of chain extension in both fatty acid synthases (FASs) and polyketide synthases (PKSs). A small α-helical acyl carrier protein (ACP) shuttles fatty acyl intermediates between enzyme active sites. To accomplish this task, the ACP relies on a series of dynamic interactions with multiple partner enzymes of FAS and associated FAS-dependent pathways. Recent structures of the Escherichia coli FAS ACP, AcpP, in covalent complexes with its two cognate elongating KSs, FabF and FabB, provide high-resolution details of these interfaces, but a systematic analysis of specific interfacial interactions responsible for stabilizing these complexes has not yet been undertaken. Here, we use site-directed mutagenesis with both in vitro and in vivo activity analyses to quantitatively evaluate these contacting surfaces between AcpP and FabF. We delineate the FabF interface into three interacting regions and demonstrate the effects of point mutants, double mutants, and region deletion variants. Results from these analyses reveal a robust and modular FabF interface capable of tolerating seemingly critical interface mutations with only the deletion of an entire region significantly compromising activity. Structure and sequence analyses of FabF orthologs from related type II FAS pathways indicate significant conservation of type II FAS KS interface residues and, overall, support its delineation into interaction regions. These findings strengthen our mechanistic understanding of molecular recognition events between ACPs and FAS enzymes and provide a blueprint for engineering ACP-dependent biosynthetic pathways.

Structural basis for selectivity in a highly reducing type II polyketide synthase

In type II polyketide synthases (PKSs), the ketosynthase-chain length factor (KS-CLF) complex catalyzes polyketide chain elongation with the acyl carrier protein (ACP). Highly reducing type II PKSs, represented by IgaPKS, produce polyene structures instead of the well-known aromatic skeletons. Here, we report the crystal structures of the Iga11-Iga12 (KS-CLF) heterodimer and the covalently cross-linked Iga10=Iga11-Iga12 (ACP=KS-CLF) tripartite complex. The latter structure revealed the molecular basis of the interaction between Iga10 and Iga11-Iga12, which differs from that between the ACP and KS of Escherichia coli fatty acid synthase. Furthermore, the reaction pocket structure and site-directed mutagenesis revealed that the negative charge of Asp 113 of Iga11 prevents further condensation using a β-ketoacyl product as a substrate, which distinguishes IgaPKS from typical type II PKSs. This work will facilitate the future rational design of PKSs.

Gating mechanism of elongating β-ketoacyl-ACP synthases

Carbon-carbon bond forming reactions are essential transformations in natural product biosynthesis. During de novo fatty acid and polyketide biosynthesis, β-ketoacyl-acyl carrier protein (ACP) synthases (KS), catalyze this process via a decarboxylative Claisen-like condensation reaction. KSs must recognize multiple chemically distinct ACPs and choreograph a ping-pong mechanism, often in an iterative fashion. Here, we report crystal structures of substrate mimetic bearing ACPs in complex with the elongating KSs from Escherichia coli, FabF and FabB, in order to better understand the stereochemical features governing substrate discrimination by KSs. Complemented by molecular dynamics (MD) simulations and mutagenesis studies, these structures reveal conformational states accessed during KS catalysis. These data taken together support a gating mechanism that regulates acyl-ACP binding and substrate delivery to the KS active site. Two active site loops undergo large conformational excursions during this dynamic gating mechanism and are likely evolutionarily conserved features in elongating KSs.

Colorimetric Assay Reports on Acyl Carrier Protein Interactions

The ability to produce new molecules of potential pharmaceutical relevance via combinatorial biosynthesis hinges on improving our understanding of acyl-carrier protein (ACP)-protein interactions. However, the weak and transient nature of these interactions makes them difficult to study using traditional spectroscopic approaches. Herein we report that converting the terminal thiol of the E. coli ACP 4'-phosphopantetheine arm into a mixed disulfide with 2-nitro-5-thiobenzoate ion (TNB-) activates this site to form a selective covalent cross-link with the active site cysteine of a cognate ketoacyl synthase (KS). The concomitant release of TNB2-, which absorbs at 412 nm, provides a visual and quantitative measure of mechanistically relevant ACP-KS interactions. The colorimetric assay can propel the engineering of biosynthetic routes to novel chemical diversity by providing a high-throughput screen for functional hybrid ACP-KS partnerships as well as the discovery of novel antimicrobial agents by enabling the rapid identification of small molecule inhibitors of ACP-KS interactions.

Isolation of colonization-defective Escherichia coli mutants reveals critical requirement for fatty acids in bacterial colony formation

Most bacterial cells in nature exhibit extremely low colony-forming activity, despite showing various signs of viability, impeding the isolation and utilization of many bacterial resources. However, the general causes responsible for this state of low colony formation are largely unknown. Because liquid cultivation typically yields more bacterial cell cultures than traditional solid cultivation, we hypothesized that colony formation requires one or more specific gene functions that are dispensable or less important for growth in liquid media. To verify our hypothesis and reveal the genetic background limiting colony formation among bacteria in nature, we isolated Escherichia coli mutants that had decreased frequencies of colony formation but could grow in liquid medium from a temperature-sensitive mutant collection. Mutations were identified in fabB, which is essential for the synthesis of long unsaturated fatty acids. We then constructed a fabB deletion mutant in a wild-type background. Detailed behavioural analysis of the mutant revealed that under fatty acid-limited conditions, colony formation on solid media was more sensitively and seriously impaired than growth in liquid media. Furthermore, growth under partial inhibition of fatty acid synthesis with cerulenin or triclosan brought about similar phenotypes, not only in E. coli but also in Bacillus subtilis and Corynebacterium glutamicum. These results indicate that fatty acids have a critical importance in colony formation and that depletion of fatty acids in the environment partly accounts for the low frequency of bacterial colony formation.

N-Acylated Derivatives of Sulfamethoxazole Block Chlamydia Fatty Acid Synthesis and Interact with FabF

The type II fatty acid synthesis (FASII) pathway is essential for bacterial lipid biosynthesis and continues to be a promising target for novel antibacterial compounds. Recently, it has been demonstrated that Chlamydia is capable of FASII and this pathway is indispensable for Chlamydia growth. Previously, a high-content screen with Chlamydia trachomatis-infected cells was performed, and acylated sulfonamides were identified to be potent growth inhibitors of the bacteria. C. trachomatis strains resistant to acylated sulfonamides were isolated by serial passage of a wild-type strain in the presence of low compound concentrations. Results from whole-genome sequencing of 10 isolates from two independent drug-resistant populations revealed that mutations that accumulated in fabF were predominant. Studies of the interaction between the FabF protein and small molecules showed that acylated sulfonamides directly bind to recombinant FabF in vitro and treatment of C. trachomatis-infected HeLa cells with the compounds leads to a decrease in the synthesis of Chlamydia fatty acids. This work demonstrates the importance of FASII for Chlamydia development and may lead to the development of new antimicrobials.

Malonyl-CoA pathway: a promising route for 3-hydroxypropionate biosynthesis

3-Hydroxypropionate (3HP) is an attractive platform chemical, serving as a precursor to a variety of commodity chemicals like acrylate and acrylamide, as well as a monomer of a biodegradable plastic. To establish a sustainable way to produce these commercially important chemicals and materials, fermentative production of 3HP is widely investigated in recent years. It is reported that 3HP can be produced from several intermediates, such as glycerol, malonyl-CoA, and β-alanine. Among all these biosynthetic routes, the malonyl-CoA pathway has some distinct advantages, including a broad feedstock spectrum, thermodynamic feasibility, and redox neutrality. To date, this pathway has been successfully constructed in various species including Escherichia coli, yeast and cyanobacteria, and optimized through carbon flux redirection, enzyme screening and engineering, and an increasing supply of energy and cofactors, resulting in significantly enhanced 3HP titer up to 40 g/L. These results show the feasibility of commercial manufacturing of 3HP and its derivatives in the future.

Structural characterisation of the fatty acid biosynthesis enzyme FabF from the pathogen Listeria monocytogenes

Development of new antimicrobial agents is required against the causative agent for listeriosis, Listeria monocytogenes, as the number of drug resistant strains continues to increase. A promising target is the β-ketoacyl-acyl carrier protein synthase FabF, which participates in the catalysis of fatty acid synthesis and elongation, and is required for the production of phospholipid membranes, lipoproteins, and lipopolysaccharides. In this study, we report the 1.35 Å crystal structure of FabF from L. monocytogenes, providing an excellent platform for the rational design of novel inhibitors. By comparing the structure of L. monocytogenes FabF with other published bacterial FabF structures in complex with known inhibitors and substrates, we highlight conformational changes within the active site, which will need to be accounted for during drug design and virtual screening studies. This high-resolution structure of FabF represents an important step in the development of new classes of antimicrobial agents targeting FabF for the treatment of listeriosis.

The stringent response plays a key role in Bacillus subtilis survival of fatty acid starvation

The stringent response is a universal adaptive mechanism to protect bacteria from nutritional and environmental stresses. The role of the stringent response during lipid starvation has been studied only in Gram-negative bacteria. Here, we report that the stringent response also plays a crucial role in the adaptation of the model Gram-positive Bacillus subtilis to fatty acid starvation. B. subtilis lacking all three (p)ppGpp-synthetases (Rel_Bs , RelP and RelQ) or bearing a Rel_Bs variant that no longer synthesizes (p)ppGpp suffer extreme loss of viability on lipid starvation. Loss of viability is paralleled by perturbation of membrane integrity and function, with collapse of membrane potential as the likely cause of death. Although no increment of (p)ppGpp could be detected in lipid starved B. subtilis, we observed a substantial increase in the GTP/ATP ratio of strains incapable of synthesizing (p)ppGpp. Artificially lowering GTP with decoyinine rescued viability of such strains, confirming observations that low intracellular GTP is important for survival of nutritional stresses. Altogether, our results show that activation of the stringent response by lipid starvation is a broadly conserved response of bacteria and that a key role of (p)ppGpp is to couple biosynthetic processes that become detrimental if uncoordinated.

DHA Production in Escherichia coli by Expressing Reconstituted Key Genes of Polyketide Synthase Pathway from Marine Bacteria

The gene encoding phosphopantetheinyl transferase (PPTase), pfaE, a component of the polyketide synthase (PKS) pathway, is crucial for the production of docosahexaenoic acid (DHA, 22:6ω3), along with the other pfa cluster members pfaA, pfaB, pfaC and pfaD. DHA was produced in Escherichia coli by co-expressing pfaABCD from DHA-producing Colwellia psychrerythraea 34H with one of four pfaE genes from bacteria producing arachidonic acid (ARA, 20:4ω6), eicosapentaenoic acid (EPA, 20:5ω3) or DHA, respectively. Substitution of the pfaE gene from different strain source in E. coli did not influence the function of the PKS pathway producing DHA, although they led to different DHA yields and fatty acid profiles. This result suggested that the pfaE gene could be switchable between these strains for the production of DHA. The DHA production by expressing the reconstituted PKS pathway was also investigated in different E. coli strains, at different temperatures, or with the treatment of cerulenin. The highest DHA production, 2.2 mg of DHA per gram of dry cell weight or 4.1% of total fatty acids, was obtained by co-expressing pfaE(EPA) from the EPA-producing strain Shewanella baltica with pfaABCD in DH5α. Incubation at low temperature (10–15°C) resulted in higher accumulation of DHA compared to higher temperatures. The addition of cerulenin to the medium increased the proportion of DHA and saturated fatty acids, including C12:0, C14:0 and C16:0, at the expense of monounsaturated fatty acids, including C16:1 and C18:1. Supplementation with 1 mg/L cerulenin resulted in the highest DHA yield of 2.4 mg/L upon co-expression of pfaE(DHA) from C. psychrerythraea.

Antibacterial FabH Inhibitors with Mode of Action Validated in Haemophilus influenzae by in Vitro Resistance Mutation Mapping

Fatty acid biosynthesis is essential to bacterial growth in Gram-negative pathogens. Several small molecules identified through a combination of high-throughput and fragment screening were cocrystallized with FabH (β-ketoacyl-acyl carrier protein synthase III) from Escherichia coli and Streptococcus pneumoniae. Structure-based drug design was used to merge several scaffolds to provide a new class of inhibitors. After optimization for Gram-negative enzyme inhibitory potency, several compounds demonstrated antimicrobial activity against an efflux-negative strain of Haemophilus influenzae. Mutants resistant to these compounds had mutations in the FabH gene near the catalytic triad, validating FabH as a target for antimicrobial drug discovery.

Effect of cerulenin on fatty acid composition and gene expression pattern of DHA-producing strain Colwellia psychrerythraea strain 34H

Background

Colwellia psychrerythraea 34H is a psychrophilic bacterium able to produce docosahexaenoic acid (DHA). Polyketide synthase pathway is assumed to be responsible for DHA production in marine bacteria.

Results

Five pfa genes from strain 34H were confirmed to be responsible for DHA formation by heterogeneous expression in Escherichia coli. The complexity of fatty acid profile of this strain was revealed by GC and GC–MS. Treatment of cells with cerulenin resulted in significantly reduced level of C16 monounsaturated fatty acid (C16:1^Δ9t, C16:1^Δ7). In contrast, the amount of saturated fatty acids (C10:0, C12:0, C14:0), hydroxyl fatty acids (3-OH C10:0 and 3-OH C12:0), as well as C20:4ω3, C20:5ω3 and C22:6ω3 were increased. RNA sequencing (RNA-Seq) revealed the altered gene expression pattern when C. psychrerythraea cells were treated with cerulenin. Genes involved in polyketide synthase pathway and fatty acid biosynthesis pathway were not obviously affected by cerulenin treatment. In contrast, several genes involved in fatty acid degradation or β-oxidation pathway were dramatically reduced at the transcriptional level.

Conclusions

Genes responsible for DHA formation in C. psychrerythraea was first cloned and characterized. We revealed the complexity of fatty acid profile in this DHA-producing strain. Cerulenin could substantially change the fatty acid composition by affecting the fatty acid degradation at transcriptional level. Acyl-CoA dehydrogenase gene family involved in the first step of β-oxidation pathway may be important to the selectivity of degraded fatty acids. In addition, inhibition of FabB protein by cerulenin may lead to the accumulation of malonyl-CoA, which is the substrate for DHA formation.

Electronic supplementary material

The online version of this article (doi:10.1186/s12934-016-0431-9) contains supplementary material, which is available to authorized users.

The Multiple DSF-family QS Signals are Synthesized from Carbohydrate and Branched-chain Amino Acids via the FAS Elongation Cycle

Members of the diffusible signal factor (DSF) family are a novel class of quorum sensing (QS) signals in diverse Gram-negative bacteria. Although previous studies have identified RpfF as a key enzyme for the biosynthesis of DSF family signals, many questions in their biosynthesis remain to be addressed. In this study with the phytopathogen Xanthomonas campestris pv. campestris (Xcc), we show that Xcc produces four DSF-family signals (DSF, BDSF, CDSF and IDSF) during cell culture, and that IDSF is a new functional signal characterized as cis-10-methyl-2-dodecenoic acid. Using a range of defined media, we further demonstrate that Xcc mainly produces BDSF in the presence of carbohydrates; leucine and valine are the primary precursor for DSF biosynthesis; isoleucine is the primary precursor for IDSF biosynthesis. Furthermore, our biochemical analyses show that the key DSF synthase RpfF has both thioesterase and dehydratase activities, and uses 3-hydroxydedecanoyl-ACP as a substrate to produce BDSF. Finally, our results show that the classic fatty acid synthesis elongation cycle is required for the biosynthesis of DSF-family signals. Taken all together, these findings establish a general biosynthetic pathway for the DSF-family quorum sensing signals.

Structures of Pseudomonas aeruginosa β-ketoacyl-(acyl-carrier-protein) synthase II (FabF) and a C164Q mutant provide templates for antibacterial drug discovery and identify a buried potassium ion and a ligand-binding site that is an artefact of the crystal form

Bacterial infections remain a serious health concern, in particular causing life-threatening infections of hospitalized and immunocompromised patients. The situation is exacerbated by the rise in antibacterial drug resistance, and new treatments are urgently sought. In this endeavour, accurate structures of molecular targets can support early-stage drug discovery. Here, crystal structures, in three distinct forms, of recombinant Pseudomonas aeruginosa β-ketoacyl-(acyl-carrier-protein) synthase II (FabF) are presented. This enzyme, which is involved in fatty-acid biosynthesis, has been validated by genetic and chemical means as an antibiotic target in Gram-positive bacteria and represents a potential target in Gram-negative bacteria. The structures of apo FabF, of a C164Q mutant in which the binding site is altered to resemble the substrate-bound state and of a complex with 3-(benzoylamino)-2-hydroxybenzoic acid are reported. This compound mimics aspects of a known natural product inhibitor, platensimycin, and surprisingly was observed binding outside the active site, interacting with a symmetry-related molecule. An unusual feature is a completely buried potassium-binding site that was identified in all three structures. Comparisons suggest that this may represent a conserved structural feature of FabF relevant to fold stability. The new structures provide templates for structure-based ligand design and, together with the protocols and reagents, may underpin a target-based drug-discovery project for urgently needed antibacterials.

FabH mutations confer resistance to FabF-directed antibiotics in Staphylococcus aureus

Delineating the mechanisms for genetically acquired antibiotic resistance is a robust approach to target validation and anticipates the evolution of clinical drug resistance. This study defines a spectrum of mutations in fabH that render Staphylococcus aureus resistant to multiple natural products known to inhibit the elongation condensing enzyme (FabF) of bacterial type II fatty acid synthesis. Twenty independently isolated clones resistant to platensimycin, platencin, or thiolactomycin were isolated. All mutants selected against one antibiotic were cross-resistant to the other two antibiotics. Mutations were not detected in fabF, but the resistant strains harbored missense mutations in fabH. The altered amino acids clustered in and around the FabH active-site tunnel. The mutant FabH proteins were catalytically compromised based on the low activities of the purified enzymes, a fatty acid-dependent growth phenotype, and elevated expression of the fabHF operon in the mutant strains. Independent manipulation of fabF and fabH expression levels showed that the FabH/FabF activity ratio was a major determinant of antibiotic sensitivity. Missense mutations that reduce FabH activity are sufficient to confer resistance to multiple antibiotics that bind to the FabF acyl-enzyme intermediate in S. aureus.

A curated C. difficile strain 630 metabolic network: prediction of essential targets and inhibitors

BACKGROUND

Clostridium difficile is the leading cause of hospital-borne infections occurring when the natural intestinal flora is depleted following antibiotic treatment. Current treatments for Clostridium difficile infections present high relapse rates and new hyper-virulent and multi-resistant strains are emerging, making the study of this nosocomial pathogen necessary to find novel therapeutic targets.

RESULTS

We present iMLTC806cdf, an extensively curated reconstructed metabolic network for the C. difficile pathogenic strain 630. iMLTC806cdf contains 806 genes, 703 metabolites and 769 metabolic, 117 exchange and 145 transport reactions. iMLTC806cdf is the most complete and accurate metabolic reconstruction of a gram-positive anaerobic bacteria to date. We validate the model with simulated growth assays in different media and carbon sources and use it to predict essential genes. We obtain 89.2% accuracy in the prediction of gene essentiality when compared to experimental data for B. subtilis homologs (the closest organism for which such data exists). We predict the existence of 76 essential genes and 39 essential gene pairs, a number of which are unique to C. difficile and have non-existing or predicted non-essential human homologs. For 29 of these potential therapeutic targets, we find 125 inhibitors of homologous proteins including approved drugs with the potential for drug repositioning, that when validated experimentally could serve as starting points in the development of new antibiotics.

CONCLUSIONS

We created a highly curated metabolic network model of C. difficile strain 630 and used it to predict essential genes as potential new therapeutic targets in the fight against Clostridium difficile infections.