Isolation and proof of structure of wildfire toxin

Characterization of Enzymes Catalyzing the Initial Steps of the β-Lactam Tabtoxin Biosynthesis

Tabtoxin is a β-lactam ring-containing phytotoxin produced by a plant pathogenic Pseudomonas species. Here, we describe the early stages of tabtoxin biosynthesis, involving a C-methylation reaction catalyzed by the S-adenosyl-l-methionine-dependent methyltransferase TblA as the initial step for the β-lactam construction. Gene deletion and in vitro biochemical assays demonstrated that the Gcn5-related N-acetyltransferase domain of TblD catalyzes the acetylation of the α-amino group of 5-methyl-l-lysine. This establishment of the early reaction steps lays the foundation for characterizing unique β-lactam biosynthesis.

Synthetic studies of biologically active natural products contributing to pesticide development

Natural product research, including total synthesis, is becoming increasingly important for the discovery of pesticide seeds and leads. Synthetic studies of biologically active compounds such as antibiotics (enacyloxins, polynactin, pamamycins, spirofungin A and B, glutarimides and antimycins), phytopathogenic toxins (pyricuol, pyriculariol, tabtoxinine-β-lactam, gigantenone, phomenone and phaseolinone), marine derived products (pteroenone, β-D-Asp-Gly, didemniselinolipid B, cortistatin A, sanctolide A and gizzerosine), POPs (dieldrin, endosulfan, HCB), plant hormones (abscisic acid and jasmonic acid), insect pheromones (endo-brevicomin etc.), especially using a variety of biotransformation are described.

Small-Molecule Acetylation by GCN5-Related N-Acetyltransferases in Bacteria

Acetylation is a conserved modification used to regulate a variety of cellular pathways, such as gene expression, protein synthesis, detoxification, and virulence. Acetyltransferase enzymes transfer an acetyl moiety, usually from acetyl coenzyme A (AcCoA), onto a target substrate, thereby modulating activity or stability. Members of the GCN5- N -acetyltransferase (GNAT) protein superfamily are found in all domains of life and are characterized by a core structural domain architecture. These enzymes can modify primary amines of small molecules or of lysyl residues of proteins. From the initial discovery of antibiotic acetylation, GNATs have been shown to modify a myriad of small-molecule substrates, including tRNAs, polyamines, cell wall components, and other toxins. This review focuses on the literature on small-molecule substrates of GNATs in bacteria, including structural examples, to understand ligand binding and catalysis. Understanding the plethora and versatility of substrates helps frame the role of acetylation within the larger context of bacterial cellular physiology.

Palladium-catalyzed formal insertion of carbenoids into N,O-aminals: direct access to α-alkoxy-β-amino acid esters

A new and efficient reaction has been developed to synthesize α-alkoxy-β-amino acid esters via palladium-catalyzed formal insertion of carbenoids into N,O-aminals.

Functional chararacterization of the enzymes TabB and TabD involved in tabtoxin biosynthesis by Pseudomonas syringae

Pseudomonas syringae pv. tabaci ATCC 11528 produces tabtoxin, a β-lactam-containing dipeptide phytotoxin. Tabtoxinine-β-lactam (TβL), one of tabtoxin's constituent amino acids, structurally mimics lysine, and many of the proteins encoded by the tabtoxin biosynthetic gene cluster are homologs of lysine biosynthetic enzymes, suggesting that the tabtoxin and lysine biosynthetic routes parallel one another. We cloned and expressed TabB and TabD, predicted homologs of tetrahydrodipicolinate (THDPA)-N-acyltransferase and N-acyl-THDPA aminotransferase, respectively, to determine their activities in vitro. We confirmed that TabB succinylates THDPA and that TabD is a PLP-dependent aminotransferase that utilizes glutamate as an amine donor. Surprisingly, we also found that though TabD could utilize the TabB product N-succinyl-THDPA as a substrate, THDPA itself was also recognized. These observations reveal that TabB functionally duplicates DapD, the THDPA-N-succinyltransferase involved in lysine biosynthesis, and reinforce the close relationship between the metabolic logics underpinning the respective biosynthetic pathways.

Mechanistic Basis for ATP-Dependent Inhibition of Glutamine Synthetase by Tabtoxinine-β-lactam

Tabtoxinine-β-lactam (TβL), also known as wildfire toxin, is a time- and ATP-dependent inhibitor of glutamine synthetase produced by plant pathogenic strains of Pseudomonas syringae. Here we demonstrate that recombinant glutamine synthetase from Escherichia coli phosphorylates the C3-hydroxyl group of the TβL 3-(S)-hydroxy-β-lactam (3-HβL) warhead. Phosphorylation of TβL generates a stable, noncovalent enzyme-ADP-inhibitor complex that resembles the glutamine synthetase tetrahedral transition state. The TβL β-lactam ring remains intact during enzyme inhibition, making TβL mechanistically distinct from traditional β-lactam antibiotics such as penicillin. Our findings could enable the design of new 3-HβL transition state inhibitors targeting enzymes in the ATP-dependent carboxylate-amine ligase superfamily with broad therapeutic potential in many disease areas.

Antibiotics from Gram-negative bacteria: a comprehensive overview and selected biosynthetic highlights

Covering: up to 2017The overwhelming majority of antibiotics in clinical use originate from Gram-positive Actinobacteria. In recent years, however, Gram-negative bacteria have become increasingly recognised as a rich yet underexplored source of novel antimicrobials, with the potential to combat the looming health threat posed by antibiotic resistance. In this article, we have compiled a comprehensive list of natural products with antimicrobial activity from Gram-negative bacteria, including information on their biosynthetic origin(s) and molecular target(s), where known. We also provide a detailed discussion of several unusual pathways for antibiotic biosynthesis in Gram-negative bacteria, serving to highlight the exceptional biocatalytic repertoire of this group of microorganisms.

Tabtoxinine-β-lactam is a “stealth” β-lactam antibiotic that evades β-lactamase-mediated antibiotic resistance

Tabtoxinine-β-lactam (TβL) is a phytotoxin produced by plant pathogenic strains of Pseudomonas syringae.

Synthesis of Naturally Derived Bioactive Compounds of Agricultural Interest

Synthetic studies on bioactive compounds are described, involving phytotoxins (tobacco wildfire disease toxin tabtoxinine-beta-lactam and rice blast disease toxin pyricuol) a glutarimide antibiotic (actiketal) black vomit toxin (gizzerosine) and marine products (antifeedant pteroenone and serinol compound didemniserinolipid).

L-amino acid ligase from Pseudomonas syringae producing tabtoxin can be used for enzymatic synthesis of various functional peptides

Functional peptides are expected to be beneficial compounds that improve our quality of life. To address the growing need for functional peptides, we have examined peptide synthesis by using microbial enzymes. l-Amino acid ligase (Lal) catalyzes the condensation of unprotected amino acids in an ATP-dependent manner and is applicable to fermentative production. Hence, Lal is a promising enzyme to achieve cost-effective synthesis. To obtain a Lal with novel substrate specificity, we focused on the putative Lal involved in the biosynthesis of the dipeptidic phytotoxin designated tabtoxin. The tabS gene was cloned from Pseudomonas syringae NBRC14081 and overexpressed in Escherichia coli cells. The recombinant TabS protein produced showed the broadest substrate specificity of any known Lal; it detected 136 of 231 combinations of amino acid substrates when dipeptide synthesis was examined. In addition, some new substrate specificities were identified and unusual amino acids, e.g., l-pipecolic acid, hydroxy-l-proline, and β-alanine, were found to be acceptable substrates. Furthermore, kinetic analysis and monitoring of the reactions over a short time revealed that TabS showed distinct substrate selectivity at the N and C termini, which made it possible to specifically synthesize a peptide without by-products such as homopeptides and heteropeptides with the reverse sequence. TabS specifically synthesized the following functional peptides, including their precursors: l-arginyl-l-phenylalanine (antihypertensive effect; yield, 62%), l-leucyl-l-isoleucine (antidepressive effect; yield, 77%), l-glutaminyl-l-tryptophan (precursor of l-glutamyl-l-tryptophan, which has antiangiogenic activity; yield, 54%), l-leucyl-l-serine (enhances saltiness; yield, 83%), and l-glutaminyl-l-threonine (precursor of l-glutamyl-l-threonine, which enhances saltiness; yield, 96%). Furthermore, our results also provide new insights into tabtoxin biosynthesis.

Pseudomonas syringae self-protection from tabtoxinine-β-lactam by ligase TblF and acetylase Ttr

Plant pathogenic Pseudomonas syringae produce the hydroxy-β-lactam antimetabolite tabtoxinine-β-lactam (TβL) as a time-dependent inactivating glutamine analogue of plant glutamine synthetases. The producing pseudomonads use multiple modes of self-protection, two of which are characterized in this study. The first is the dipeptide ligase TblF which converts tabtoxinine-β-lactam to the TβL-Thr dipeptide known as tabtoxin. The dipeptide is not recognized by glutamine synthetase. This represents a Trojan Horse strategy: the dipeptide is secreted, taken up by dipeptide permeases in neighboring cells, and TβL is released by peptidase action. The second self-protection mode is elaboration by the acetyltransferase Ttr, which acetylates the α-amino group of the proximal inactivator TβL, but not the tabtoxin dipeptide.

Genes Involved in the Production of Antimetabolite Toxins by Pseudomonas syringae Pathovars

Pseudomonas syringae is pathogenic in a wide variety of plants, causing diseases with economic impacts. Pseudomonas syringae pathovars produce several toxins that can function as virulence factors and contribute to disease symptoms. These virulence factors include antimetabolite toxins, such as tabtoxin, phaseolotoxin and mangotoxin, which target enzymes in the pathways of amino acid metabolism. The antimetabolite toxins are generally located in gene clusters present in the flexible genomes of specific strains. These gene clusters are typically present in blocks of genes that appear to be integrated into specific sites in the P. syringae core genome. A general overview of the genetic organization and biosynthetic and regulatory functions of these genetic traits of the antimetabolite toxins will be given in the present work.

Chemical and metabolic aspects of antimetabolite toxins produced by Pseudomonas syringae pathovars

Pseudomonas syringae is a phytopathogenic bacterium present in a wide variety of host plants where it causes diseases with economic impact. The symptoms produced by Pseudomonas syringae include chlorosis and necrosis of plant tissues, which are caused, in part, by antimetabolite toxins. This category of toxins, which includes tabtoxin, phaseolotoxin and mangotoxin, is produced by different pathovars of Pseudomonas syringae. These toxins are small peptidic molecules that target enzymes of amino acids' biosynthetic pathways, inhibiting their activity and interfering in the general nitrogen metabolism. A general overview of the toxins' chemistry, biosynthesis, activity, virulence and potential applications will be reviewed in this work.

Construction of tn3-containing plasmids from plant-pathogenic pseudomonads and an examination of their biological properties

Indigenous plasmids isolated from Pseudomonas tabaci ATCC 11528(pJP1), Pseudomonas angulata 45(pJP30), and P. tabaci BR2(pBPW1) (M. Obukowicz and P. D. Shaw, J. Bacteriol. 155:438-442, 1983) were labeled with Tn3, and the strains were subsequently cured of their respective plasmids. Plasmid-containing and cured isolates caused plant symptoms that were nearly indistinguishable, and the same amount of tabtoxin was produced by P. tabaci strains ATCC 11528 and BR2.

Synthesis of Enantiopure Pyrrolidine-Derived Peptidomimetics and Oligo-β-peptides via Nucleophilic Ring-Opening of β-Lactams

The synthesis of the two enantiomers of pyrrolidine-derived spiro beta-lactams by resolution with D- and L-Boc phenylalanine is described. The potential of these optically active spiro beta-lactams on the synthesis of peptidomimetics as analogues of melanostatin is evaluated. Theoretical studies of several models, at the Becke3LYP/6-31+G* level of theory, together with previous experimental evidences from our group, gathered by NMR, allow us to design structures that can efficiently mimic some biologically active peptide-type molecules. On the other hand, the spiro beta-lactams have shown their utility in the preparation of beta-peptides. As an example, a homo-tetra-beta-peptide was synthesized. This research will continue in the future in order to obtain higher peptides with potential biological activity.

Straightforward Asymmetric Entry to Highly Functionalized 3-Substituted 3-Hydroxy-β-lactams via Baylis−Hillman or Bromoallylation Reactions

The reaction of various activated vinyl systems, including 2-cyclopenten-1-one, with enantiopure azetidine-2,3-diones 1 was promoted by DABCO to afford the corresponding optically pure Baylis-Hillman adducts 2 without detectable epimerization. However, the reaction of alpha-keto lactams 1 with but-3-yn-2-one was not as successful, giving the corresponding beta-halo Baylis-Hillman adduct in low yield. Metal-mediated bromoallylation reaction between 2,3-dibromopropene and azetidine-2,3-diones 1 was investigated in aqueous media. Surprisingly, indium was unable to promote the bromoallylation reaction of alpha-keto lactams 1, but the Sn-Hf(4)Cl-promoted bromoallylation of ketones 1 proceeded efficiently to achieve bromohomoallyl alcohols 5 as single diastereomers. On this basis, simple and fast protocols for the asymmetric synthesis of the potentially bioactive 3-substituted 3-hydroxy-beta-lactam moiety were developed.

Crystal structure of tabtoxin resistance protein complexed with acetyl coenzyme A reveals the mechanism for beta-lactam acetylation

Tabtoxin resistance protein (TTR) is an enzyme that renders tabtoxin-producing pathogens, such as Pseudomonas syringae, tolerant to their own phytotoxins. Here, we report the crystal structure of TTR complexed with its natural cofactor, acetyl coenzyme A (AcCoA), to 1.55A resolution. The binary complex forms a characteristic "V" shape for substrate binding and contains the four motifs conserved in the GCN5-related N-acetyltransferase (GNAT) superfamily, which also includes the histone acetyltransferases (HATs). A single-step mechanism is proposed to explain the function of three conserved residues, Glu92, Asp130 and Tyr141, in catalyzing the acetyl group transfer to its substrate. We also report that TTR possesses HAT activity and suggest an evolutionary relationship between TTR and other GNAT members.

Apoptotic cell death is a common response to pathogen attack in oats

We have examined the characteristics of cell death induced by pathogen infection in oats with respect to following hallmark apoptotic features: DNA laddering, chromatin condensation, and electron microscopic-terminal deoxynucleotidyl transferase-mediated UTP end labeling positive response. A wide range of plant pathogens representing different levels of parasitism in susceptible and resistant interactions were used for the inocula, which include (i) an obligate parasite, Puccinia coronata f. sp. avenae (the crown rust fungus); (ii) a facultative biotroph parasite, Magnaporthe grisea (the blast fungus); (iii) pathogenic bacteria, Pseudomonas syringae pv. atropurpurea and P. syringae pv. coronafaciens (the halo or stripe blights of oats); and (iv) Ryegrass mottle virus. Surprisingly, any of the pathogens used induced most of the apoptotic features in oat cells at and around the infection sites, indicating that apoptotic cell death is a common phenomenon in oats during pathogen attack. The localization and the timing of apoptotic cell death during a course of infection were, however, quite different depending on the interactions (compatible or incompatible) and the pathogens (fungi, bacteria, or viruses). Possible roles of apoptotic cell death in the susceptible and resistant interactions are discussed.

Tabtoxin-resistant protein: overexpression, purification, and characterization

One of the self-protection mechanisms in Pseudomonas syringae pv. tabaci, a pathogen of tobacco wildfire, is thought to be due to its tabtoxin-resistance gene (ttr). In this study, the ttr gene was inserted into an expression vector, pQE30, and successfully expressed in Escherichia coli M15 at high levels. The purified recombinant tabtoxin-resistant protein (TTR) had an apparent molecular mass of about 21 kDa on SDS-PAGE as well as by mass spectroscopy and had a pI of 6.6 on isoelectric focusing-PAGE. Spectral analysis showed that TTR possesses a maximum fluorescence wavelength (lambda(max)) of 325 nm upon excitation at 282 nm and a positive band with a maximum at 195 nm and a broad negative band with a minimum at 215 nm in the far-UV CD spectrum. The spectrophotometric assay demonstrated the strong detoxification activity of TTR. These results are the first report of the characterization of the purified tabtoxin-resistant protein. Its capacity to detoxify tabtoxinine-beta-lactam shows that it must be one of the self-protection mechanisms in pv. tabaci.

Metal-mediated carbonyl-1,3-butadien-2-ylation by 1,4-bis(methanesulfonyl)-2-butyne or 1,4-dibromo-2-butyne in aqueous media: asymmetric synthesis of 3-substituted 3-hydroxy-beta-lactams

Metal-mediated 1,3-butadien-2-ylation reactions between 1,4-dibromo-2-butyne or 1,4-bis(methanesulfonyl)-2-butyne and optically pure azetidine-2,3-diones were investigated in aqueous media, offering a convenient asymmetric entry to the potentially bioactive 3-substituted 3-hydroxy-beta-lactam moiety. The diastereoselectivity of the addition reaction was controlled by the bulky chiral auxiliary at C4. However, while the regioselectivity of the process was full, the chemical yield of the addition was a function of the nature of both the metal reagent and the system solvent as well. In addition, 2-azetidinone-tethered 1,3-butadienes can easily be transformed into other functionalities via Diels-Alder reaction.

Microbial toxins in plant-pathogen interactions: Biosynthesis, resistance mechanisms, and significance

In the history of phytopathology, microbial toxins have been the objects of extensive studies as possible pathogenicity or virulence factors for the producer pathogens. The recent development of molecular genetic techniques provided an experimental basis to thoroughly test the role of these secondary metabolites in pathogenesis. Some of them did prove to be highly associated with disease initiation or enhanced virulence in certain plant-pathogen interactions. In this review, we describe recent progresses in the field of plant-pathogen interactions focusing on two toxins; i.e., tabtoxin from Pseudomonas syringae and trichothecenes from Fusarium and other fungi. These microbial toxins have convincingly been shown to play causal roles in plant disease development. Studies on the biosynthesis and resistance mechanisms of these producers are outlined, and the significance of this knowledge is discussed in relation to practical applications in agriculture.

Metal-promoted allylation, propargylation, or allenylation of azetidine-2,3-diones in aqueous and anhydrous media. Application to the asymmetric synthesis of densely functionalized 3-substituted 3-hydroxy-beta-lactams

Metal-mediated carbonyl allylation, allenylation, and propargylation of optically pure azetidine-2,3-diones were investigated in both anhydrous and aqueous environments. Different metals promoters showed varied regioselectivities on product formation during allenylation/propargylation reactions of the keto-beta-lactams. The stereochemistry of the new C3-substituted C3-hydroxy quaternary center was controlled by placing a chiral auxiliary at C4. In this way, the coupling of azetidine-2,3-diones with a variety of propenyl-, propynyl-, and allenylmetal reagents offers a convenient asymmetric entry to potentially bioactive 3-substituted 3-hydroxy-beta-lactams.

Detection of tabtoxin-producing strains of Pseudomonas syringae by PCR

AIMS

The present study describes a system based on PCR to distinguish tabtoxin-producing strains of Pseudomonas syringae from other Ps. syringae plant pathogens that produce chlorosis-inducing phytotoxins.

METHODS AND RESULTS

Thirty-two strains of Ps. syringae and related species were examined. Two sets of PCR primers were developed to amplify genes (tblA and tabA) required for tabtoxin production. Only a PCR product of 829 bp or 1020 bp was produced in PCR reactions with the tblA or tabA primer sets, respectively, and cells from tabtoxin-producing pathovars of Pseudomonas syringae. All known non-tabtoxin producing bacterial species failed to produce an amplification product with either primer set.

CONCLUSIONS

PCR of genes required for tabtoxin production is a simple, rapid and reliable method for identifying tabtoxin-producing strains of Ps. syringae.

SIGNIFICANCE AND IMPACT OF THE STUDY

The protocol can effectively distinguish tabtoxin-producing strains of Ps. syringae from other Ps. syringae pathovars and Ps. syringae pv. tabaci strains from other tabtoxin-producing Ps. syringae pathovars.

Regio- and stereocontrolled metal-mediated carbonyl propargylation or allenylation of enantiomerically pure azetidine-2,3-diones: synthesis of highly functionalized 3-substituted 3-hydroxy-beta-lactams

Regio- and stereocontrolled metal-mediated Barbier-type reactions of azetidine-2,3-diones with differently substituted propargyl bromides offer an efficient asymmetric entry to densely functionalized 3-propargyl- (or allenyl-) substituted 3-hydroxy-beta-lactams.

Biochemical and physiological aspects of brown blotch disease of Agaricus bisporus

Pseudomonas tolaasii is a bacterium endemic to the compost beds where common mushroom (Agaricus bisporus) is cultivated. Under some environmental conditions still not well-determined, but influenced by temperature and relative humidity, the bacterium can become pathogenic and provoke the brown blotch disease. This review describes the interaction between P. tolaasii and A. bisporus that results in the appearance of brown spots on the mushroom caps, typical symptoms of the disease. Firstly, P. tolaasii is studied, the changes in pathogenicity are explained, the compounds that provoke the damage are enumerated as well as various experimental methods to identify the pathogenic form of the bacteria. Secondly, mechanisms involved in the formation of the brown colour on the A. bisporus caps upon infection are briefly mentioned, taking into account the enzymes that catalyse the reaction, their mechanism, substrates and reaction products. Afterwards, a detailed description of the infection process is presented step by step, starting by the chemotactical attraction, fixation, secretion of the toxins, membrane breakdown, effect of the toxin on mushroom polyphenol oxidases and on the discolouration reaction. A possible mechanism of infection is hypothesised at the molecular level. Finally, the strategies tested until now to control the disease are discussed.

Pseudomonas syringae phytotoxins: mode of action, regulation, and biosynthesis by peptide and polyketide synthetases

Coronatine, syringomycin, syringopeptin, tabtoxin, and phaseolotoxin are the most intensively studied phytotoxins of Pseudomonas syringae, and each contributes significantly to bacterial virulence in plants. Coronatine functions partly as a mimic of methyl jasmonate, a hormone synthesized by plants undergoing biological stress. Syringomycin and syringopeptin form pores in plasma membranes, a process that leads to electrolyte leakage. Tabtoxin and phaseolotoxin are strongly antimicrobial and function by inhibiting glutamine synthetase and ornithine carbamoyltransferase, respectively. Genetic analysis has revealed the mechanisms responsible for toxin biosynthesis. Coronatine biosynthesis requires the cooperation of polyketide and peptide synthetases for the assembly of the coronafacic and coronamic acid moieties, respectively. Tabtoxin is derived from the lysine biosynthetic pathway, whereas syringomycin, syringopeptin, and phaseolotoxin biosynthesis requires peptide synthetases. Activation of phytotoxin synthesis is controlled by diverse environmental factors including plant signal molecules and temperature. Genes involved in the regulation of phytotoxin synthesis have been located within the coronatine and syringomycin gene clusters; however, additional regulatory genes are required for the synthesis of these and other phytotoxins. Global regulatory genes such as gacS modulate phytotoxin production in certain pathovars, indicating the complexity of the regulatory circuits controlling phytotoxin synthesis. The coronatine and syringomycin gene clusters have been intensively characterized and show potential for constructing modified polyketides and peptides. Genetic reprogramming of peptide and polyketide synthetases has been successful, and portions of the coronatine and syringomycin gene clusters could be valuable resources in developing new antimicrobial agents.

Expression of an Engineered Cecropin Gene Cassette in Transgenic Tobacco Plants Confers Disease Resistance to Pseudomonas syringae pv. tabaci

ABSTRACT A chimeric gene fusion cassette, consisting of a secretory sequence from barley alpha-amylase joined to a modified cecropin (MB39) coding sequence and placed under control of the promoter and terminator from the potato proteinase inhibitor II (PiII) gene, was introduced into tobacco by Agrobacterium-mediated transformation. Transgenic and control plants reacted differently when inoculated with tobacco wildfire pathogen Pseudomonas syringae pv. tabaci at various cell concentrations. With control plants (transformed with a PiII-GUS [beta-D-glucuronidase] gene fusion), necrosis was clearly visible in leaf tissue infiltrated with bacterial inoculum levels of 10(2), 10(3), 10(4), 10(5), and 10(6) CFU/ml. With MB39-transgenic plants, however, necrosis was observed only in the areas infiltrated with the two highest levels (10(5) and 10(6) CFU/ml). No necrosis was evident in areas infiltrated with bacterial concentrations of 10(4) CFU/ml or less. Bacterial multiplication in leaves of MB39-transgenic plants was suppressed more than 10-fold compared to control plants, and absence of disease symptom development was associated with this growth suppression. We conclude that the pathogen-induced promoter and the secretory sequence were competent elements for transforming a cecropin gene into an effective disease-control gene for plants.

Stereoselective Synthesis of 3-Substituted 4-(Formyloxy)-2-azetidinones by the Unusual Baeyer-Villiger Reaction of beta-Lactam Aldehydes. Scope and Synthetic Applications

The Baeyer-Villiger oxidation of 4-formyl-beta-lactams 1with m-CPBA gave 4-(formyloxy) beta-lactams 2 in a simple, efficient, and totally stereoselective process. This reaction is one of the scarce examples of the preferred migration of a carbon moiety in an aliphatic aldehyde. The influence of the substituents at N1 and C3 of the four-membered ring in the Baeyer-Villiger rearrangement has been studied. Thus, alkyl, alkenyl, aryl, and alkyloxy 3-substituted-1-(p-anisyl)-2-azetidinones 1 form exclusively 4-(formyloxy) beta-lactams 2. Amide or acetoxy substituents at C3 of the four-membered ring produce mixtures of 4-(formyloxy) beta-lactams 2and 4-carboxy beta-lactams 5. The exclusive formation of carboxy derivatives is observed sometimes for 1-alkyl-substituted-2-azetidinones 1. 4-(Formyloxy) beta-lactams 2 are suitable starting materials to prepare different 4-unsubstituted beta-lactams 9 using beta-hydroxy amides 8 as isolable intermediates. The overall transformation 4-formyl-2-azetidinone to 4-unsubstituted beta-lactam is an easy and convenient stereoselective route to these interesting types of compounds.

Expression of giant silkmoth cecropin B genes in tobacco

Cecropin B is a small antibacterial peptide from the giant silkmoth Hyalophora cecropia. To reveal the potential of this peptide for engineering bacterial disease resistance into crops, several cecropin B gene constructs were made either for expression in the cytosol or for secretion. All constructs were cloned in a plant expression vector and introduced in tobacco via Agrobacterium tumefaciens. A cDNA-derived cecropin B gene construct lacking the amino-terminal signal peptide was poorly expressed in transgenic plants at the mRNA level, whereas plants harbouring a full-length cDNA-derived construct containing the insect signal peptide, showed increased cecropin B-mRNA levels. Highest expression was found in plants harbouring a construct with a plant-gene-derived signal peptide. In none of the transgenic plants could the cecropin B peptide be detected. This is most likely caused by breakdown of the peptide by plant endogenous proteases, since a chemically synthesized cecropin B peptide was degraded within seconds in various plant cell extracts. This degradation could be prevented by the addition of specific protease inhibitors and by boiling the extract prior to adding the peptide. In addition, anionic detergents, in contrast to cationic, zwitter-ionic or non-ionic detergents, could prevent this degradation. Nevertheless, transgenic tobacco plants were evaluated for resistance to Pseudomonas solanacearum, the causal agent of bacterial wilt of many crops, and P. syringae pv. tabaci, the causal agent of bacterial wildfire, which are highly susceptible to cecropin B in vitro. No resistance was found. These experiments indicate that introduction and expression of cecropin B genes in tobacco does not result in detectable cecropin B protein levels and resistance to bacterial infections, most likely due to degradation of the protein by endogenous proteases.