LytB, a novel gene of the 2-C-methyl-D-erythritol 4-phosphate pathway of isoprenoid biosynthesis in Escherichia coli

Metabolic engineering of terpene metabolism in lavender

Background

Several members of the Lamiaceae family of plants produce large amounts of essential oil [EO] that find extensive applications in the food, cosmetics, personal hygiene, and alternative medicine industries. There is interest in enhancing EO metabolism in these plants.

Main body

Lavender produces a valuable EO that is highly enriched in monoterpenes, the C10 class of the isoprenoids or terpenoids. In recent years, substantial effort has been made by researchers to study terpene metabolism and enhance lavender EO through plant biotechnology. This paper reviews recent advances related to the cloning of lavender monoterpene biosynthetic genes and metabolic engineering attempts aimed at improving the production of lavender monoterpenes in plants and microbes.

Conclusion

Metabolic engineering has led to the improvement of EO quality and yield in several plants, including lavender. Furthermore, several biologically active EO constituents have been produced in microorganisms.

Human Vγ9Vδ2 T cells exhibit antifungal activity against Aspergillus fumigatus and other filamentous fungi

Invasive aspergillosis (IA) and mucormycosis are life-threatening diseases, especially among immunocompromised patients. Drug-resistant Aspergillus fumigatus strains have been isolated worldwide, which can pose a serious clinical problem. As IA mainly occurs in patients with compromised immune systems, the ideal therapeutic approach should aim to bolster the immune system. In this study, we focused on Vγ9Vδ2 T cells that exhibit immune effector functions and examined the possibility of harnessing this unconventional T cell subset as a novel therapeutic modality for IA. A potent antifungal effect was observed when A. fumigatus (Af293) hyphae were challenged by Vγ9Vδ2 T cells derived from peripheral blood. In addition, Vγ9Vδ2 T cells exhibited antifungal activity against hyphae of all Aspergillus spp., Cunninghamella bertholletiae, and Rhizopus microsporus but not against their conidia. Furthermore, Vγ9Vδ2 T cells also exhibited antifungal activity against azole-resistant A. fumigatus, indicating that Vγ9Vδ2 T cells could be used for treating drug-resistant A. fumigatus. The antifungal activity of Vγ9Vδ2 T cells depended on cell-to-cell contact with A. fumigatus hyphae, and degranulation characterized by CD107a mobilization seems essential for this activity against A. fumigatus. Vγ9Vδ2 T cells could be developed as a novel modality for treating IA or mucormycosis.

IMPORTANCE

Invasive aspergillosis (IA) and mucormycosis are often resistant to treatment with conventional antifungal agents and have a high mortality rate. Additionally, effective antifungal treatment is hindered by drug toxicity, given that both fungal and human cells are eukaryotic, and antifungal agents are also likely to act on human cells, resulting in adverse effects. Therefore, the development of novel therapeutic agents specifically targeting fungi is challenging. This study demonstrated the antifungal activity of Vγ9Vδ2 T cells against various Aspergillus spp. and several Mucorales in vitro and discussed the mechanism underlying their antifungal activity. We indicate that adoptive immunotherapy using Vγ9Vδ2 T cells may offer a new therapeutic approach to IA.

The Reductive Dehydroxylation Catalyzed by IspH, a Source of Inspiration for the Development of Novel Anti-Infectives

The non-mevalonate or also called MEP pathway is an essential route for the biosynthesis of isoprenoid precursors in most bacteria and in microorganisms belonging to the Apicomplexa phylum, such as the parasite responsible for malaria. The absence of this pathway in mammalians makes it an interesting target for the discovery of novel anti-infectives. As last enzyme of this pathway, IspH is an oxygen sensitive [4Fe-4S] metalloenzyme that catalyzes 2H⁺/2e⁻ reductions and a water elimination by involving non-conventional bioinorganic and bioorganometallic intermediates. After a detailed description of the discovery of the [4Fe-4S] cluster of IspH, this review focuses on the IspH mechanism discussing the results that have been obtained in the last decades using an approach combining chemistry, enzymology, crystallography, spectroscopies, and docking calculations. Considering the interesting druggability of this enzyme, a section about the inhibitors of IspH discovered up to now is reported as well. The presented results constitute a useful and rational help to inaugurate the design and development of new potential chemotherapeutics against pathogenic organisms.

Acceleration of target production in co-culture by enhancing intermediate consumption through adaptive laboratory evolution

Co-culture is a promising way to alleviate metabolic burden by dividing the metabolic pathways into several modules and sharing the conversion processes with multiple strains. Since an intermediate is passed from the donor to the recipient via the extracellular environment, it is inevitably diluted. Therefore, enhancing the intermediate consumption rate is important for increasing target productivity. In the present study, we demonstrated the enhancement of mevalonate consumption in Escherichia coli by adaptive laboratory evolution and applied the evolved strain to isoprenol production in an E. coli (upstream: glucose to mevalonate)-E. coli (downstream: mevalonate to isoprenol) co-culture. An engineered mevalonate auxotroph strain was repeatedly sub-cultured in a synthetic medium supplemented with mevalonate, where the mevalonate concentration was decreased stepwise from 100 to 20 µM. In five parallel evolution experiments, all growth rates gradually increased, resulting in five evolved strains. Whole-genome re-sequencing and reverse engineering identified three mutations involved in enhancing mevalonate consumption. After introducing nudF gene for producing isoprenol, the isoprenol-producing parental and evolved strains were respectively co-cultured with a mevalonate-producing strain. At an inoculation ratio of 1:3 (upstream:downstream), isoprenol production using the evolved strain was 3.3 times higher than that using the parental strain.

Toxoplasma gondii apicoplast-resident ferredoxin is an essential electron transfer protein for the MEP isoprenoid-biosynthetic pathway

Apicomplexan parasites, such as Toxoplasma gondii, are unusual in that each cell contains a single apicoplast, a plastid-like organelle that compartmentalizes enzymes involved in the essential 2C-methyl-D-erythritol 4-phosphate pathway of isoprenoid biosynthesis. The last two enzymatic steps in this organellar pathway require electrons from a redox carrier. However, the small iron-sulfur cluster-containing protein ferredoxin, a likely candidate for this function, has not been investigated in this context. We show here that inducible knockdown of T. gondii ferredoxin results in progressive inhibition of growth and eventual parasite death. Surprisingly, this phenotype is not accompanied by ultrastructural changes in the apicoplast or overall cell morphology. The knockdown of ferredoxin was instead associated with a dramatic decrease in cellular levels of the last two metabolites in isoprenoid biosynthesis, 1-hydroxy-2-methyl-2-(E)- butenyl-4-pyrophosphate, and isomeric dimethylallyl pyrophosphate/isopentenyl pyrophosphate. Ferredoxin depletion was also observed to impair gliding motility, consistent with isoprenoid metabolites being important for dolichol biosynthesis, protein prenylation, and modification of other proteins involved in motility. Significantly, pharmacological inhibition of isoprenoid synthesis of the host cell exacerbated the impact of ferredoxin depletion on parasite replication, suggesting that the slow onset of parasite death after ferredoxin depletion is because of isoprenoid scavenging from the host cell and leading to partial compensation of the depleted parasite metabolites upon ferredoxin knockdown. Overall, these findings show that ferredoxin has an essential physiological function as an electron donor for the 2C-methyl-D-erythritol 4-phosphate pathway and is a potential drug target for apicomplexan parasites.

Cancer immunotherapy harnessing γδ T cells and programmed death-1

Cancer immunotherapy has received increasing attention since the success of CTLA-4 and programmed death-1 (PD-1) immune checkpoint inhibitors and CAR-T cells. One of the most promising next-generation cancer treatments is adoptive transfer of immune effector cells. Developing an efficacious adoptive transfer therapy requires growing large numbers of highly purified immune effector cells in a short period of time. γδ T cells can be effectively expanded using synthetic antigens such as pyrophosphomonoesters and nitrogen-containing bisphosphonates (N-BPs). Pyrophosphomonoester antigens, initially identified in mycobacterial extracts, were used for this purpose in the early years of the development of γδ T cell-based therapy. GMP-grade N-BPs, which are now commercially available, are used in many clinical trials worldwide. In order to develop N-BPs for cancer immunotherapy, N-BP prodrugs have been synthesized; among these, tetrakis-pivaloyloxymethyl 2-(thiazole-2-ylamino)ethylidene-1,1-bisphosphonate (PTA) is the most potent compound for stimulating γδ T cells. The activated γδ T cells express high levels of PD-1, suggesting the potential for a combination therapy harnessing γδ T cells and PD-1 immune checkpoint inhibitors. In addition, the functions of γδ T cells can be modified by IL-18. Collectively, the recent findings show that γδ T cells are one of the most promising immune effector subsets for the development of novel cancer immunotherapy.

Adaptation of hydroxymethylbutenyl diphosphate reductase enables volatile isoprenoid production

Volatile isoprenoids produced by plants are emitted in vast quantities into the atmosphere, with substantial effects on global carbon cycling. Yet, the molecular mechanisms regulating the balance between volatile and non-volatile isoprenoid production remain unknown. Isoprenoids are synthesised via sequential condensation of isopentenyl pyrophosphate (IPP) to dimethylallyl pyrophosphate (DMAPP), with volatile isoprenoids containing fewer isopentenyl subunits. The DMAPP:IPP ratio could affect the balance between volatile and non-volatile isoprenoids, but the plastidic DMAPP:IPP ratio is generally believed to be similar across different species. Here we demonstrate that the ratio of DMAPP:IPP produced by hydroxymethylbutenyl diphosphate reductase (HDR/IspH), the final step of the plastidic isoprenoid production pathway, is not fixed. Instead, this ratio varies greatly across HDRs from phylogenetically distinct plants, correlating with isoprenoid production patterns. Our findings suggest that adaptation of HDR plays a previously unrecognised role in determining in vivo carbon availability for isoprenoid emissions, directly shaping global biosphere-atmosphere interactions.

Medically Useful Plant Terpenoids: Biosynthesis, Occurrence, and Mechanism of Action

Specialized plant terpenoids have found fortuitous uses in medicine due to their evolutionary and biochemical selection for biological activity in animals. However, these highly functionalized natural products are produced through complex biosynthetic pathways for which we have a complete understanding in only a few cases. Here we review some of the most effective and promising plant terpenoids that are currently used in medicine and medical research and provide updates on their biosynthesis, natural occurrence, and mechanism of action in the body. This includes pharmacologically useful plastidic terpenoids such as p-menthane monoterpenoids, cannabinoids, paclitaxel (taxol®), and ingenol mebutate which are derived from the 2-C-methyl-d-erythritol-4-phosphate (MEP) pathway, as well as cytosolic terpenoids such as thapsigargin and artemisinin produced through the mevalonate (MVA) pathway. We further provide a review of the MEP and MVA precursor pathways which supply the carbon skeletons for the downstream transformations yielding these medically significant natural products.

Strain-specific response to ampicillin in Wolbachia-infected mosquito cell lines

Wolbachia pipientis (Rickettsiales; Anaplasmataceae) is an obligate intracellular alpha proteobacterium that occurs in arthropods and filarial worms. Some strains of Wolbachia can be maintained as persistent infections in insect cell lines. C/wStr1 cells from the mosquito Aedes albopictus maintain a robust infection with Wolbachia strain wStr, originally isolated from the planthopper, Laodelphax striatellus. To explore possible functions of penicillin-binding proteins expressed from the wStr genome, C/wStr1 cells were exposed to ampicillin. Absolute levels of Wolbachia increased 3.5-fold in ampicillin-treated cells and fivefold in naive cells newly infected with wStr. Because cell numbers were depressed by ampicillin treatment, Wolbachia yield on a per-cell basis increased by 15-fold. The absence of a similar effect on wAlbB in Aa23 host cells suggests that the Wolbachia strain, the presence/absence of genes encoding penicillin-binding proteins, or the interaction between wAlbB and its host cells may modulate the effects of ampicillin.

A high-throughput screening campaign to identify inhibitors of DXP reductoisomerase (IspC) and MEP cytidylyltransferase (IspD)

The rise of antibacterial resistance among human pathogens represents a problem that could change the landscape of healthcare unless new antibiotics are developed. The methyl erythritol phosphate (MEP) pathway represents an attractive series of targets for novel antibiotic design, considering each enzyme of the pathway is both essential and has no human homologs. Here we describe a pilot scale high-throughput screening (HTS) campaign against the first and second committed steps in the pathway, catalyzed by DXP reductoisomerase (IspC) and MEP cytidylyltransferase (IspD), using compounds present in the commercially available LOPAC1280 library as well as in an in-house natural product extract library. Hit compounds were characterized to deduce their mechanism of inhibition; most function through aggregation. The HTS workflow outlined here is useful for quickly screening a chemical library, while effectively identifying false positive compounds associated with assay constraints and aggregation.

Exceptionally high percentage of IPP synthesis by Ginkgo biloba IspH is mainly due to Phe residue in the active site

(E)-4-Hydroxy-3-methylbut-2-enyl diphosphate (HMBPP) reductase (IspH, HDR or LytB) is an Fe/S enzyme catalyzing the reductive dehydroxylation of HMBPP to isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP) in the last step of methylerythritol phosphate (MEP) pathway. The MEP pathway is known to produce 4-6:1 ratio of IPP and DMAPP mixture by the last enzyme, IspH. Plant IspH in plastids follows same catalytic mechanism as others, but GbIspH (Ginkgo biloba IspH) was reported to produce a mixture of IPP and DMAPP in a ratio of 16:1. Present catalytic mechanisms of IspH involve a common allyl anion intermediate, and the intramolecular proton transfer to the allyl moiety is considered as the key reaction step determining the product between IPP and DMAPP. The F212 residue in plant IspH was found as a potential amino acid residue that could mediate the proton transfer to the allyl anion intermediate before the product release. In this report, catalytic function of GbIspH F212 residue (H74 in E. coli), especially during the product formation in the active site, was studied by means of site-directed mutation. The product ratio of IPP/DMAPP was measured as 6.5 ± 0.1 for F212H GbIspH, and the value was close to the reported bacterial IspH having His residue on that specific position. Along with the other F212Y mutant, of which ratio was determined as 10.9 ± 0.1, the results strongly support that the Phe residue in plant IspH is the key amino acid residue that allows exclusive production of IPP in plant chloroplast.

New Insight into Isoprenoids Biosynthesis Process and Future Prospects for Drug Designing in Plasmodium

The MEP (Methyl Erythritol Phosphate) isoprenoids biosynthesis pathway is an attractive drug target to combat malaria, due to its uniqueness and indispensability for the parasite. It is functional in the apicoplast of Plasmodium and its products get transported to the cytoplasm, where they participate in glycoprotein synthesis, electron transport chain, tRNA modification and several other biological processes. Several compounds have been tested against the enzymes involved in this pathway and amongst them Fosmidomycin, targeted against IspC (DXP reductoisomerase) enzyme and MMV008138 targeted against IspD enzyme have shown good anti-malarial activity in parasite cultures. Fosmidomycin is now-a-days prescribed clinically, however, less absorption, shorter half-life, and toxicity at higher doses, limits its use as an anti-malarial. The potential of other enzymes of the pathway as candidate drug targets has also been determined. This review details the various drug molecules tested against these targets with special emphasis to Plasmodium. We corroborate that MEP pathway functional within the apicoplast of Plasmodium is a major drug target, especially during erythrocytic stages. However, the major bottlenecks, bioavailability and toxicity of the new molecules needs to be addressed, before considering any new molecule as a potent antimalarial.

Genetics of Capsular Polysaccharides and Cell Envelope (Glyco)lipids

This article summarizes what is currently known of the structures, physiological roles, involvement in pathogenicity, and biogenesis of a variety of noncovalently bound cell envelope lipids and glycoconjugates of Mycobacterium tuberculosis and other Mycobacterium species. Topics addressed in this article include phospholipids; phosphatidylinositol mannosides; triglycerides; isoprenoids and related compounds (polyprenyl phosphate, menaquinones, carotenoids, noncarotenoid cyclic isoprenoids); acyltrehaloses (lipooligosaccharides, trehalose mono- and di-mycolates, sulfolipids, di- and poly-acyltrehaloses); mannosyl-beta-1-phosphomycoketides; glycopeptidolipids; phthiocerol dimycocerosates, para-hydroxybenzoic acids, and phenolic glycolipids; mycobactins; mycolactones; and capsular polysaccharides.

Proteomic Analysis of the Relationship between Metabolism and Nonhost Resistance in Soybean Exposed to Bipolaris maydis

Nonhost resistance (NHR) pertains to the most common form of plant resistance against pathogenic microorganisms of other species. Bipolaris maydis is a non-adapted pathogen affecting soybeans, particularly of maize/soybean intercropping systems. However, no experimental evidence has described the immune response of soybeans against B. maydis. To elucidate the molecular mechanism underlying NHR in soybeans, proteomics analysis based on two-dimensional polyacrylamide gel electrophoresis (2-DE) was performed to identify proteins involved in the soybean response to B. maydis. The spread of B. maydis spores across soybean leaves induced NHR throughout the plant, which mobilized almost all organelles and various metabolic processes in response to B. maydis. Some enzymes, including ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO), mitochondrial processing peptidase (MPP), oxygen evolving enhancer (OEE), and nucleoside diphosphate kinase (NDKs), were found to be related to NHR in soybeans. These enzymes have been identified in previous studies, and STRING analysis showed that most of the protein functions related to major metabolic processes were induced as a response to B. maydis, which suggested an array of complex interactions between soybeans and B. maydis. These findings suggest a systematic NHR against non-adapted pathogens in soybeans. This response was characterized by an overlap between metabolic processes and response to stimulus. Several metabolic processes provide the soybean with innate immunity to the non-adapted pathogen, B. maydis. This research investigation on NHR in soybeans may foster a better understanding of plant innate immunity, as well as the interactions between plant and non-adapted pathogens in intercropping systems.

Silencing of Essential Genes within a Highly Coordinated Operon in Escherichia coli

Essential bacterial genes located within operons are particularly challenging to study independently because of coordinated gene expression and the nonviability of knockout mutants. Essentiality scores for many operon genes remain uncertain. Antisense RNA (asRNA) silencing or in-frame gene disruption of genes may help establish essentiality but can lead to polar effects on genes downstream or upstream of the target gene. Here, the Escherichia coli ribF-ileS-lspA-fkpB-ispH operon was used to evaluate the possibility of independently studying an essential gene using expressed asRNA and target gene overexpression to deregulate coupled expression. The gene requirement for growth in conditional silencing strains was determined by the relationship of target mRNA reduction with growth inhibition as the minimum transcript level required for 50% growth (MTL50). Mupirocin and globomycin, the protein inhibitors of IleS and LspA, respectively, were used in sensitization assays of strains containing both asRNA-expressing and open reading frame-expressing plasmids to examine deregulation of the overlapping ileS-lspA genes. We found upstream and downstream polar silencing effects when either ileS or lspA was silenced, indicating coupled expression. Weighted MTL50 values (means and standard deviations) of ribF, ileS, and lspA were 0.65 ± 0.18, 0.64 ± 0.06, and 0.76 ± 0.10, respectively. However, they were not significantly different (P = 0.71 by weighted one-way analysis of variance). The gene requirement for ispH could not be determined due to insufficient growth reduction. Mupirocin and globomycin sensitization experiments indicated that ileS-lspA expression could not be decoupled. The results highlight the inherent challenges associated with genetic analyses of operons; however, coupling of essential genes may provide opportunities to improve RNA-silencing antimicrobials.

Functional evidence for the critical amino-terminal conserved domain and key amino acids of Arabidopsis 4-HYDROXY-3-METHYLBUT-2-ENYL DIPHOSPHATE REDUCTASE

The plant 4-HYDROXY-3-METHYLBUT-2-ENYL DIPHOSPHATE REDUCTASE (HDR) catalyzes the last step of the methylerythritol phosphate pathway to synthesize isopentenyl diphosphate and its allyl isomer dimethylallyl diphosphate, which are common precursors for the synthesis of plastid isoprenoids. The Arabidopsis (Arabidopsis thaliana) genomic HDR transgene-induced gene-silencing lines are albino, variegated, or pale green, confirming that HDR is essential for plants. We used Escherichia coli isoprenoid synthesis H (Protein Data Bank code 3F7T) as a template for homology modeling to identify key amino acids of Arabidopsis HDR. The predicted model reveals that cysteine (Cys)-122, Cys-213, and Cys-350 are involved in iron-sulfur cluster formation and that histidine (His)-152, His-241, glutamate (Glu)-242, Glu-243, threonine (Thr)-244, Thr-312, serine-379, and asparagine-381 are related to substrate binding or catalysis. Glu-242 and Thr-244 are conserved only in cyanobacteria, green algae, and land plants, whereas the other key amino acids are absolutely conserved from bacteria to plants. We used site-directed mutagenesis and complementation assay to confirm that these amino acids, except His-152 and His-241, were critical for Arabidopsis HDR function. Furthermore, the Arabidopsis HDR contains an extra amino-terminal domain following the transit peptide that is highly conserved from cyanobacteria, and green algae to land plants but not existing in the other bacteria. We demonstrated that the amino-terminal conserved domain was essential for Arabidopsis and cyanobacterial HDR function. Further analysis of conserved amino acids in the amino-terminal conserved domain revealed that the tyrosine-72 residue was critical for Arabidopsis HDR. These results suggest that the structure and reaction mechanism of HDR evolution have become specific for oxygen-evolving photosynthesis organisms and that HDR probably evolved independently in cyanobacteria versus other prokaryotes.

Kinetic characterization and allosteric inhibition of the Yersinia pestis 1-deoxy-D-xylulose 5-phosphate reductoisomerase (MEP synthase)

The methylerythritol phosphate (MEP) pathway found in many bacteria governs the synthesis of isoprenoids, which are crucial lipid precursors for vital cell components such as ubiquinone. Because mammals synthesize isoprenoids via an alternate pathway, the bacterial MEP pathway is an attractive target for novel antibiotic development, necessitated by emerging antibiotic resistance as well as biodefense concerns. The first committed step in the MEP pathway is the reduction and isomerization of 1-deoxy-D-xylulose-5-phosphate (DXP) to methylerythritol phosphate (MEP), catalyzed by MEP synthase. To facilitate drug development, we cloned, expressed, purified, and characterized MEP synthase from Yersinia pestis. Enzyme assays indicate apparent kinetic constants of KMDXP = 252 µM and KMNADPH = 13 µM, IC50 values for fosmidomycin and FR900098 of 710 nM and 231 nM respectively, and Ki values for fosmidomycin and FR900098 of 251 nM and 101 nM respectively. To ascertain if the Y. pestis MEP synthase was amenable to a high-throughput screening campaign, the Z-factor was determined (0.9) then the purified enzyme was screened against a pilot scale library containing rationally designed fosmidomycin analogs and natural product extracts. Several hit molecules were obtained, most notably a natural product allosteric affector of MEP synthase and a rationally designed bisubstrate derivative of FR900098 (able to associate with both the NADPH and DXP binding sites in MEP synthase). It is particularly noteworthy that allosteric regulation of MEP synthase has not been described previously. Thus, our discovery implicates an alternative site (and new chemical space) for rational drug development.

Metabolic engineering of Salmonella vaccine bacteria to boost human Vγ2Vδ2 T cell immunity

Human Vγ2Vδ2 T cells monitor isoprenoid metabolism by recognizing foreign (E)-4-hydroxy-3-methyl-but-2-enyl pyrophosphate (HMBPP), a metabolite in the 2-C-methyl-D-erythritol-4-phosphate pathway used by most eubacteria and apicomplexan parasites, and self isopentenyl pyrophosphate, a metabolite in the mevalonate pathway used by humans. Whereas microbial infections elicit prolonged expansion of memory Vγ2Vδ2 T cells, immunization with prenyl pyrophosphates or aminobisphosphonates elicit short-term Vγ2Vδ2 expansion with rapid anergy and deletion upon subsequent immunizations. We hypothesized that a live, attenuated bacterial vaccine that overproduces HMBPP would elicit long-lasting Vγ2Vδ2 T cell immunity by mimicking a natural infection. Therefore, we metabolically engineered the avirulent aroA(-) Salmonella enterica serovar Typhimurium SL7207 strain by deleting the gene for LytB (the downstream enzyme from HMBPP) and functionally complementing for this loss with genes encoding mevalonate pathway enzymes. LytB(-) Salmonella SL7207 had high HMBPP levels, infected human cells as efficiently as did the wild-type bacteria, and stimulated large ex vivo expansions of Vγ2Vδ2 T cells from human donors. Importantly, vaccination of a rhesus monkey with live lytB(-) Salmonella SL7207 stimulated a prolonged expansion of Vγ2Vδ2 T cells without significant side effects or anergy induction. These studies provide proof-of-principle that metabolic engineering can be used to derive live bacterial vaccines that boost Vγ2Vδ2 T cell immunity. Similar engineering of metabolic pathways to produce lipid Ags or B vitamin metabolite Ags could be used to derive live bacterial vaccine for other unconventional T cells that recognize nonpeptide Ags.

Biosynthesis pathways of ginkgolides

The ginkgolides, acting as anti-platelet-activating factors, have been studied for many years. The biosynthetic pathway of ginkgolides is still far away from unveiling at the level of molecular genetics and biochemistry. There are at least 11 kinds of enzymes having been cloned from Ginkgo biloba L., which catalyze the formation of ginkgolides via a series of reactions. Some researchers have indicated that the addition of precursors and elicitors can influence the accumulation of ginkgolides in the suspension cell cultures of G. biloba. There are also other factors that can influence the production of ginkgolides. This review focuses on the aforementioned aspects to discuss the biosynthetic pathways of the ginkgolides.

Map-based cloning of zb7 encoding an IPP and DMAPP synthase in the MEP pathway of maize

IspH is a key enzyme in the last step of the methyl-D-erythritol-4-phosphate (MEP) pathway. Loss of function of IspH can often result in complete yellow or albino phenotype in many plants. Here, we report the characterization of a recessive mutant of maize, zebra7 (zb7), showing transverse green/yellow striped leaves in young plants. The yellow bands of the mutant have decreased levels of chlorophylls and carotenoids with delayed chloroplast development. Low temperature suppressed mutant phenotype, while alternate light/dark cycle or high temperature enlarged the yellow section. Map-based cloning demonstrated that zb7 encodes the IspH protein with a mis-sense mutation in a conserved region. Transgenic silencing of Zb7 in maize resulted in complete albino plantlets that are aborted in a few weeks, confirming that Zb7 is important in the early stages of maize chloroplast development. Zb7 is constitutively expressed and its expression subject to a 16-h light/8-h dark cycle regulation. Our results suggest that the less effective or unstable IspH in zb7 mutant, together with its diurnal expression, are mechanistically accounted for the zebra phenotype. The increased IspH mRNA in the leaves of zb7 at the late development stage may explain the restoration of mutant phenotype in mature stages.

Biochemistry of the non-mevalonate isoprenoid pathway

The non-mevalonate pathway of isoprenoid (terpenoid) biosynthesis is essential in many eubacteria including the major human pathogen, Mycobacterium tuberculosis, in apicomplexan protozoa including the Plasmodium spp. causing malaria, and in the plastids of plants. The metabolic route is absent in humans and is therefore qualified as a promising target for new anti-infective drugs and herbicides. Biochemical and structural knowledge about all enzymes involved in the pathway established the basis for discovery and development of inhibitors by high-throughput screening of compound libraries and/or structure-based rational design.

Francisella tularensis 2-C-methyl-D-erythritol 4-phosphate cytidylyltransferase: kinetic characterization and phosphoregulation

Deliberate and natural outbreaks of infectious disease, the prevalence of antibiotic resistant strains, and the ease by which antibiotic resistant bacteria can be intentionally engineered all underscore the necessity of effective vaccines and continued development of novel antimicrobial/antiviral therapeutics. Isoprenes, a group of molecules fundamentally involved in a variety of crucial biological functions, are derived from either the mevalonic acid (MVA) or methylerythritol phosphate (MEP) pathway. While mammals utilize the MVA pathway, many bacteria utilize the MEP pathway, highlighting the latter as an attractive target for antibiotic development. In this report we describe the cloning and characterization of Francisella tularensis MEP cytidylyltransferase, a MEP pathway enzyme and potential target for antibiotic development. Size exclusion chromatography indicates the protein exists as a dimer in solution. Enzyme assays produced an apparentK(MEP)(M) = 178 μM, K(CTP)(M) = 73 μM , k(MEP)(cat) = 1(s-1), k(CTP)(cat) = 0.8( s-1), and a k(MEP)(cat)/ K(MEP)(M) = 3.4 x 10(5) M(-1) min(-1). The enzyme exhibits a strict preference for Mg(+2) as a divalent cation and CTP as the nucleotide. Titanium dioxide chromatography-tandem mass spectrometry identified Thr141 as a site of phosphorylation. T141D and T141E site-directed mutants are catalytically inactive, suggesting a mechanism for post-translational control of metabolic flux through the F. tularensis MEP pathway. Overall, our study suggests that MEP cytidylyltransferase is an excellent target for the development of novel antibiotics against F. tularensis.

A hybrid two-component system of Tannerella forsythia affects autoaggregation and post-translational modification of surface proteins

Tannerella forsythia is a Gram-negative oral anaerobe closely associated with both periodontal and periapical diseases. The ORF TF0022 of strain ATCC 43037 encodes a hybrid two-component system consisting of an N-terminal histidine kinase and a C-terminal response regulator. Disruption of the TF0022 locus enhanced autoaggregation of the broth-cultured cells. Comparative proteome analyses revealed that two S-layer proteins in the TF0022 mutant exhibited decreased apparent masses by denaturing gel electrophoresis, suggesting a deficiency in post-translational modification. Furthermore, the mutant decreased the production of a glycosyltransferase encoded by TF1061 that is located in a putative glycosylation-related gene cluster. Quantitative real-time PCR revealed reduced transcription of TF1061 and the associated genes in the TF0022 mutant. These results indicate that TF0022 upregulates the expression of the glycosylation-related genes and suggest modulation of the autoaggregation of T. forsythia cells by a possible post-translational modification of cell-surface components.

Kinetic characterization and phosphoregulation of the Francisella tularensis 1-deoxy-D-xylulose 5-phosphate reductoisomerase (MEP synthase)

Deliberate and natural outbreaks of infectious disease underscore the necessity of effective vaccines and antimicrobial/antiviral therapeutics. The prevalence of antibiotic resistant strains and the ease by which antibiotic resistant bacteria can be intentionally engineered further highlights the need for continued development of novel antibiotics against new bacterial targets. Isoprenes are a class of molecules fundamentally involved in a variety of crucial biological functions. Mammalian cells utilize the mevalonic acid pathway for isoprene biosynthesis, whereas many bacteria utilize the methylerythritol phosphate (MEP) pathway, making the latter an attractive target for antibiotic development. In this report we describe the cloning and characterization of Francisella tularensis MEP synthase, a MEP pathway enzyme and potential target for antibiotic development. In vitro growth-inhibition assays using fosmidomycin, an inhibitor of MEP synthase, illustrates the effectiveness of MEP pathway inhibition with F. tularensis. To facilitate drug development, F. tularensis MEP synthase was cloned, expressed, purified, and characterized. Enzyme assays produced apparent kinetic constants (K_M^DXP = 104 µM, K_M^NADPH = 13 µM, k_cat^DXP = 2 s⁻¹, k_cat^NADPH = 1.3 s⁻¹), an IC₅₀ for fosmidomycin of 247 nM, and a K_i for fosmidomycin of 99 nM. The enzyme exhibits a preference for Mg⁺² as a divalent cation. Titanium dioxide chromatography-tandem mass spectrometry identified Ser177 as a site of phosphorylation. S177D and S177E site-directed mutants are inactive, suggesting a mechanism for post-translational control of metabolic flux through the F. tularensis MEP pathway. Overall, our study suggests that MEP synthase is an excellent target for the development of novel antibiotics against F. tularensis.

Molecular and functional characterization of a cDNA encoding 4-hydroxy-3-methylbut-2-enyl diphosphate reductase from Dunaliella salina

In green algae, the final step of the plastidial methylerythritol phosphate (MEP) pathway is catalyzed by 4-hydroxy-3-methylbut-2-enyl diphosphate reductase (HDR; EC: 1.17.1.2), an enzyme proposed to play a key role in the regulation of isoprenoid biosynthesis. Here we report the isolation and functional characterization of a 1959-bp Dunaliella salina HDR (DsHDR) cDNA encoding a deduced polypeptide of 474 amino acid residues. Phylogenetic analysis implied a cyanobacterial origin for plant and algal HDR genes. Steady-state DsHDR transcript levels were higher in D. salina cells submitted to nutritional depletion, high salt and/or high light, suggesting that DsHDR may respond to the same environmental cues as genes involved in carotenoid biosynthesis.

Changes in protein expression in Burkholderia vietnamiensis PR1 301 at pH 5 and 7 with and without nickel

Burkholderia vietnamiensis PR1(301) (PR1) exhibits pH-dependent nickel (Ni) tolerance, with lower Ni toxicity observed at pH 5 than at pH 7. The Ni tolerance mechanism in PR1 is currently unknown, and traditional mechanisms of Ni resistance do not appear to be present. Therefore, 2D gel electrophoresis was used to examine changes in protein expression in PR1 with and without Ni (3.4 mM) at pH 5 and 7. Proteins with both a statistically significant and at least a twofold difference in expression level between conditions (pH, Ni) were selected and identified using MALDI-TOF-MS or LC-MS. Results showed increased expression of proteins involved in cell shape and membrane composition at pH 5 compared with pH 7. Scanning electron microscopy indicated elongated cells at pH 5 and 6 compared with pH 7 in the absence of Ni. Fatty acid methyl ester analysis showed a statistically significant difference in the percentages of long- and short-chain fatty acids at pH 5 and 7. These findings suggest that changes in membrane structure and function may be involved in the ability of PR1 to grow at higher concentrations of Ni at pH 5 than at pH 7.

Heterologous production of polyketides in bacteria

Polyketide natural products are among the most important microbial metabolites in human medicine and are widely used to treat both acute and degenerative diseases. The need to develop new drugs has prompted the idea of using heterologous systems for the expression of polyketide biosynthetic pathways. The basic idea behind this approach is to use heterologous bacterial systems with better growth and genetic characteristics that could support better production of a certain compound than the original host or that could allow the generation of novel analogues through combinatorial biosynthesis. Moreover, these hosts could be used to express "cryptic" secondary metabolic pathways or serve as surrogate hosts in metagenomics experiments in order to find potential new bioactive compounds. In this chapter we discuss recent advances in the heterologous production of polyketides in bacteria and describe some methodological improvements of the systems.

From molecular fossils of bacterial hopanoids to the formation of isoprene units: discovery and elucidation of the methylerythritol phosphate pathway

Investigations on the biosynthesis of bacterial triterpenoids of the hopane series led to the unexpected discovery of an alternative mevalonate independent pathway for the formation of isoprene units. Methylerythritol phosphate, already presenting the C5 branched isoprene skeleton, is the key intermediate. This pathway was independently characterized in ginkgo embryos for the formation of diterpenoids. It is present in most bacteria and in the plastids of all organisms belonging to phototrophic phyla. The key steps of the discovery and elucidation of this metabolic route are presented in this review.

The Mycobacterium tuberculosis MEP (2C-methyl-d-erythritol 4-phosphate) pathway as a new drug target

Tuberculosis (TB) is still a major public health problem, compounded by the human immunodeficiency virus (HIV)-TB co-infection and recent emergence of multidrug-resistant (MDR) and extensively drug resistant (XDR)-TB. Novel anti-TB drugs are urgently required. In this context, the 2C-methyl-d-erythritol 4-phosphate (MEP) pathway of Mycobacterium tuberculosis has drawn attention; it is one of several pathways vital for M. tuberculosis viability and the human host lacks homologous enzymes. Thus, the MEP pathway promises bacterium-specific drug targets and the potential for identification of lead compounds unencumbered by target-based toxicity. Indeed, fosmidomycin is now known to inhibit the second step in the MEP pathway. This review describes the cardinal features of the main enzymes of the MEP pathway in M. tuberculosis and how these can be manipulated in high throughput screening campaigns in the search for new anti-infectives against TB.

Analysis of the isoprenoid biosynthesis pathways in Listeria monocytogenes reveals a role for the alternative 2-C-methyl-D-erythritol 4-phosphate pathway in murine infection

Most bacteria synthesize isoprenoids through one of two essential pathways which provide the basic building block, isopentyl diphosphate (IPP): either the classical mevalonate pathway or the alternative non-mevalonate 2-C-methyl-D-erythritol 4-phosphate (MEP) pathway. However, postgenomic analyses of the Listeria monocytogenes genome revealed that this pathogen possesses the genetic capacity to produce the complete set of enzymes involved in both pathways. The nonpathogenic species Listeria innocua naturally lacks the last two genes (gcpE and lytB) of the MEP pathway, and bioinformatic analyses strongly suggest that the genes have been lost through evolution. In the present study we show that heterologous expression of gcpE and lytB in L. innocua can functionally restore the MEP pathway in this organism and confer on it the ability to induce Vgamma9 Vdelta2 T cells. We have previously confirmed that both pathways are functional in L. monocytogenes and can provide sufficient IPP for normal growth in laboratory media (M. Begley, C. G. Gahan, A. K. Kollas, M. Hintz, C. Hill, H. Jomaa, and M. Eberl, FEBS Lett. 561:99-104, 2004). Here we describe a targeted mutagenesis strategy to create a double pathway mutant in L. monocytogenes which cannot grow in the absence of exogenously provided mevalonate, confirming the requirement for at least one intact pathway for growth. In addition, murine studies revealed that mutants lacking the MEP pathway were impaired in virulence relative to the parent strain during intraperitoneal infection, while mutants lacking the classical mevalonate pathway were not impaired in virulence potential. In vivo bioluminescence imaging also confirmed in vivo expression of the gcpE gene (MEP pathway) during murine infection.

Biosynthesis of isoprenoids: characterization of a functionally active recombinant 2-C-methyl-D-erythritol 4-phosphate cytidyltransferase (IspD) from Mycobacterium tuberculosis H37Rv

Tuberculosis, caused by Mycobacterium tuberculosis, continues to be one of the leading infectious diseases to humans. It is urgent to discover novel drug targets for the development of antitubercular agents. The 2-C-methyl-D-erythritol-4-phosphate (MEP) pathway for isoprenoid biosynthesis has been considered as an attractive target for the discovery of novel antibiotics for its essentiality in bacteria and absence in mammals. MEP cytidyltransferase (IspD), the third-step enzyme of the pathway, catalyzes MEP and CTP to form 4-diphosphocytidyl-2-C-methylerythritol (CDP-ME) and PPi. In the work, ispD gene from M. tuberculosis H37Rv (MtIspD) was cloned and expressed. With N-terminal fusion of a histidine-tagged sequence, MtIspD could be purified to homogeneity by one-step nickel affinity chromatography. MtIspD exists as a homodimer with an apparent molecular mass of 52 kDa. Enzyme property analysis revealed that MtIspD has high specificity for pyrimidine bases and narrow divalent cation requirements, with maximal activity found in the presence of CTP and Mg(2+). The turnover number of MtIspD is 3.4 s(-1). The Km for MEP and CTP are 43 and 92 muM, respectively. Furthermore, MtIspD shows thermal instable above 50 degrees C. Circular dichroism spectra revealed that the alteration of tertiary conformation is closely related with sharp loss of enzyme activity at higher temperature. This study is expected to help better understand the features of IspD and provide useful information for the development of novel antibiotics to treat M. tuberculosis.

Practical and theoretical advances in predicting the function of a protein by its phylogenetic distribution

The gap between the amount of genome information released by genome sequencing projects and our knowledge about the proteins' functions is rapidly increasing. To fill this gap, various 'genomic-context' methods have been proposed that exploit sequenced genomes to predict the functions of the encoded proteins. One class of methods, phylogenetic profiling, predicts protein function by correlating the phylogenetic distribution of genes with that of other genes or phenotypic characteristics. The functions of a number of proteins, including ones of medical relevance, have thus been predicted and subsequently confirmed experimentally. Additionally, various approaches to measure the similarity of phylogenetic profiles and to account for the phylogenetic bias in the data have been proposed. We review the successful applications of phylogenetic profiling and analyse the performance of various profile similarity measures with a set of one microsporidial and 25 fungal genomes. In the fungi, phylogenetic profiling yields high-confidence predictions for the highest and only the highest scoring gene pairs illustrating both the power and the limitations of the approach. Both practical examples and theoretical considerations suggest that in order to get a reliable and specific picture of a protein's function, results from phylogenetic profiling have to be combined with other sources of evidence.

Bacterial hosts for natural product production

Four bacterial hosts are reviewed in the context of either native or heterologous natural product production. E. coli, B. subtilis, pseudomonads, and Streptomyces bacterial systems are presented with each having either a long-standing or more recent application to the production of therapeutic natural compounds. The four natural product classes focused upon include the polyketides, nonribosomal peptides, terpenoids, and flavonoids. From the perspective of both innate and heterologous production potential, each bacterial host is evaluated according to biological properties that would either hinder or facilitate natural product biosynthesis.

1-Hydroxy-2-methyl-2-(E)-butenyl 4-diphosphate reductase (IDS) is encoded by multicopy genes in gymnosperms Ginkgo biloba and Pinus taeda

Isoprenoids are synthesized through the condensation of five-carbon intermediates, isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP), derived from two distinct biosynthetic routes: cytosolic mevalonate (MVA) and plastidial 2-C-methyl-D: -erythritol 4-phosphate (MEP) pathways. 1-Hydroxy-2-methyl-2-(E)-butenyl 4-diphosphate reductase (IDS; EC 1.17.1.2), which catalyzes the last step of MEP pathway, was cloned as a multicopy gene from gymnosperms Ginkgo biloba (GbIDS1, GbIDS2, and GbIDS2-1) and Pinus taeda (PtIDS1 and PtIDS2), and characterized. Phylogenetic tree constructed with other plant IDSs demonstrated gymnosperm IDSs were distinctively different from angiosperm IDSs. The gymnosperm IDS clade contained two subclades, one composed of GbIDS1 and PtIDS1, and the other composed of GbIDS2, GbIDS2-1, and PtIDS2. G. biloba IDSs, except GbIDS2-1, successfully complemented Escherichia coli DLYT1, a lytB disruptant, confirming the in vivo competency of isozymes. During the 4 weeks study period, although transcript levels of GbIDS1s were similar both in roots and leaves of cultured G. biloba embryo, the transcripts of GbIDS2 predominantly occurred in the embryo roots, where diterpene ginkgolides are biosynthesized. Levels of PtIDS2 transcripts in the diterpenoid resin-producing wood were 4-5 times higher than those in other tissues. Higher levels of GbIDS1 transcripts were induced by light, whereas those of GbIDS2 were increased by methyl jasmonate treatment. These results strongly imply GbIDS2 and PtIDS2 have high correlation with secondary metabolism. In Arabidopsis transient expression system, N-terminal 100 amino acid residues of GbIDS1 delivered fused GFP protein into chloroplast as well as cytosol and nucleus, whereas those of GbIDS2, GbIDS2-1, and two PtIDSs delivered GFP only into chloroplast.

Diversity in isoprene unit biosynthesis: The methylerythritol phosphate pathway in bacteria and plastids

The long-overlooked methylerythritol phosphate (MEP) pathway represents an alternative to the mevalonate route for the formation of isoprene units. It is found in most bacteria as well as in the plastids of all phototrophic organisms. A selection of significant steps of its discovery and elucidation are presented in this contribution, as well as a complete hypothetical biogenetic scheme for the last reduction step.

Human gamma delta T cells and tumor immunotherapy

Human Vgamma2Vdelta2 T cells recognize nonpeptide antigens derived from pathogenic microbes in a TCR-dependent manner, such as pyrophosphomonoester compounds from mycobacteria and malaria parasite and alkyl amines from Proteus, suggesting that this subset of gamma delta T cells is involved in infectious immunity. The precise recognition mechanism has been delineated using a site-directed mutagenesis strategy based on crystal structure of gamma delta TCR. On the other hand, several lines of evidence indicate that human gamma delta T cells are involved in tumor immunity. Although activated gamma delta T cells exhibit a cytolytic activity against most of tumor cells, only a small fraction of tumor cells, like Burkitt lymphoma cells and multiple myeloid cells, is recognized by human gamma delta T cells in a TCR-dependent manner. This implicates that human gamma delta T cells have two distinct pathways for anti-tumor immunity. One is a natural killer-like pathway and the other is a TCR-dependent pathway. Recently, it was shown that treatment of human tumor cells with nitrogen-containing bisphosphonates, therapeutic drugs for hypercalcemia in malignancy, generated antigenic structure on the surface of tumor cells, which could be recognized by human gamma delta T cells in a TCR-dependent manner. This tumor labeling system may lead to a novel strategy for cancer immunotherapy.

Cloning and expression of IspDF from Mesorhizobium loti. Characterization of a bifunctional protein that catalyzes non-consecutive steps in the methylerythritol phosphate pathway

Gram-negative bacteria, plant chloroplasts, green algae and some Gram-positive bacteria utilize the 2-C-methyl-d-erythritol phosphate (MEP) pathway for the biosynthesis of isoprenoids. IspD, ispE, and ispF encode the enzymes required to convert MEP to 2-C-methyl-d-erythritol 2,4-cyclodiphosphate (cMEDP) during the biosynthesis of isopentenyl diphosphate and dimethylallyl diphosphate in the MEP pathway. Upon analysis of the Mesorhizobium loti genome, ORF mll0395 showed homology to both ispD and ispF and appeared to encode a fusion protein. M. loti ispE was located elsewhere on the chromosome. Purified recombinant IspDF protein was mostly a homodimer, MW approximately 46 kDa/subunit. Incubation of IspDF with MEP, CTP, and ATP gave 4-diphosphocytidyl-2-C-methyl-d-erythritol (CDP-ME) as the only product. When Escherichia coli IspE protein was added to the incubation mixture, cMEDP was formed. In addition, M. loti ORF mll0395 complements lethal disruptions in both ispD and ispF in Salmonella typhimurium. These results indicate that IspDF is a bifunctional protein, which catalyzes the first and third steps in the conversion of MEP to cMEDP.

Microbial Isoprenoid Production: An Example of Green Chemistry through Metabolic Engineering

Saving energy, cost efficiency, producing less waste, improving the biodegradability of products, potential for producing novel and complex molecules with improved properties, and reducing the dependency on fossil fuels as raw materials are the main advantages of using biotechnological processes to produce chemicals. Such processes are often referred to as green chemistry or white biotechnology. Metabolic engineering, which permits the rational design of cell factories using directed genetic modifications, is an indispensable strategy for expanding green chemistry. In this chapter, the benefits of using metabolic engineering approaches for the development of green chemistry are illustrated by the recent advances in microbial production of isoprenoids, a diverse and important group of natural compounds with numerous existing and potential commercial applications. Accumulated knowledge on the metabolic pathways leading to the synthesis of the principal precursors of isoprenoids is reviewed, and recent investigations into isoprenoid production using engineered cell factories are described.

The Arabidopsis IspH homolog is involved in the plastid nonmevalonate pathway of isoprenoid biosynthesis

Plant isoprenoids are synthesized via two independent pathways, the cytosolic mevalonate (MVA) pathway and the plastid nonmevalonate pathway. The Escherichia coli IspH (LytB) protein is involved in the last step of the nonmevalonate pathway. We have isolated an Arabidopsis (Arabidopsis thaliana) ispH null mutant that has an albino phenotype and have generated Arabidopsis transgenic lines showing various albino patterns caused by IspH transgene-induced gene silencing. The initiation of albino phenotypes rendered by IspH gene silencing can arise independently from multiple sites of the same plant. After a spontaneous initiation, the albino phenotype is systemically spread toward younger tissues along the source-to-sink flow relative to the initiation site. The development of chloroplasts is severely impaired in the IspH-deficient albino tissues. Instead of thylakoids, mutant chloroplasts are filled with vesicles. Immunoblot analysis reveals that Arabidopsis IspH is a chloroplast stromal protein. Expression of Arabidopsis IspH complements the lethal phenotype of an E. coli ispH mutant. In 2-week-old Arabidopsis seedlings, the expression of 1-deoxy-d-xylulose 5-phosphate synthase (DXS), 1-deoxy-d-xylulose 5-phosphate reductoisomerase (DXR), IspD, IspE, IspF, and IspG genes is induced by light, whereas the expression of the IspH gene is constitutive. The addition of 3% sucrose in the media slightly increased levels of DXS, DXR, IspD, IspE, and IspF mRNA in the dark. In a 16-h-light/8-h-dark photoperiod, the accumulation of the IspH transcript oscillates with the highest levels detected in the early light period (2-6 h) and the late dark period (4-6 h). The expression patterns of DXS and IspG are similar to that of IspH, indicating that these genes are coordinately regulated in Arabidopsis when grown in a 16-h-light/8-h-dark photoperiod.

Characterization of the Arabidopsis clb6 mutant illustrates the importance of posttranscriptional regulation of the methyl-D-erythritol 4-phosphate pathway

The biosynthesis of isopentenyl diphosphate and dimethylallyl diphosphate, the two building blocks for isoprenoid biosynthesis, occurs by two independent pathways in plants. The mevalonic pathway operates in the cytoplasm, and the methyl-d-erythritol 4-phosphate (MEP) pathway operates in plastids. Plastidic isoprenoids play essential roles in plant growth and development. Plants must regulate the biosynthesis of isoprenoids to fulfill metabolic requirements in specific tissues and developmental conditions. The regulatory events that modulate the plant MEP pathway are not well understood. In this article, we demonstrate that the CHLOROPLAST BIOGENESIS6 (CLB6) gene, previously shown to be required for chloroplast development, encodes 1-hydroxy-2-methyl-butenyl 4-diphosphate reductase, the last-acting enzyme of the MEP pathway. Comparative analysis of the expression levels of all MEP pathway gene transcripts and proteins in the clb6-1 mutant background revealed that posttranscriptional control modulates the levels of different proteins in this central pathway. Posttranscriptional regulation was also found during seedling development and during fosmidomycin inhibition of the pathway. Our results show that the first enzyme of the pathway, 1-deoxy-d-xylulose 5-phosphate synthase, is feedback regulated in response to the interruption of the flow of metabolites through the MEP pathway.

Perspectives in anti-infective drug design. The late steps in the biosynthesis of the universal terpenoid precursors, isopentenyl diphosphate and dimethylallyl diphosphate

A mevalonate-independent pathway for the biosynthesis of isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP) that has been elucidated during the last decade is essential in plants, many eubacteria and apicomplexan parasites, but is absent in Archaea and animals. The enzymes of the pathway are potential targets for the development of novel antibiotic, antimalarial and herbicidal agents. This review is focused on the late steps of this pathway. The intermediate 2C-methyl-D-erythritol 2,4-cyclodiphosphate is converted into IPP and DMAPP via 1-hydroxy-2-methyl-2-(E)-butenyl 4-diphosphate by the consecutive action of the iron-sulfur proteins IspG and IspH. IPP and DMAPP can be interconverted by IPP isomerase which is essential in microorganisms using the mevalonate pathway, whereas its presence is optional in microorganisms using the non-mevalonate pathway. A hitherto unknown family of IPP isomerases using FMN as coenzyme has been discovered recently in Archaea and certain eubacteria.

Non-peptide antigens activating human Vγ9/Vδ2 T lymphocytes

Various non-peptidic ligands which specifically activate most of circulating human Vgamma9/Vdelta2 T lymphocytes are now known. Most of these are so-called phosphoantigens and directly trigger the Vgamma9/Vdelta2 TCR expressing cells, without need for MHC-restricted presentation molecules. Although some potent phosphoantigens currently involved in clinical trials are chemically-synthesized molecules, most of the natural antigens were isolated from microbial cultures. The structures and biosynthesis of phosphoantigens are reviewed here and the possible physiological significance of their recognition by gammadelta T lymphocytes is discussed.

Biosynthesis of isoprenoids: a bifunctional IspDF enzyme from Campylobacter jejuni

In the nonmevalonate pathway of isoprenoid biosynthesis, the conversion of 2C-methyl-d-erythritol 4-phosphate into its cyclic diphosphate proceeds via nucleotidyl intermediates and is catalyzed by the products of the ispD, ispE and ispF genes. An open reading frame of Campylobacter jejuni with similarity to the ispD and ispF genes of Escherichia coli was cloned into an expression vector directing the formation of a 42 kDa protein in a recombinant E. coli strain. The purified protein was shown to catalyze the transformation of 2C-methyl-D-erythritol 4-phosphate into 4-diphosphocytidyl-2C-methyl-D-erythritol and the conversion of 4-diphosphocytidyl-2C-methyl-D-erythritol 2-phosphate into 2C-methyl-D-erythritol 2,4-cyclodiphosphate at catalytic rates of 19 micro mol x mg(-1) x min(-1) and 7 micro mol x mg(-1) x min(-1), respectively. Both enzyme-catalyzed reactions require divalent metal ions. The C. jejuni enzyme does not catalyze the formation of 2C-methyl-D-erythritol 3,4-cyclophosphate from 4-diphosphocytidyl-2C-methyl-D-erythritol, a side reaction catalyzed in vitro by the IspF proteins of E. coli and Plasmodium falciparum. Comparative genomic analysis show that all sequenced alpha- and epsilon-proteobacteria have fused ispDF genes. These bifunctional proteins are potential drug targets in several human pathogens (e.g. Helicobacter pylori, C. jejuni and Treponema pallidum).