Trichodiene biosynthesis and the stereochemistry of the enzymatic cyclization of farnesyl pyrophosphate

Targeting active site residues and structural anchoring positions in terpene synthases

The sesterterpene synthase SmTS1 from Streptomyces mobaraensis contains several unusual residues in positions that are otherwise highly conserved. Site-directed mutagenesis experiments for these residues are reported that showed different effects, resulting in some cases in an improved catalytic activity, but in other cases in a loss of enzyme function. For other enzyme variants a functional switch was observed, turning SmTS1 from a sesterterpene into a diterpene synthase. This article gives rational explanations for these findings that may generally allow for protein engineering of other terpene synthases to improve their catalytic efficiency or to change their functions.

Trichothecenes in Cereal Grains - An Update

Trichothecenes are sesquiterpenoid mycotoxins produced by fungi from the order Hypocreales, including members of the Fusarium genus that infect cereal grain crops. Different trichothecene-producing Fusarium species and strains have different trichothecene chemotypes belonging to the Type A and B class. These fungi cause a disease of small grain cereals, called Fusarium head blight, and their toxins contaminate host tissues. As potent inhibitors of eukaryotic protein synthesis, trichothecenes pose a health risk to human and animal consumers of infected cereal grains. In 2009, Foroud and Eudes published a review of trichothecenes in cereal grains for human consumption. As an update to this review, the work herein provides a comprehensive and multi-disciplinary review of the Fusarium trichothecenes covering topics in chemistry and biochemistry, pathogen biology, trichothecene toxicity, molecular mechanisms of resistance or detoxification, genetics of resistance and breeding strategies to reduce their contamination of wheat and barley.

Enhanced structural diversity in terpenoid biosynthesis: enzymes, substrates and cofactors

The enormous diversity of terpenes found in nature is generated by enzymes known as terpene synthases, or cyclases. Some are also known for their ability to convert a single substrate into multiple products. This review comprises monoterpene and sesquiterpene synthases that are multiproduct in nature along with the regulation factors that can alter the product specificity of multiproduct terpene synthases without genetic mutations. Variations in specific assay conditions with focus on shifts in product specificity based on change in metal cofactors, assay pH and substrate geometry are described. Alterations in these simple cellular conditions provide the organism with enhanced chemodiversity without investing into new enzymatic architecture. This versatility to modulate product diversity grants organisms, especially immobile ones like plants with access to an enhanced defensive repertoire by simply altering cofactors, pH level and substrate geometry.

Enzymatic synthesis of natural (+)-aristolochene from a non-natural substrate

Aristolochene synthase from Penicillium roqueforti converts 7-methylene-FDP, a substrate the enzyme never encounters in nature, to the natural product (+)-aristolochene.

Traversing the fungal terpenome

Fungi (Ascomycota and Basidiomycota) are prolific producers of structurally diverse terpenoid compounds. Classes of terpenoids identified in fungi include the sesqui-, di- and triterpenoids. Biosynthetic pathways and enzymes to terpenoids from each of these classes have been described. These typically involve the scaffold generating terpene synthases and cyclases, and scaffold tailoring enzymes such as e.g. cytochrome P450 monoxygenases, NAD(P)+ and flavin dependent oxidoreductases, and various group transferases that generate the final bioactive structures. The biosynthesis of several sesquiterpenoid mycotoxins and bioactive diterpenoids has been well-studied in Ascomycota (e.g. filamentous fungi). Little is known about the terpenoid biosynthetic pathways in Basidiomycota (e.g. mushroom forming fungi), although they produce a huge diversity of terpenoid natural products. Specifically, many trans-humulyl cation derived sesquiterpenoid natural products with potent bioactivities have been isolated. Biosynthetic gene clusters responsible for the production of trans-humulyl cation derived protoilludanes, and other sesquiterpenoids, can be rapidly identified by genome sequencing and bioinformatic methods. Genome mining combined with heterologous biosynthetic pathway refactoring has the potential to facilitate discovery and production of pharmaceutically relevant fungal terpenoids.

Molecular and Genetic Studies ofFusariumTrichothecene Biosynthesis: Pathways, Genes, and Evolution

Trichothecenes are a large family of sesquiterpenoid secondary metabolites of Fusarium species (e.g., F. graminearum) and other molds. They are major mycotoxins that can cause serious problems when consumed via contaminated cereal grains. In the past 20 years, an outline of the trichothecene biosynthetic pathway has been established based on the results of precursor feeding experiments and blocked mutant analyses. Following the isolation of the pathway gene Tri5 encoding the first committed enzyme trichodiene synthase, 10 biosynthesis genes (Tri genes; two regulatory genes, seven pathway genes, and one transporter gene) were functionally identified in the Tri5 gene cluster. At least three pathway genes, Tri101 (separated alone), and Tri1 and Tri16 (located in the Tri1-Tri16 two-gene cluster), were found outside of the Tri5 gene cluster. In this review, we summarize the current understanding of the pathways of biosynthesis, the functions of cloned Tri genes, and the evolution of Tri genes, focusing on Fusarium species.

A single amino acid determines the site of deprotonation in the active center of sesquiterpene synthases SbTPS1 and SbTPS2 from Sorghum bicolor

The multitude of terpene carbon skeletons found in nature is formed by enzymes known as terpene synthases (TPSs). These proteins are often multiproduct enzymes converting a single prenyl diphosphate substrate into a mixture of terpene products. The recently identified sesquiterpene synthases SbTPS1 and SbTPS2 from Sorghum bicolor produce terpene blends containing the same products, but in different proportions. A single amino acid in the active site was reported to determine the different product specificities of SbTPS1 and SbTPS2. In this study we examined the reaction mechanism of the Sorghum TPSs. Feeding experiments with deuterium-labeled substrates and chiral analysis of the enzyme products zingiberene, β-sesquiphellandrene and β-bisabolene revealed that the reactions catalyzed by both enzymes proceeded via (S)-nerolidyl diphosphate and the cyclic (6S)-bisabol-7-yl and (6R)-bisabol-1-yl cation intermediates. The site of deprotonation of the final cation was shown to be the only catalytic difference between SbTPS1 and SbTPS2. Docking of the (6R)-bisabol-1-yl cation into structural models of SbTPS1 and SbTPS2 indicated a potential role of initially cleaved pyrophosphate group as a proton acceptor.

Biosynthesis of sesquiterpenes by the fungus Fusarium verticillioides

The volatiles of the fungus Fusarium verticillioides were analysed by GC-MS. Sesquiterpenes dominated, with trichodiene as the principle component. Several other sesquiterpenes were detected in low amounts that were unambiguously identified from their mass spectra and retention indices. The absolute configurations of (R)-β-bisabolene, (R)-cuparene, (+)-β-barbatene, (-)-α-cedrene, (+)-β-cedrene, and (+)-α-funebrene originating from different key cationic intermediates, were determined by chiral GC-MS and proved to be related to the trichodiene stereostructure. The unusual compound (E)-iso-γ-bisabolene was also found corroborating a previously suggested mechanism for the cyclisation of the bisabolyl to the cuprenyl cation that is based on quantum mechanical calculations (Y. J. Hong, D. J. Tantillo, Org. Lett. 2006, 8, 4601-4604). These analyses resulted in a revised biosynthesis scheme to trichodiene and the side products of the responsible terpene cyclase, trichodiene synthase, an enzyme that is well characterised from Fusarium sporotrichioides. Feeding studies with several deuterated mevalonolactone isotopomers unravelled stereochemical aspects of the late cyclisations towards trichodiene.

Two distinct mechanisms for TIM barrel prenyltransferases in bacteria

The reactions of two bacterial TIM barrel prenyltransferases (PTs), MoeO5 and PcrB, were explored. MoeO5, the enzyme responsible for the first step in moenomycin biosynthesis, catalyzes the transfer of farnesyl to 3-phosphoglyceric acid (3PG) to give a product containing a cis-allylic double bond. We show that this reaction involves isomerization to a nerolidyl pyrophosphate intermediate followed by bond rotation prior to attack by the nucleophile. This mechanism is unprecedented for a prenyltransferase that catalyzes an intermolecular coupling. We also show that PcrB transfers geranyl and geranylgeranyl groups to glycerol-1-phosphate (G1P), making it the first known bacterial enzyme to use G1P as a substrate. Unlike MoeO5, PcrB catalyzes prenyl transfer without isomerization to give products that retain the trans-allylic bond of the prenyl donors. The TIM barrel family of PTs is unique in including enzymes that catalyze prenyl transfer by distinctly different reaction mechanisms.

Terpenoid cyclases: design and function of electrophilic catalysts

Terpenoid cyclases catalyse the cyclization of the universal acyclic precursors geranyl and farnesyl diphosphate to monoterpenes and sesquiterpenes, respectively. All such cyclases investigated to date are operationally soluble, moderately lipophilic proteins of relative molecular weight 40,000-100,000, requiring no cofactors other than a divalent metal, usually Mg2+ and occasionally Mn2+. The focus of most work has been on the mechanisms of the cyclization reactions themselves. It is currently proposed that the cyclase binds the acyclic substrate in a suitable conformation and initiates the cyclization by ionization of the labile allylic diphosphate moiety. The use of stereospecifically labelled substrates and analysis of the sites of labelling in the derived cyclization products has allowed the proposal of detailed cyclization mechanisms. Further insight into the architecture and function of the cyclase active site has come from the study of substrate and intermediate analogues designed to act as potential inhibitors or anomalous substrates of the normal cyclization reaction. Progress has also been made on the cloning of the relevant structural genes for sesquiterpene cyclases. This has led to new insights into the basic requirements for cyclase catalysis and specificity.

The diverse sesquiterpene profile of patchouli, Pogostemon cablin, is correlated with a limited number of sesquiterpene synthases

Pogostemon cablin (patchouli), like many plants within the Lamiaceae, accumulates large amounts of essential oil. Patchouli oil is unique because it consists of over 24 different sesquiterpenes, rather than a blend of different mono-, sesqui- and di-terpene compounds. To determine if this complex mixture of sesquiterpenes arises from an equal number of unique sesquiterpene synthases, we developed a RT-PCR strategy to isolate and functionally characterize the respective patchouli oil synthase genes. Unexpectedly, only five terpene synthase cDNA genes were isolated. Four of the cDNAs encode for synthases catalyzing the biosynthesis of one major sesquiterpene, including a gamma-curcumene synthase, two germacrene D synthases, and a germacrene A synthase. The fifth cDNA encodes for a patchoulol synthase, which catalyzes the conversion of FPP to patchoulol plus at least 13 additional sesquiterpene products. Equally intriguing, the yield of the different in vitro reaction products resembles quantitatively and qualitatively the profile of sesquiterpenes found in patchouli oil extracted from plants, suggesting that a single terpene synthase is responsible for the bulk and diversity of terpene products produced in planta.

Trichodiene synthase: mechanism-based inhibition of a sesquiterpene cyclase

The 10-cyclopropylidene analog of farnesyl diphosphate was shown to be a mechanism-based inhibitor of trichodiene synthase with an inactivation rate (k(inact)) of 0.010 +/- 0.0003 min(-1) and an apparent Ki of 663 +/- 75 nM. The presence of three anomalous sesquiterpene products detected in incubation mixtures indicate that the compound also serves as a substrate of the enzyme.

Tri6 encodes an unusual zinc finger protein involved in regulation of trichothecene biosynthesis in Fusarium sporotrichioides

In Fusarium sporotrichioides, several genes required for biosynthesis of the trichothecene mycotoxin T-2 toxin are closely linked. Further characterization of this gene cluster has revealed a gene, Tri6, that specifies a 217-amino-acid protein with regions similar to Cys2His2 zinc finger proteins. Temporal expression of Tri6 is similar to that of trichothecene biosynthetic pathway genes. Analysis of Tri6 transcripts indicated that transcription is initiated in two regions and that within each region there may be at least four initiation sites. Disruption of Tri6 resulted in a mutant that did not produce trichothecenes but that did accumulate low levels of the trichothecene precursor trichodiene. The Tri6 mutant was unable to convert six trichothecene biosynthetic intermediates to T-2 toxin, and transcription of two biosynthetic genes, Tri4 and Tri5, was greatly reduced in the mutant relative to the wild type. In addition, the product of Tri6 functioned as a transcriptional activator in Saccharomyces cerevisiae when fused to the DNA binding region of GAL4. These results indicate that Tri6 encodes a protein involved in the transcriptional regulation of trichothecene biosynthetic genes in F. sporotrichioides.

Cloning and bacterial expression of a sesquiterpene cyclase from Hyoscyamus muticus and its molecular comparison to related terpene cyclases

Genomic and cDNA clones for vetispiradiene synthase, a sesquiterpene cyclase found in Hyoscyamus muticus, were isolated using a combination of reverse transcription-polymerase chain reactions and conventional cloning procedures. RNA blot hybridization demonstrated an induction of mRNA consistent with the induction of cyclase enzyme activity in elicitor-treated cells, DNA blot hybridization indicated a gene family of 6 to 8 members, and bacterial expression of 3 cDNA clones indicated that each coded for a vetispiradiene synthase enzyme activity catalyzing the synthesis of a single reaction product. Intron-exon organization of the vetispiradiene synthase gene was identical with that previously described for 5-epi-aristolochene synthase (tobacco sesquiterpene cyclase) and casbene synthase (castor bean diterpene cyclase), and the vetispiradiene synthase amino acid sequence was 77% identical with and 81% similar to the tobacco sesquiterpene cyclase. Regions of the vetispiradiene synthase sequence centered around amino acids 60, 100, and 370 were conspicuously different relative to the tobacco sesquiterpene cyclase. The sequence similarity between the tobacco and H. muticus enzymes is suggested to be reflective of the conservation of several partial reactions common to both enzymes, and the differences may be reflective of a partial reaction unique to each enzyme.

Evidence for a gene cluster involving trichothecene-pathway biosynthetic genes in Fusarium sporotrichioides

Two overlapping cosmid clones (Cos1-1 and Cos9-1) carrying the Tox5 gene were isolated from a library of F. sporotrichioides strain NRRL 3299 genomic DNA. These cosmids were used to transform three T-2 toxin-deficient mutants that are blocked at different steps in the trichothecene pathway. Both cosmids restored T-2 toxin production to Tox3-1- or Tox4-1- mutants but neither restored T-2 toxin production to a Tox1-2- mutant. The production of T-2 toxin by the complemented Tox3-1- and Tox4-1- mutants, as well as the production of diacetoxycirpenol by the cosmid-transformed Tox1-2- mutant, were 2- to 10-fold higher than in strain NRRL 3299. In addition, those transformants carrying Cos9-1 produced significantly higher levels of trichothecenes than transformants carrying Cos1-1. Two different DNA fragments (FSC13-9 and FSC14-5), representing the region of overlap between the cosmid clones, were isolated. These fragments specifically complemented either the Tox3-1- mutant (FSC14-5) or the Tox4-1- mutant (FSC13-9). The trichothecene-production phenotype of these transformants was similar to NRRL 3299. These results suggest that two or more genes involved in the biosynthesis of trichothecenes are closely linked to Tox5.

Purification and characterization of the monoterpene cyclase gamma-terpinene synthase from Thymus vulgaris

The monoterpene cyclase, gamma-terpinene synthase, from Thymus vulgaris (thyme) leaves was purified to apparent homogeneity by isoelectric focusing and dye-ligand, anion-exchange, hydrophobic interaction, and gel permeation chromatography. The enzyme has a native molecular weight of 96,000 as determined by gel permeation chromatography, and exhibited a specific activity of 538 nmol/h.mg protein (turnover number of approximately 0.01/s). Sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed the enzyme to be composed of two apparently identical subunits of Mr approximately 55,000. The protein was very hydrophobic, and possessed a pI value of 4.85 as determined by isoelectric focusing. Maximum activity was observed at pH 6.8 in the presence of 20 mM Mg2+; 5 mM Mn2+ could support catalysis, albeit at a much lower rate. The Km value for the substrate, geranyl pyrophosphate, was 2.6 microM. Cyclase activity was inhibited by cysteine- and histidine-directed reagents. Purified gamma-terpinene synthase also possessed the ability to cyclize geranyl pyrophosphate to small amounts of alpha-thujene and to lesser quantities of myrcene, alpha-terpinene, limonene, linalool, terpinen-4-ol, and alpha-terpineol, all of which appear to be coproducts of the reaction sequence leading to gamma-terpinene. In general properties, the gamma-terpinene synthase from thyme leaves resembles other monoterpene cyclases as well as sesquiterpene and diterpene cyclases.

Purification and characterization of the sesquiterpene cyclase patchoulol synthase from Pogostemon cablin

The sesquiterpene cyclase, patchoulol synthase, from Pogostemon cablin (patchouli) leaves was purified to apparent homogeneity by chromatofocusing, anion exchange, gel permeation, and hydroxylapatite chromatography. The enzyme showed a maximum specific activity of about 20 nmol/min/mg protein, and a native molecular weight of 80,000 as determined by gel permeation chromatography. The protein was very hydrophobic, as judged by chromatographic behavior on several matrices, and possessed a pI value of about 5.0, as determined by isoelectric and chromatofocusing. SDS-PAGE showed the enzyme to be composed of two apparently identical subunits of Mr approximately 40,000. Maximum activity was observed at pH 6.7 in the presence of Mg2+ (Km approximately 1.7 mM); other divalent metal ions were ineffective in promoting catalysis. The Km value for the substrate, farnesyl pyrophosphate, was 6.8 microM. Patchoulol synthase copurified with the ability to transform farnesyl pyrophosphate to cyclic olefins (alpha- and beta-patchoulene, alpha-bulnesene, and alpha-guiaene) and this observation, plus evidence based on differential inhibition and inactivation studies, suggested that these structurally related products are synthesized by the same cyclase enzyme. In general properties, the patchoulol synthase from patchouli leaves resembles fungal sesquiterpene olefin cyclases except for the ability to synthesize multiple products, a property more typical of monoterpene cyclases of higher plant origin.

Studies of the cryptic allylic pyrophosphate isomerase activity of trichodiene synthase using the anomalous substrate 6,7-dihydrofarnesyl pyrophosphate

Two enantiomeric analogues of farnesyl pyrophosphate (1) were tested as inhibitors and anomalous substrates of trichodiene synthase, which catalyzes the cyclization of trans,trans-farnesyl pyrophosphate (1) to the sesquiterpene hydrocarbon trichodiene (2). The reaction has been shown to involve preliminary isomerization of 1 to the tertiary allylic isomer nerolidyl pyrophosphate (3) which is cyclized without detectable release of the intermediate from the active site of the cyclase. Both (7S)-trans-6,7-dihydrofarnesyl pyrophosphate (7a) and (7R)-trans-6,7-dihydrofarnesyl pyrophosphate (7b), prepared from (3R)- and (3S)- citronellol (9a and 9b), respectively, proved to be modest competitive inhibitors of trichodiene synthase. The values of Ki(7a), 395 nM, and Ki(7b), 220 nM, were 10-15 times the observed Km for 1 and half the Ki of inorganic pyrophosphate alone. Incubation of either 7a or 7b with trichodiene synthase resulted in formation of a mixture of products which by radio/gas-liquid chromatographic and GC/selected ion mass spectrometric analysis was shown to be composed of 80-85% isomeric trienes 19-21 and 15-20% allylic alcohols 12 and 18. Examination of the water-soluble products resulting from incubation of 7a also revealed the generation of 24% of the isomeric cis-6,7-dihydrofarnesyl pyrophosphate (26). The combined rate of formation of anomalous alcoholic and olefinic products was 10% the Vmax determined for the conversion of 1 to 2. The results can be explained by initial enzyme-catalyzed isomerization of dihydrofarnesyl pyrophosphate (7) to the corresponding tertiary allylic isomer dihydronerolidyl pyrophosphate (8). Since the latter intermediate is unable to cyclize due to the absence of the 6,7-double bond, ionization of 8 and quenching of the resulting ion pair by deprotonation, capture of water, or collapse to the isomeric primary pyrophosphate esters will generate the observed spectrum of anomalous products.

Biosynthesis of monoterpenes: stereochemistry of the coupled isomerization and cyclization of geranyl pyrophosphate to camphane and isocamphane monoterpenes

The conversion of geranyl pyrophosphate to (+)-bornyl pyrophosphate and (+)-camphene is considered to proceed by the initial isomerization of the substrate to (-)-(3R)-linalyl pyrophosphate and the subsequent cyclization of this bound intermediate. In the case of (-)-bornyl pyrophosphate and (-)-camphene, isomerization of the substrate to the (+)-(3S)-linalyl intermediate precedes cyclization. The geranyl and linalyl precursors were shown to be mutually competitive substrates (inhibitors) of the relevant cyclization enzymes isolated from Salvia officinalis (sage) and Tanacetum vulgare (tansy) by the mixed substrate analysis method, demonstrating that isomerization and cyclization take place at the same active site. Incubation of partially purified enzyme preparations with (3R)-[1Z-3H]linalyl pyrophosphate plus [1-14C]geranyl pyrophosphate gave rise to double-labeled (+)-bornyl pyrophosphate and (+)-camphene, whereas incubation of enzyme preparations catalyzing the antipodal cyclizations with (3S)-[1Z-3H]-linalyl pyrophosphate plus [1-14C]geranyl pyrophosphate yielded double-labeled (-)-bornyl pyrophosphate and (-)-camphene. Each product was then transformed to the corresponding (+)- or (-)-camphor without change in the 3H:14C isotope ratio, and the location of the tritium label was deduced in each case by stereoselective, base-catalyzed exchange of the exo-alpha-hydrogen of the derived ketone. The finding that the 1Z-3H of the linalyl precursor was positioned at the endo-alpha-hydrogen of the corresponding camphor in all cases, coupled to the previously demonstrated retention of configuration at C1 of the geranyl substrate in these transformations, confirmed the syn-isomerization of geranyl pyrophosphate to linalyl pyrophosphate and the cyclization of the latter via the anti,endo- conformer. These relative stereochemical elements, in combination with the observed enantiospecificities of the enzymes for the linalyl intermediates, allows definition of the overall absolute stereochemistry of the coupled isomerization and cyclization of geranyl pyrophosphate to the antipodal camphane (bornane) and isocamphane monoterpenoids.

Expression of the trichodiene synthase gene of Fusarium sporotrichioides in Escherichia coli results in sesquiterpene production

Trichodiene synthase is a sesquiterpene cyclase involved in the biosynthesis of trichothecene mycotoxins. We report that insertion of the unaltered trichodiene synthase gene of Fusarium sporotrichioides into the Escherichia coli expression vector pDR540 produced an inactive polypeptide with a molecular weight approximately 2000 greater than that of trichodiene synthase. This result is consistent with the presence of an intron in the trichodiene synthase gene, and prompted us to specifically delete a putative 60-nucleotide intron sequence. Insertion of the intron-deleted open reading frame into pDR540 resulted in the production of active enzyme. Trichodiene synthase activity in crude extracts from induced cultures was 0.07 nmol/min/mg of protein and represented 0.05-0.10% of the total cell protein. A cross-reactive protein was present with the same apparent molecular weight as the subunit of native trichodiene synthase. The recombinant enzyme was partially purified and shown to have properties closely resembling those of the native enzyme. Trichodiene was detected in ethyl acetate extracts from induced cultures at a concentration of 60 micrograms/liter after 4.5 h. These findings support the primary structure recently reported for trichodiene synthase and demonstrate that the expression of a sesquiterpene cyclase in E. coli results in sesquiterpene production.

Isolation and nucleotide sequence of a sesquiterpene cyclase gene from the trichothecene-producing fungus Fusarium sporotrichioides

The trichodiene synthase gene (Tox5) has been isolated from the fungus Fusarium sporotrichioides, and its nucleotide (nt) sequence determined. A lambda gt11 library of F. sporotrichioides DNA was screened with antiserum against trichodiene synthase (TS). DNA fragments were isolated which encode a portion of the Tox5 gene. In subsequent screening of the library we employed one of these DNAs as a probe and identified several recombinant phage containing the entire Tox5 gene. The gene consists of a 1182-nt open reading frame (ORF) which contains a 60-nt intron and specifies a Mr 43,999 protein. The deduced amino acid sequence of the ORF was identical to sequences determined for several CNBr peptides from purified TS. Southern and Northern analyses indicated that the Tox5 gene is present in a single copy and is transcribed into an mRNA of about 1450 nt. Upstream from the start codon, 'TATA'-like sequences and a short repeated sequence resembling the 'CCAAT' box were observed. The primary structure described for TS is the first such report for a member of the terpene cyclase group of enzymes.

Regulation of Trichodiene Synthase in Fusarium sporotrichioides and Gibberella pulicaris ( Fusarium sambucinum )

The regulation of trichodiene synthase (TS) and its relationship to trichothecene biosynthesis was investigated in Fusarium sporotrichioides NRRL 3299 and Gibberella pulicaris R-6380. Cultures were analyzed for the presence of TS activity, trichothecenes, and immunodetectable TS polypeptide over a time period of 144 h. Enzyme activity increased from barely detectable to maximum levels over a period of 3 h for F. sporotrichioides , while in G. pulicaris , a steady increase was observed over 144 h. Increases in TS activity of 50-fold for F. sporotrichioides and 10-fold for G. pulicaris R-6380 preceded by several hours the detection of trichothecenes. Immunoblot analysis employing polyclonal serum specific for the enzyme from F. sporotrichioides showed that increases in the levels of TS polypeptide corresponded to the observed changes in enzyme activity for both organisms. These data indicate that the regulation of TS activity is accomplished through increases in its cellular concentration and that TS may serve as a useful indicator of trichothecene biosynthetic activity.

Partial purification and characterization of two sesquiterpene cyclases from sage (Salvia officinalis) which catalyze the respective conversion of farnesyl pyrophosphate to humulene and caryophyllene

Humulene cyclase and caryophyllene cyclase, two enzymes which catalyze the cyclization of farnesyl pyrophosphate to the respective sesquiterpene olefins, have been partially purified from the supernatant fraction of a sage (Salvia officinalis) leaf epidermis extract and separated from each other by a combination of hydrophobic interaction, gel filtration, and ion-exchange chromatography. The molecular weight of both cyclases was estimated by gel filtration to be 57,000 and both cyclases exhibited a pH optimum of 6.5 and preferred Mg2+ (Km approximately 1.5 mM) as the required divalent metal cation. Both enzymes possessed a Km of about 1.7 microM for farnesyl pyrophosphate, were strongly inhibited by p-hydroxymercuribenzoate, and exhibited comparable sensitivities to a variety of other potential inhibitors. The properties of the two sesquiterpene olefin cyclases, which are the first from a higher plant source to be examined in detail, were very similar to each other and to other monoterpene, sesquiterpene, and diterpene cyclases previously described.

Isolation and characterization of mutants blocked in T-2 toxin biosynthesis

Mutants of Fusarium sporotrichioides NRRL 3299 that were blocked or altered in the biosynthesis of the trichothecene T-2 toxin were generated by UV treatment and identified by a rapid screen in which monoclonal antibodies to T-2 were used. Three stable mutants were isolated and chemically characterized. Two mutants accumulated diacetoxyscirpenol, which suggests that they were defective in the step required for the addition of a hydroxyl group to the C-8 position in the trichothecene core structure. The third mutant appeared to be partially blocked at an early step or regulatory point in the pathway. This represents the first isolation of mutants in a trichothecene biosynthetic pathway.

Partial purification and characterization of pentalenene synthase

Pentalenene synthase, an enzyme which catalyzes the cyclization of farnesyl pyrophosphate to the sesquiterpene hydrocarbon pentalenene, has been partially purified from the supernatant fraction of Streptomyces UC5319 by a combination of anion-exchange, hydroxylapatite, and gel-filtration chromatography. The molecular weight of the partially purified synthase was estimated by gel filtration chromatography to be 57,000 and the cyclase activity was shown to be associated with a major protein band among eight visible by nondenaturing polyacrylamide disc gel electrophoresis. The Km for farnesyl pyrophosphate was 0.77 +/- 0.21 microM and the Vmax for the partially purified synthase was 287 +/- 21 nmol of pentalenene/mg protein per hour. Cyclase activity required the presence of a divalent metal cation. Although either Mg2+ or Mn2+ could be used, Mn2+ was inhibitory at concentrations above 2.5 mM. No other cofactors were required. Whereas neither product, pentalenene nor inorganic pyrophosphate, showed significant inhibition of cyclase activity at concentrations of ca 10 microM, the combination of the two resulted in an approximate sevenfold increase in the apparent Km for farnesyl pyrophosphate, suggesting that both products can bind cooperatively at the active site to inhibit pentalenene synthase competitively.

Purification and characterization of the sesquiterpene cyclase trichodiene synthetase from Fusarium sporotrichioides

The sesquiterpene cyclase, trichodiene synthetase, has been purified from a supernatant fraction of Fusarium sporotrichioides by hydrophobic interaction, anion exchange, and gel filtration chromatography. Purified enzyme had a specific activity 15-fold higher than that previously reported for preparations of terpene cyclases. Molecular weight determinations by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and gel filtration chromatography indicated the enzyme to be a dimer with a subunit of Mr 45,000. The requirement of Mg2+ (Km 0.1 mM) for activity could be partially substituted with Mn2+ at a concentration of 0.01 mM, but higher concentrations of Mn2+ were inhibitory. Maximum activity was observed between pH 6.75 and pH 7.75. The Km for farnesyl pyrophosphate was 0.065 microM.