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

The mystery of the trichothecene 3-O-acetyltransferase gene. Analysis of the region around Tri101 and characterization of its homologue from Fusarium sporotrichioides

The trichothecene 3-O-acetyltransferase gene, Tri101, plays a pivotal role for the well-being of the type B trichothecene producer Fusarium graminearum. We have analyzed the cosmids containing Tri101 and found that this resistance gene is not in the biosynthetic gene cluster reported so far. It was located between the UTP-ammonia ligase gene and the phosphate permease gene which are not related to trichothecene biosynthesis. These two 'house-keeping' genes were also linked in Fusarium species that do not produce trichothecenes. The result suggests that the isolated occurrence of Tri101 is attributed to horizontal gene transfer and not to the reciprocal translocation of the chromosome containing the gene cluster. Interestingly, 3-O-acetylation was not always a primary self-defensive strategy for all the t-type trichothecene producers; i.e. the type A trichothecene producer Fusarium sporotrichioides did not acetylate T-2 toxin in vivo although the fungus possessed a functional 3-O-acetyltransferase gene. Thus Tri101 appears to be a defense option which the producers have independently acquired in addition to their original resistance mechanisms.

Trichothecene 3-O-acetyltransferase protects both the producing organism and transformed yeast from related mycotoxins. Cloning and characterization of Tri101

Trichothecene mycotoxins such as deoxynivalenol, 4,15-diacetoxyscirpenol, and T-2 toxin, are potent protein synthesis inhibitors for eukaryotic organisms. The 3-O-acetyl derivatives of these toxins were shown to reduce their in vitro activity significantly as assessed by assays using a rabbit reticulocyte translation system. The results suggested that the introduction of an O-acetyl group at the C-3 position in the biosynthetic pathway works as a resistance mechanism for Fusarium species that produce t-type trichothecenes (trichothecenes synthesized via the precursor trichotriol). A gene responsible for the 3-O-acetylation reaction, Tri101, has been successfully cloned from a Fusarium graminearum cDNA library that was designed to be expressed in Schizosaccharomyces pombe. Fission yeast transformants were selected for their ability to grow in the presence of T-2 toxin, and this strategy allowed isolation of 25 resistant clones, all of which contained a cDNA for Tri101. This is the first drug-inactivating O-acetyltransferase gene derived from antibiotic-producing organisms. The open reading frame of Tri101 codes for a polypeptide of 451 amino acid residues, which shows no similarity to any other proteins reported so far. TRI101 from recombinant Escherichia coli catalyzes O-acetylation of the trichothecene ring specifically at the C-3 position in an acetyl-CoA-dependent manner. By using the Tri101 cDNA as a probe, two least overlapping cosmid clones that cover a region of 70 kilobase pairs have been isolated from the genome of F. graminearum. Other trichothecene biosynthetic genes, Tri4, Tri5, and Tri6, were not clustered in the region covered by these cosmid clones. These new cosmid clones are considered to be located in other parts of the large biosynthetic gene cluster and might be useful for the study of trichothecene biosynthesis.

Restoration of wild-type virulence to Tri5 disruption mutants of Gibberella zeae via gene reversion and mutant complementation

Gibberella zeae is a pathogen of small grain crops and produces trichothecene mycotoxins in infected host tissue. The role of trichothecenes in the virulence of G. zeae was previously investigated using trichothecene-non-producing mutants that were generated via transformation-mediated disruption of a gene (Tri5) that encodes the first enzyme in the trichothecene biosynthetic pathway. The mutants were less virulent on some hosts than the wild-type strain from which they were derived. Here, we used two approaches to determine whether the reduced virulence of mutants was due specifically to Tri5 disruption or to non-target effects caused by the transformation process. First, we generated a revertant from a Tri5 disruption mutant by allowing the mutant to pass through the sexual phase of its life cycle. In approximately 2% of the resulting progeny the disrupted Tri5 had reverted to wild-type; however, only one of three revertant progeny also regained the ability to produce trichothecenes. In the second approach, we complemented the Tri5 mutation in a disruption mutant by transforming the mutant with a plasmid carrying a functional copy of Tri5. In all transformants examined, the ability to produce trichothecenes was restored. The restoration of trichothecene production in the revertant progeny and in the complemented mutant was accompanied by restoration of wild-type or near wild-type levels of virulence on wheat seedlings (cultivar Wheaton). The results indicate that the reduced virulence of the mutants was caused by disruption of Tri5 rather than non-target effects resulting from the transformation process. The results also provide further evidence that trichothecenes contribute to the virulence of plant-pathogenic fungi.

Prospects for the bioengineering of isoprenoid biosynthesis

Over the last decade, our understanding of isoprenoid biosynthesis has progressed to the stage where specific strategies for the bioengineering of essential oil production can be considered. This review provides a current overview of the enzymology and regulation of essential oil isoprenoid biosynthesis. The reaction mechanisms of the synthases which produce many of the basic isoprenoid skeletons are described in detail. Coverage is also provided of the regulation of isoprenoid biosynthesis, including the roles played by tissue and subcellular compartmentation, and by partitioning of intermediates between different branches of isoprenoid metabolism. This provides necessary context for rationally targeting specific enzymes of metabolic pathways for bioengineering essential oil production. Wherever possible, emphasis is placed on research specific to essential oil isoprenoid biosynthesis, although relevant work related to other isoprenoids is also considered when it can provide useful insights. Finally, building upon this understanding of essential oil isoprenoid biosynthesis, several approaches to the bioengineering of isoprenoid metabolism are considered.

Identifying functional domains within terpene cyclases using a domain-swapping strategy

Cyclic terpenes and terpenoids are found throughout nature. They comprise an especially important class of compounds from plants that mediate plant- environment interactions, and they serve as pharmaceutical agents with antimicrobial and anti-tumor activities. Molecular comparisons of several terpene cyclases, the key enzymes responsible for the multistep cyclization of C10, C15, and C20 allylic diphosphate substrates, have revealed a striking level of sequence similarity and conservation of exon position and size within the genes. Functional domains responsible for a terminal enzymatic step were identified by swapping regions approximating exons between a Nicotiana tabacum 5-epi-aristolochene synthase (TEAS) gene and a Hyoscyamus muticus vetispiradiene synthase (HVS) gene and by characterization of the resulting chimeric enzymes expressed in bacteria. While exon 4 of the TEAS gene conferred specificity for the predominant reaction products of the tobacco enzyme, exon 6 of the HVS gene conferred specificity for the predominant reaction products of the Hyoscyamus enzyme. Combining these two functional domains of the TEAS and HVS genes resulted in a novel enzyme capable of synthesizing reaction products reflective of both parent enzymes. The relative ratio of the TEAS and HVS reaction products was also influenced by the source of exon 5 present in the new chimeric enzymes. The association of catalytic activities with conserved but separate exonic domains suggests a general means for generating additional novel terpene cyclases.

Resolution of Fusarium sporotrichioides Proteins by Two-Dimensional Polyacrylamide Gel Electrophoresis and Identification by Sequence Homology Comparison in Protein Data Base

Proteins from Fusarium sporotrichioides M-1-1, a T2-toxin-producing strain, were separated by two-dimensional polyacrylamide gel electrophoresis. One thousand two hundred and forty-four protein spots were resolved and 103 protein spots were subjected to N-terminal sequencing. Fifty-eight protein spots were sequenced and 48 proteins were observed to have blocked N termini. Forty out of 58 sequenced proteins were identified by homology search against the PIR protein sequence data base and protein superfamily data base, while the residual 18 sequences were not identified. Twenty-seven of the N-terminal-blocked proteins were subjected to mild anhydrous hydrazine vapor deblocking. Twenty-four spots were not deblocked indicating the presence of acyl groups at the N termini, while 3 proteins were deblocked showing the blocked group to be pyrroglutamyl carboxylic acid residues. The results can provide a more global view of cellular genetic expression than any other technique. The created data may offer a unique opportunity to link information with DNA sequence data. Copyright 1995 S. Karger AG, Basel

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.

Trichothecene biosynthesis in Fusarium species: chemistry, genetics, and significance

Several species of the genus Fusarium and related fungi produce trichothecenes which are sesquiterpenoid epoxides that act as potent inhibitors of eukaryotic protein synthesis. Interest in the trichothecenes is due primarily to their widespread contamination of agricultural commodities and their adverse effects on human and animal health. In this review, we describe the trichothecene biosynthetic pathway in Fusarium species and discuss genetic evidence that several trichothecene biosynthetic genes are organized in a gene cluster. Trichothecenes are highly toxic to a wide range of eukaryotes, but their specific function, if any, in the survival of the fungi that produce them is not obvious. Trichothecene gene disruption experiments indicate that production of trichothecenes can enhance the severity of disease caused by Fusarium species on some plant hosts. Understanding the regulation and function of trichothecene biosynthesis may aid in development of new strategies for controlling their production in food and feed products.

Analysis of Tox5 gene expression in Gibberella pulicaris strains with different trichothecene production phenotypes

The Tox5 gene encodes trichodiene synthase, the first unique enzyme in the trichothecene biosynthetic pathway. In Gibberella pulicaris R-6380, the level of Tox5 mRNA was found to increase 47-fold in early stationary phase. Sequence analysis of the Tox5 promoter regions from geographically distinct strains of G. pulicaris revealed the existence of two Tox5 alleles (Tox5-1 and Tox5-2). All G. pulicaris strains that produce high levels of trichothecenes in liquid culture carry a 42-nucleotide (nt) tandem repeat sequence (Tox5-1) in the Tox5 promoter region, whereas strains that produce low levels of trichothecenes carry a single copy of this sequence (Tox5-2). A genetic cross between high- and low-level trichothecene producers resulted in the cosegregation of higher-level trichothecene production with the Tox5-1 allele. To determine the importance of the 42-nt repeat sequence in the regulation of Tox5 expression, reporter gene constructs carrying either the Tox5-1 or the Tox5-2 promoter region fused to the beta-galactosidase gene of Escherichia coli were introduced into the high-level-trichothecene-producing strain, R-6380. Expression of reporter gene activity in transformants was found to be regulated in a manner similar to Tox5 expression but appeared to be independent of the 42-nt sequence copy number. These results indicate that transcriptional controls play an important role in the regulation of Tox5 expression and that genes involved in trichothecene biosynthesis in G. pulicaris may be linked to Tox5.

Characterization and mechanism of (4S)-limonene synthase, a monoterpene cyclase from the glandular trichomes of peppermint (Mentha x piperita)

(4S)-Limonene synthase, a monoterpene cyclase isolated from the secretory cells of the glandular trichomes of Mentha x piperita (peppermint), catalyzes the cyclization of geranyl pyrophosphate to (4S)-limonene, a key intermediate in the biosynthesis of p-menthane monoterpenes in Mentha species. The enzyme synthesizes principally (-)-(4S)-limonene (greater than 94% of the total products), plus several other monoterpene olefins. The general properties of (4S)-limonene synthase resemble those of other monoterpene cyclases. The enzyme shows a pH optimum near 6.7, an isoelectric point of 4.35, and requires a divalent metal ion for catalysis, either Mg2+ or Mn2+, with Mn2+ preferred. The Km value measured for geranyl pyrophosphate was 1.8 microM. The activity of (4S)-limonene synthase was inhibited by sodium phosphate, sodium pyrophosphate, and reagents directed against the amino acids cysteine, methionine, and histidine. In the presence of Mn2+, geranyl pyrophosphate protected against cysteine-directed inhibition, suggesting that at least one cysteine residue is located at or near the active site. Experiments with alternate substrates and substrate analogs confirmed many elements of the proposed reaction mechanism, including the binding of geranyl pyrophosphate in the form of a complex with the divalent metal ion, the preliminary isomerization of geranyl pyrophosphate to linalyl pyrophosphate (a bound intermediate capable of cyclization), and the participation of a series of carbocation:pyrophosphate anion pairs in the reaction sequence.

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.

Bioconversion of possible T-2 toxin precursors by a mutant strain of Fusarium sporotrichioides NRRL 3299

Liquid cultures of a mutant strain of Fusarium sporotrichioides NRRL 3299 that accumulates trichodiene rather than T-2 toxin converted tricho-9-ene-2 alpha,3 alpha,11 alpha-triol, trichotriol (tricho-10-ene-2 alpha,3 alpha,9 alpha-triol), tricho-10-ene-2 alpha,3 alpha,9 beta-triol, 3 alpha-hydroxytrichothecene, and 3 alpha-acetoxytrichothecene to T-2 toxin. Other possible oxygenated precursors of T-2 toxin, including trichodiol (tricho-10-ene-2 alpha,9 alpha-diol), trichothecene, 4 alpha-hydroxytrichothecene, and 15-hydroxytrichothecene, were not metabolized. The results indicate that in the biosynthesis of T-2 toxin by F. sporotrichioides, (i) oxygenation at C-3 occurs prior to the second cyclization, (ii) this second cyclization involves two steps that may be nonenzymatic, and (iii) oxidation at C-3 precedes that at C-4 or C-15.

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.

Genetic and mutational tools for investigating the genetics and molecular biology of trichothecene production in Gibberella pulicaris (Fusarium sambucinum)

Gibberella pulicaris (Fusarium sambucinum) is a promising organism for studying the genetics and regulation of trichothecene biosynthesis; conditions for obtaining fertile crosses have been defined (Desjardins & Beremand, 1987) and crosses between natural variants have provided some information about the number, location, arrangement, and role of genes which determine trichothecene production (Desjardins & Beremand, 1987; Beremand & Desjardins, 1988). The development of some additional experimental tools and methodologies required for the further genetic analysis of trichothecene production in G. pulicaris are described in the present study. A highly fertile, isogenic line was constructed for G. pulicaris strain R-6380. The ability to readily generate mutants in this strain was also demonstrated. Both biochemical and morphological mutants were obtained following UV-mutagenesis. The inheritance of some of these mutations through meiosis indicated that they will be useful genetic markers for crosses and mapping studies. Since strain R-6380 is also transformable (Salch & Beremand, 1988), it is an excellent choice for transmission and molecular genetic studies involving trichothecene production.

Purification and characterization of the sesquiterpene cyclase aristolochene synthase from Penicillium roqueforti

The sesquiterpene cyclase, aristolochene synthase, has been purified from Penicillium roqueforti by gel filtration and anion-exchange chromatography. Isolation was facilitated by a change in the elution behavior of the enzyme during gel filtration at different steps in the purification. The purified enzyme had a specific activity of 70 nmol/min/mg protein. The molecular weight as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis was Mr 37,000. The native molecular weight as determined by gel filtration chromatography was Mr 48,000. The requirement for Mg2+ could be partially substituted with 0.01 mM Mn2+, but higher concentrations were inhibitory. Pyrophosphate, a competitive inhibitor of most terpene cyclases, had no effect on enzyme activity up to a concentration of 5.0 mM. The maximum activity was observed between pH 6.25 and pH 7.50, and the Km for farnesyl pyrophosphate was 0.55 +/- 0.06 microM.

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.

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.