Arrangement of catalytic sites in the multifunctional enzyme enniatin synthetase

Unnatural verticilide enantiomer inhibits type 2 ryanodine receptor-mediated calcium leak and is antiarrhythmic

Ca²⁺ leak via ryanodine receptor type 2 (RyR2) can cause potentially fatal arrhythmias in a variety of heart diseases and has also been implicated in neurodegenerative and seizure disorders, making RyR2 an attractive therapeutic target for drug development. Here we synthesized and investigated the fungal natural product and known insect RyR antagonist (-)-verticilide and several congeners to determine their activity against mammalian RyR2. Although the cyclooligomeric depsipeptide natural product (-)-verticilide had no effect, its nonnatural enantiomer [ent-(+)-verticilide] significantly reduced RyR2-mediated spontaneous Ca²⁺ leak both in cardiomyocytes from wild-type mouse and from a gene-targeted mouse model of Ca²⁺ leak-induced arrhythmias (Casq2^-/-). ent-(+)-verticilide selectively inhibited RyR2-mediated Ca²⁺ leak and exhibited higher potency and a distinct mechanism of action compared with the pan-RyR inhibitors dantrolene and tetracaine and the antiarrhythmic drug flecainide. ent-(+)-verticilide prevented arrhythmogenic membrane depolarizations in cardiomyocytes without significant effects on the cardiac action potential and attenuated ventricular arrhythmia in catecholamine-challenged Casq2^-/- mice. These findings indicate that ent-(+)-verticilide is a potent and selective inhibitor of RyR2-mediated diastolic Ca²⁺ leak, making it a molecular tool to investigate the therapeutic potential of targeting RyR2 hyperactivity in heart and brain pathologies. The enantiomer-specific activity and straightforward chemical synthesis of (unnatural) ent-(+)-verticilide provides a compelling argument to prioritize ent-natural product synthesis. Despite their general absence in nature, the enantiomers of natural products may harbor unprecedented activity, thereby leading to new scaffolds for probe and therapeutic development.

Bacillus subtilis as heterologous host for the secretory production of the non-ribosomal cyclodepsipeptide enniatin

The heterologous expression of genes or gene clusters in microbial hosts, followed by metabolic engineering of biosynthetic pathways, is key to access industrially and pharmaceutically relevant compounds in an economically affordable and sustainable manner. Therefore, platforms need to be developed, which provide tools for the controlled synthesis of bioactive compounds. The Gram-positive bacterium Bacillus subtilis is a promising candidate for such applications, as it is generally regarded as a safe production host, its physiology is well investigated and a variety of tools is available for its genetic manipulation. Furthermore, this industrially relevant bacterium provides a high secretory potential not only for enzymes but also for primary and secondary metabolites. In this study, we present the first heterologous expression of an eukaryotic non-ribosomal peptide synthetase gene (esyn) coding for the biosynthesis of the small molecule enniatin in B. subtilis. Enniatin is a pharmaceutically used cyclodepsipeptide for treatment of topical bacterial and fungal infections. We generated various enniatin-producing B. subtilis strains, allowing for either single chromosomal or plasmid-based multi-copy expression of the esyn cluster under the control of an acetoin-inducible promoter system. Optimization of cultivation conditions, combined with modifications of the genetic background and multi-copy plasmid-based esyn expression, resulted in a secretory production of enniatin B. This work presents B. subtilis as a suitable host for the expression of heterologous eukaryotic non-ribosomal peptide synthetases (NRPS) clusters.

Biosynthesis of crocacin involves an unusual hydrolytic release domain showing similarity to condensation domains

The crocacins are potent antifungal and cytotoxic natural compounds from myxobacteria of the genus Chondromyces. Although total synthesis approaches have been reported, the molecular and biochemical basis guiding the formation of the linear crocacin scaffold has remained unknown. Along with the identification and functional analysis of the crocacin biosynthetic gene cluster from Chondromyces crocatus Cm c5, we here present the identification and biochemical characterization of an unusual chain termination domain homologous to condensation domains responsible for hydrolytic release of the product from the assembly line. In particular, gene inactivation studies and in vitro experiments using the heterologously produced domain CroK-C2 confirm this surprising role giving rise to the linear carboxylic acid. Additionally, we determined the kinetic parameters of CroK-C2 by monitoring hydrolytic cleavage of the substrate mimic N-acetylcysteaminyl-crocacin B using an innovative high-performance liquid chromatography mass spectrometry-based assay.

Cloning and characterization of the gene cluster required for beauvericin biosynthesis in Fusarium proliferatum

Beauvericin, a cyclohexadepsipeptide-possessing natural product with synergistic antifungal, insecticidal, and cytotoxic activities. We isolated and characterized the fpBeas gene cluster, devoted to beauvericin biosynthesis, from the filamentous fungus Fusarium proliferatum LF061. Targeted inactivation of the F. proliferatum genomic copy of fpBeas abolished the production of beauvericin. Comparative sequence analysis of the FpBEAS showed 74% similarity with the BbBEAS that synthesizes the cyclic trimeric ester beauvericin in Beauveria bassiana, which assembles N-methyl-dipeptidol monomer intermediates by the programmed iterative use of the nonribosomal peptide synthetase modules. Differences between the organization of the beauvericin loci in F. proliferaturm and B. bassiana revealed the mechanism for high production of beauvericin in F. proliferatum. Our work provides new insights into beauvericin biosynthesis, and may lead to beauvericin overproduction and creation of new analogs via synthetic biology approaches.

Antifungal effects of the bioactive compounds enniatins A, A(1), B, B(1)

To produce enniatin (ENs), Fusarium tricinctum CECT 20150 was grown in a liquid medium of potato (PDB), being mycotoxin purified by high performance liquid chromatography (HPLC) with a reverse phase semipreparative column using a mobile phase of acetonitrile/water using gradient condition. The purity of the ENs fractions was verified by analytical HPLC and LC/MS-MS. The pure fractions of ENs were utilized to study the biological activity on several mycotoxigenic moulds as Fusarium verticilloides, Fusarium sporotrichioides, Fusarium tricinctum, Fusarium poae, Fusarium oxysporum, Fusarium proliferatum, Beauveria bassiana, Trichoderma harzianum, Aspergillus flavus, Aspergillus parasiticus, Aspergillus fumigatus, Aspergillus ochraceus and Penicillium expansum. The results obtained demonstrated that in several antibiograms, ENs induced the inhibition of the grown microorganisms tested.

Biosynthesis of the cyclooligomer depsipeptide bassianolide, an insecticidal virulence factor of Beauveria bassiana

Beauveria bassiana is a facultative entomopathogen with an extremely broad host range that is used as a commercial biopesticide for the control of insects of agricultural, veterinary and medical significance. B. bassiana produces bassianolide, a cyclooligomer depsipeptide secondary metabolite. We have cloned the bbBsls gene of B. bassiana encoding a nonribosomal peptide synthetase (NRPS). Targeted inactivation of the B. bassiana genomic copy of bbBsls abolished bassianolide production, but did not affect the biosynthesis of beauvericin, another cyclodepsipeptide produced by the strain. Comparative sequence analysis of the BbBSLS bassianolide synthetase revealed enzymatic domains for the iterative synthesis of an enzyme-bound dipeptidol monomer intermediate from d-2-hydroxyisovalerate and l-leucine. Further BbBSLS domains are predicted to catalyze the formation of the cyclic tetrameric ester bassianolide by recursive condensations of this monomer. Comparative infection assays against three selected insect hosts established bassianolide as a highly significant virulence factor of B. bassiana.

Biosynthesis of the cyclooligomer depsipeptide beauvericin, a virulence factor of the entomopathogenic fungus Beauveria bassiana

Beauvericin, a cyclohexadepsipeptide ionophore from the entomopathogen Beauveria bassiana, shows antibiotic, antifungal, insecticidal, and cancer cell antiproliferative and antihaptotactic (cell motility inhibitory) activity in vitro. The bbBeas gene encoding the BbBEAS nonribosomal peptide synthetase was isolated from B. bassiana and confirmed to be responsible for beauvericin biosynthesis by targeted disruption. BbBEAS utilizes D-2-hydroxyisovalerate (D-Hiv) and L-phenylalanine (Phe) for the iterative synthesis of a predicted N-methyl-dipeptidol intermediate, and forms the cyclic trimeric ester beauvericin from this intermediate in an unusual recursive process. Heterologous expression of the bbBeas gene in Escherichia coli to produce the 3189 amino acid, 351.9 kDa BbBEAS enzyme provided a strain proficient in beauvericin biosynthesis. Comparative infection assays with a BbBEAS knockout B. bassiana strain against three insect hosts revealed that beauvericin plays a highly significant but not indispensable role in virulence.

Deciphering the biosynthetic codes for the potent anti-SARS-CoV cyclodepsipeptide valinomycin in Streptomyces tsusimaensis ATCC 15141

Valinomycin was recently reported to be the most potent agent against severe acute respiratory-syndrome coronavirus (SARS-CoV) in infected Vero E6 cells. Aimed at generating analogues by metabolic engineering, the valinomycin biosynthetic gene cluster has been cloned from Streptomyces tsusimaensis ATCC 15141. Targeted disruption of a nonribosomal peptide synthetase (NRPS) gene abolishes valinomycin production, which confirms its predicted nonribosomal-peptide origin. Sequence analysis of the NRPS system reveals four distinctive modules, two of which contain unusual domain organizations that are presumably involved in the generation of biosynthetic precursors D-alpha-hydroxyisovaleric acid and L-lactic acid. The respective adenylation domains in these two modules contain novel substrate-specificity-conferring codes that might specify for a class of hydroxyl acids for the biosynthesis of the depsipeptide natural products.

Mapping and molecular modeling of S-adenosyl-L-methionine binding sites in N-methyltransferase domains of the multifunctional polypeptide cyclosporin synthetase

We employed a highly specific photoaffinity labeling procedure, using (14)C-labeled S-adenosyl-l-methionine (AdoMet) to define the chemical structure of the AdoMet binding centers on cyclosporin synthetase (CySyn). Tryptic digestion of CySyn photolabeled with either [methyl-(14)C]AdoMet or [carboxyl-(14)C]AdoMet yielded the sequence H(2)N-Asn-Asp-Gly-Leu-Glu-Ser-Tyr-Val-Gly-Ile-Glu-Pro-Ser-Arg-COOH (residues 10644-10657), situated within the N-methyltransferase domain of module 8 of CySyn. Radiosequencing detected Glu(10654) and Pro(10655) as the major sites of derivatization. [carboxyl-(14)C]AdoMet in addition labeled Tyr(10650). Chymotryptic digestion generated the radiolabeled peptide H(2)N-Ile-Gly-Leu-Glu-Pro-Ser-Gln-Ser-Ala-Val-Gln-Phe-COOH, corresponding to amino acids 2125-2136 of the N-methyltransferase domain of module 2. The radiolabeled amino acids were identified as Glu(2128) and Pro(2129), which are equivalent in position and function to the modified residues identified with tryptic digestions in module 8. Homology modeling of the N-methyltransferase domains indicates that these regions conserve the consensus topology of the AdoMet binding fold and consensus cofactor interactions seen in structurally characterized AdoMet-dependent methyltransferases. The modified sequence regions correspond to the motif II consensus sequence element, which is involved in directly complexing the adenine and ribose components of AdoMet. We conclude that the AdoMet binding to nonribosomal peptide synthetase N-methyltransferase domains obeys the consensus cofactor interactions seen among most structurally characterized low molecular weight AdoMet-dependent methyltransferases.

Non-ribosomal peptide synthetases as technological platforms for the synthesis of highly modified peptide bioeffectors – Cyclosporin synthetase as a complex example

Many microbial peptide secondary metabolites possess important medicinal properties, of which the immunosuppressant cyclosporin A is an example. The enormous structural and functional diversity of these low-molecular weight peptides is attributable to their mode of biosynthesis. Peptide secondary metabolites are assembled non-ribosomally by multi-functional enzymes, termed non-ribosomal peptide synthetases. These systems consist of a multi-modular arrangement of the functional domains responsible for the catalysis of the partial reactions of peptide assembly. The extensive homology shared among NRPS systems allows for the generalisation of the knowledge garnered from studies of systems of diverse origins. In this review we shall focus the contemporary knowledge of non-ribosomal peptide biosynthesis on the structure and function of the cyclosporin biosynthetic system, with some emphasis on the re-direction of the biosynthetic potential of this system by combinatorial approaches.

Mutational analysis of the N-methyltransferase domain of the multifunctional enzyme enniatin synthetase

N-Methylcyclopeptides like cyclosporins and enniatins are synthesized by multifunctional enzymes representing hybrid systems of peptide synthetases and S-adenosyl-l-methionine (AdoMet)-dependent N-methyltransferases. The latter constitute a new family of N-methyltransferases sharing high homology within procaryotes and eucaryotes. Here we describe the mutational analysis of the N-methyltransferase domain of enniatin synthetase from Fusarium scirpi to gain insight into the assembly of the AdoMet-binding site. The role of four conserved motifs (I, (2085)VLEIGTGSGMIL; II/Y, (2105)SYVGLDPS; IV, (2152)DLVVFNSVVQYFTPPEYL; and V, (2194)ATNGHFLAARA) in cofactor binding as measured by photolabeling was studied. Deletion of the first 21 N-terminal amino acid residues of the N-methyltransferase domain did not affect AdoMet binding. Further shortening close to motif I resulted in loss of binding activity. Truncation of 38 amino acids from the C terminus and also internal deletions containing motif V led to complete loss of AdoMet-binding activity. Point mutations converting the conserved Tyr(223) (corresponding to position 2106 in enniatin synthetase) in motif II/Y (close to motif I) into Val, Ala, and Ser, respectively, strongly diminished AdoMet binding, whereas conversion of this residue to Phe restored AdoMet-binding activity to approximately 70%, indicating that Tyr(223) is important for AdoMet binding and that the aromatic Tyr(223) may be crucial for AdoMet binding in N-methylpeptide synthetases.

Genes encoding synthetases of cyclic depsipeptides, anabaenopeptilides, in Anabaena strain 90

Anabaena strain 90 produces three hepatotoxic heptapeptides (microcystins), two seven-residue depsipeptides called anabaenopeptilide 90A and 90B, and three six-residue peptides called anabaenopeptins. The anabaenopeptilides belong to a group of cyanobacterial depsipeptides that share the structure of a six-amino-acid ring with a side-chain. Despite their similarity to known cyclic peptide toxins, no function has been assigned to the anabaenopeptilides. Degenerate oligonucleotide primers based on the conserved amino acid sequences of other peptide synthetases were used to amplify DNA from Anabaena 90, and the resulting polymerase chain reaction (PCR) products were used to identify a peptide synthetase gene cluster. Four genes encoding putative anabaenopeptilide synthetase domains were characterized. Three genes, apdA, apdB and apdD, contain two, four and one module, respectively, encoding a total of seven modules for activation and peptide bond formation of seven L-amino acids. Modules five and six also carry methyltransferase-like domains. Before the first module, there is a region similar in amino acid sequence to formyltransferases. A fourth gene (apdC), between modules six and seven, is similar in sequence to halogenase genes. Thus, the order of domains is co-linear with the positions of amino acid residues in the finished peptide. A mutant of Anabaena 90 was made by inserting a chloramphenicol resistance gene into the apdA gene. DNA amplification by PCR confirmed the insertion. Mass spectrometry analysis showed that anabaenopeptilides are not made in the mutant strain, but other peptides, such as microcystins and anabaenopeptins, are still produced by the mutant.

Biosynthesis of PF1022A and related cyclooctadepsipeptides

PF1022A belongs to a recently identified class of N-methylated cyclooctadepsipeptides (CODPs) with strong anthelmintic properties. Described here is the cell-free synthesis of this CODP and related structures, as well as the purification and enzymatic characterization of the responsible synthetase. For PF1022A synthesis extracts of Mycelia sterilia were incubated with the precursors L-leucine, D-lactate, D-phenyllactate, and S-adenosyl-L-methionine in the presence of ATP and MgCl(2). A 350-kDa depsipeptide synthetase, PFSYN, responsible for PF1022A synthesis was purified to electrophoretic homogeneity. Like other peptide synthetases, PFSYN follows a thiotemplate mechanism in which the substrates are activated as thioesters via adenylation. N-Methylation of the substrate L-leucine takes place after covalent binding prior to peptide bond formation. The enzyme is capable of synthesizing all known natural cyclooctadepsipeptides of the PF1022 type (A, B, C, and D) differing in the content of D-lactate and D-phenyllactate. In addition to PF1022 types A, B, C, and D, the in vitro incubations produced PF1022F (a CODP consisting of D-lactate and N-methyl-L-leucine), as well as di-, tetra-, and hexa-PF1022 homologs. PFSYN strongly resembles the well documented enniatin synthetase in size and mechanism. Our results suggest that PFSYN, like enniatin synthetase, is an enzyme with two peptide synthetase domains and forms CODP by repeated condensation of dipeptidol building blocks. Due to the low specificity of the d-hydroxy acid binding site, D-lactate or D-phenyllactate can be incorporated into the dipeptidols depending on the concentration of these substrates in the reaction mixture.

Probing the domain structure and ligand-induced conformational changes by limited proteolysis of tyrocidine synthetase 1

The boundaries of the structural domains in peptide synthetases and the conformational changes related to catalysis were investigated by limited proteolysis of tyrocidine synthetase 1 (TY1). Four regions sensitive to proteolysis were detected (cleavage site at Arg13, Arg424, Arg509 and Arg602) that, in addition to an N-terminal extension, accurately delineate the domain boundaries of the adenylate-forming domain, the aminoacyl carrier domain, and the epimerisation domain. Limited proteolysis of an active N-terminal truncated deletion mutant, His6DeltaTY1, generated two stable and structurally independent subunits, corresponding to the subdomains of the adenylation domain. The structural integrity of the carrier domain was substantiated by its resistance to proteolytic degradation. Evidence is provided that the C-terminal "spacer" region with epimerising and/or condensing activity folds into an autonomous domain stable against degradation by limited proteoly sis. In the presence of substrates, reduced susceptibility to proteolysis was observed in the linker region connecting the subdomains of the adenylation domain, and corresponding to a peptide stretch of low electron density in the X-ray structure of the homologous firefly luciferase. Sequence analysis has shown that the respective linker contains conserved residues, whereas the linker regions connecting the structural domains are of low homology with a significant content of Pro, Ala, Glu and polar residues. A combination of kinetic and proteolytic studies using ATP analogues with substitutions in the phosphate chain, AMP-PcP, AMP-PNP and AMP-cPP, strongly suggests that the generation of a productive complex is associated with the ability of the beta, gamma-pyrophosphate moiety of ATP to adopt the proper active-site conformation. These data substantiate the observation that peptide synthetases undergo a series of conformational changes in the process of adenylate formation and product release.

The nonribosomal peptide synthetase HMWP2 forms a thiazoline ring during biogenesis of yersiniabactin, an iron-chelating virulence factor of Yersinia pestis

Pathogenic Yersinia species have been shown to synthesize a siderophore molecule, yersiniabactin, as a virulence factor during iron starvation. Here we provide the first biochemical evidence for the role of the Yersinia pestis high molecular weight protein 2 (HMWP2), a nonribosomal peptide synthetase homologue, and YbtE in the initiation of yersiniabactin biosynthesis. YbtE catalyzes the adenylation of salicylate and the transfer of this activated salicyl group to the N-terminal aryl carrier protein domain (ArCP; residues 1-100) of HMWP2. A fragment of HMWP2, residues 1-1491, can adenylate cysteine and with the resulting cysteinyl-AMP autoaminoacylate the peptidyl carrier protein domain (PCP1; residues 1383-1491) either in cis or in trans. Catalytic release of hydroxyphenylthiazoline carboxylic acid (HPT-COOH) and/or N-(hydroxyphenylthiazolinylcarbonyl)cysteine (HPT-cys) is observed upon incubation of YbtE, HMWP2 1-1491, L-cysteine, salicylate, and ATP. These products presumably arise from nucleophilic attack by water or cysteine of a stoichiometric hydroxyphenylthiazolinylcarbonyl-S-PCP1-HMWP2 intermediate. Detection of the heterocyclization capacity of HMWP2 1-1491 implies salicyl-transferring and thiazoline-forming activity for the HMWP2 condensation domain (residues 101-544) and is the first demonstration of such heterocyclization ability in a nonribosomal peptide synthetase enzyme.

Acylation of Escherichia coli hemolysin: a unique protein lipidation mechanism underlying toxin function

The pore-forming hemolysin (HlyA) of Escherichia coli represents a unique class of bacterial toxins that require a posttranslational modification for activity. The inactive protoxin pro-HlyA is activated intracellularly by amide linkage of fatty acids to two internal lysine residues 126 amino acids apart, directed by the cosynthesized HlyC protein with acyl carrier protein as the fatty acid donor. This action distinguishes HlyC from all bacterial acyltransferases such as the lipid A, lux-specific, and nodulation acyltransferases, and from eukaryotic transferases such as N-myristoyl transferases, prenyltransferases, and thioester palmitoyltransferases. Most lipids directly attached to proteins may be classed as N-terminal amide-linked and internal ester-linked acyl groups and C-terminal ether-linked isoprenoid groups. The acylation of HlyA and related toxins does not equate to these but does appear related to a small number of eukaryotic proteins that include inflammatory cytokines and mitogenic and cholinergic receptors. While the location and structure of lipid moieties on proteins vary, there are common effects on membrane affinity and/or protein-protein interactions. Despite being acylated at two residues, HlyA does not possess a "double-anchor" motif and does not have an electrostatic switch, although its dependence on calcium binding for activity suggests that the calcium-myristoyl switch may have relevance. The acyl chains on HlyA may provide anchorage points onto the surface of the host cell lipid bilayer. These could then enhance protein-protein interactions either between HlyA and components of a host signal transduction pathway to influence cytokine production or between HlyA monomers to bring about oligomerization during pore formation.

Rhizobium leguminosarum bv. viciae produces a novel cyclic trihydroxamate siderophore, vicibactin

Trihydroxamate siderophores were isolated from iron-deficient cultures of three strains of Rhizobium leguminosarum biovar viciae, two from Japan (WSM709, WSM710) and one from the Mediterranean (WU235), and from a Tn5-induced mutant of WSM710 (MNF7101). The first three all produced the same compound (vicibactin), which was uncharged and could be purified by solvent extraction into benzyl alcohol. The gallium and ferric complexes of vicibactin were extractable into benzyl alcohol at pH 5.0, while metal-free vicibactin could be extracted with good yield at pH 8.0. The trihydroxamate from MNF7101 (vicibactin 7101) could not be extracted into benzyl alcohol, but its cationic nature permitted purification by chromatography on Sephadex CM-25 (NH+ 4 form). Relative molecular masses and empirical formulae were obtained from fast-atom-bombardment MS. The structures were derived from one- and two-dimensional 1H and 13C NMR spectroscopy, using DQF-COSY, NOESY, HMQC and HMBC techniques on the compounds dissolved in methanol-d 4 and DMSO-d 6. Vicibactin proves to be a cyclic molecule containing three residues each of (R)-2,5-diamino-N 2-acetyl-N 5-hydroxypentanoic acid (N 2-acetyl-N 5-hydroxy-D-ornithine) and (R)-3-hydroxybutanoic acid, arranged alternately, with alternating ester and peptide bonds. Vicibactin 7101 differed only in lacking the acetyl substitution on the N2 of the N 5-hydroxyornithine, resulting in net positive charge; it was still functional as a siderophore and promoted 55Fe uptake by iron-starved cells of WSM710 in the presence of an excess of phosphate. The rate of vicibactin biosynthesis by iron-deficient cells of WSM710 was essentially constant between pH 5.5 and 7.0, but much decreased at pH 5.0. When iron-starved cultures were supplemented with potential precursors for vicibactin, the rates of its synthesis were consistent with both β-hydroxybutyrate and ornithine being precursors. At least three genes seem likely to be involved in synthesis of vicibactin from ornithine and β-hydroxybutyrate: a hydroxylase adding the -OH group to the N5 of ornithine, an acetylase adding the acetyl group to the N2 of ornithine, and a peptide synthetase system.

ACV synthetase: expression of amino acid activating domains of the Penicillium chrysogenum enzyme in Aspergillus nidulans

Fragments of ACV synthetase of Penicillium chrysogenum carrying partial activities of amino acid activation were expressed under the alcA promoter in an acvA-deletion mutant of Aspergillus nidulans. The 210 kDa domain A-beta-galactosidase fusion protein was partially cleaved to fragments of 200 and 97 kDa. The domain A fragment and the 312 kDa domain BC construct were identified by peptide specific antibodies and shown to catalyze alpha-aminoadipate-, cysteine-, and valine-dependent ATP/[32P]PPi exchange activity. Substrate specificities were investigated using amino acid analogues. Unexpectedly neither alpha-aminoadipate nor valine activation was exclusive, implying possible misactivations and proof reading functions. Both fragments were only expressed in limited amounts and found to be unstable.

Enniatin production by fusarium strains and its effect on potato tuber tissue

Several Fusarium strains produce the cyclohexadepsipeptide enniatin, a host-nonspecific phytotoxin. Enniatins are synthesized by the 347-kDa multifunctional enzyme enniatin synthetase. In the present study, 36 Fusarium strains derived from a wide range of host plants were characterized with respect to enniatin production in different media. Thirteen of these strains produced enniatins on one or more of these media. To determine whether enniatin production affected virulence, an assay on potato tuber tissue was performed. Seven enniatin-producing and 16 nonproducing strains induced necrosis of potato tuber tissue, so that enniatin synthesis is not essential for the infection of potato tuber tissue. The application of a mixture of enniatins to slices of potato tuber, however, caused necrosis of the tissue. Therefore, enniatin production by the enniatin-synthesizing strains may affect their pathogenicity. The enniatin synthetase gene (esyn1) of Fusarium scirpi ETH 1536 was used as a probe to determine if similar sequences were present in the strains examined. In Southern blot analyses, DNA sequences hybridizing with the esyn1 probe were present in all but two of the strains examined. In some cases, enniatin-nonproducing strains had the same hybridization pattern as enniatin producers.