De novo biosynthesis of secondary metabolism enzymes in homogeneous cultures of Penicillium urticae

Arenicolins: C-Glycosylated Depsides from Penicillium arenicola

During investigation of the secondary metabolism of four strains of Penicillium arenicola, two new depsides, arenicolins A (1) and B (2), were isolated and characterized. Their structures were established mainly by analysis of NMR and HRMS data and by comparison with known compounds. These depsides incorporate intriguing structural features, including dual alkyl side chains and a C-glycosyl unit, with 1 also containing an acylated 2-hydroxymethyl-4,5,6-trihydroxycyclohexenone moiety. Although the arenicolins were produced by all strains tested, arenicolin A (1) was obtained using only one of five medium compositions employed, while arenicolin B (2) was produced in all media tested. Neither compound showed antibacterial or antifungal activity, but 1 exhibited cytotoxicity toward mammalian cell lines, including colorectal carcinoma (HCT-116), neuroblastoma (IMR-32), and ductal carcinoma (BT-474), with IC50 values of 7.3, 6.0, and 9.7 μM, respectively.

Accumulation of the mycotoxin patulin in the presence of gluconic acid contributes to pathogenicity of Penicillium expansum

Penicillium expansum, the causal agent of blue mold rot, causes severe postharvest fruit maceration through secretion of D-gluconic acid (GLA) and secondary metabolites such as the mycotoxin patulin in colonized tissue. GLA involvement in pathogenicity has been suggested but the mechanism of patulin accumulation and its contribution to P. expansum pathogenicity remain unclear. The roles of GLA and patulin accumulation in P. expansum pathogenicity were studied using i) glucose oxidase GOX2-RNAi mutants exhibiting decreased GOX2 expression, GLA accumulation, and reduced pathogenicity; ii) IDH-RNAi mutants exhibiting downregulation of IDH (the last gene in patulin biosynthesis), reduced patulin accumulation, and no effect on GLA level; and iii) PACC-RNAi mutants exhibiting downregulation of both GOX2 and IDH that reduced GLA and patulin production. Present results indicate that conditions enhancing the decrease in GLA accumulation by GOX2-RNAi and PACC-RNAi mutants, and not low pH, affected patulin accumulation, suggesting GLA production as the driving force for further patulin accumulation. Thus, it is suggested that GLA accumulation may modulate patulin synthesis as a direct precursor under dynamic pH conditions modulating the activation of the transcription factor PACC and the consequent pathogenicity factors, which contribute to host-tissue colonization by P. expansum.

Small chemical chromatin effectors alter secondary metabolite production in Aspergillus clavatus

The filamentous fungus Aspergillus clavatus is known to produce a variety of secondary metabolites (SM) such as patulin, pseurotin A, and cytochalasin E. In fungi, the production of most SM is strongly influenced by environmental factors and nutrients. Furthermore, it has been shown that the regulation of SM gene clusters is largely based on modulation of a chromatin structure. Communication between fungi and bacteria also triggers chromatin-based induction of silent SM gene clusters. Consequently, chemical chromatin effectors known to inhibit histone deacetylases (HDACs) and DNA-methyltransferases (DNMTs) influence the SM profile of several fungi. In this study, we tested the effect of five different chemicals, which are known to affect chromatin structure, on SM production in A. clavatus using two growth media with a different organic nitrogen source. We found that production of patulin was completely inhibited and cytochalasin E levels strongly reduced, whereas growing A. clavatus in media containing soya-derived peptone led to substantially higher pseurotin A levels. The HDAC inhibitors valproic acid, trichostatin A and butyrate, as well as the DNMT inhibitor 5-azacytidine (AZA) and N-acetyl-d-glucosamine, which was used as a proxy for bacterial fungal co-cultivation, had profound influence on SM accumulation and transcription of the corresponding biosynthetic genes. However, the repressing effect of the soya-based nitrogen source on patulin production could not be bypassed by any of the small chemical chromatin effectors. Interestingly, AZA influenced some SM cluster genes and SM production although no Aspergillus species has yet been shown to carry detectable DNA methylation.

Cytotoxicity and mycotoxin production of shellfish-derived Penicillium spp., a risk for shellfish consumers

In order to assess the putative toxigenic risk associated with the presence of fungal strains in shellfish-farming areas, Penicillium strains were isolated from bivalve molluscs and from the surrounding environment, and the influence of the sample origin on the cytotoxicity of the extracts was evaluated. Extracts obtained from shellfish-derived Penicillia exhibited higher cytotoxicity than the others. Ten of these strains were grown on various media including a medium based on mussel extract (Mytilus edulis), mussel flesh-based medium (MES), to study the influence of the mussel flesh on the production of cytotoxic compounds. The MES host-derived medium was created substituting the yeast extract of YES medium by an aqueous extract of mussel tissues, with other constituent identical to YES medium. When shellfish-derived strains of fungi were grown on MES medium, extracts were found to be more cytotoxic than on the YES medium for some of the strains. HPLC-UV/DAD-MS/MS dereplication of extracts from Penicillium marinum and P. restrictum strains grown on MES medium showed the enhancement of the production of some cytotoxic compounds. The mycotoxin patulin was detected in some P. antarcticum extracts, and its presence seemed to be related to their cytotoxicity. Thus, the enhancement of the toxicity of extracts obtained from shellfish-derived Penicillium strains grown on a host-derived medium, and the production of metabolites such as patulin suggests that a survey of mycotoxins in edible shellfish should be considered.

Mycophenolic Acid Production by Penicillium brevicompactum on Solid Media

When grown on Czapek-Dox agar, Penicillium brevicompactum produced mycophenolic acid after a vegetative mycelium had been formed and as aerial hyphae were developing. Nutrients were still plenteous in the agar when the synthesis began. If aerial hyphal development was prevented by placing a dialysis membrane over the growing fungus, no mycophenolic acid was produced. When the dialysis membrane was peeled back and, as a consequence, production of aerial hyphae began, mycophenolic acid biosynthesis was observed. We concluded that mycophenolic acid was produced only by P. brevicompactum colonies that possessed an aerial mycelium.

Biosynthesis and toxicological effects of patulin

Patulin is a toxic chemical contaminant produced by several species of mold, especially within Aspergillus, Penicillium and Byssochlamys. It is the most common mycotoxin found in apples and apple-derived products such as juice, cider, compotes and other food intended for young children. Exposure to this mycotoxin is associated with immunological, neurological and gastrointestinal outcomes. Assessment of the health risks due to patulin consumption by humans has led many countries to regulate the quantity in food. A full understanding of the molecular genetics of patulin biosynthesis is incomplete, unlike other regulated mycotoxins (aflatoxins, trichothecenes and fumonisins), although the chemical structures of patulin precursors are now known. The biosynthetic pathway consists of approximately 10 steps, as suggested by biochemical studies. Recently, a cluster of 15 genes involved in patulin biosynthesis was reported, containing characterized enzymes, a regulation factor and transporter genes. This review includes information on the current understanding of the mechanisms of patulin toxinogenesis and summarizes its toxicological effects.

Production of indole diterpenes by Aspergillus alliaceus

Production of two related indole diterpenes (differing by a dimethyl leucine side chain) by Aspergillus alliaceus was improved through several pilot scale fermentations. Media were optimized through focus primarily on initial increases, as well as mid-cycle additions, of carbon and nitrogen sources. Fermentation conditions were improved by varying ventilation conditions using various combinations of air flowrate and back-pressure set points. Production improvements were quantified based on total indole diterpene concentration as well as the ratio of the major-to-minor by-product components. Those changes with a positive substantial impact primarily on total indole diterpene concentration included early cycle glycerol shots and enhanced ventilation conditions (high air flowrate, low back-pressure). Those changes with a significant impact primarily on ratio included higher initial cerelose, soybean oil, monosodium glutamate, tryptophan, or ammonium sulfate concentrations, higher broth pH, and enhanced ventilation conditions. A few changes (higher initial glycerol and monosodium glutamate concentrations) resulted in less notable and desirable titer or ratio changes when implemented individually, but they were adopted to more fully realize the impact of other improvements or to simplify processing. Overall, total indole diterpene titers were improved at the 600 L pilot scale from 125-175 mg/L with a ratio of about 2.1 to 200-260 mg/L with a ratio of about 3.3-4.5. Thus, the ability to optimize total indole diterpene titer and/or ratio readily exists for secondary metabolite production using Aspergillus cultures.

Rational elimination of Aspergillus terreus sulochrin production

Elimination of undesirable co-metabolites from industrial fermentations is often required due to the toxicities associated with the contaminants and/or due to difficulties in removing the contaminants during downstream processing. Sulochrin is a co-metabolite produced during the Aspergillus terreus lovastatin fermentation. Examination of the sulochrin biosynthetic pathway identifies the emodin anthrone polyketide synthase (PKS) at the origin. Thus, genetically disrupting the emodin anthrone PKS gene was expected to result in the elimination of sulochrin biosynthesis. To perform the disruption by homologous recombination, a fragment of the emodin anthrone PKS gene first needed to be isolated. Analysis of several reported fungal PKS amino acid sequences has identified three subfamilies of related sequences (called the Patulin subfamily, the Pigment subfamily, and the Reduction subfamily). PCR primers specific for the Pigment subfamily (of which the emodin anthrone PKS is expected to belong) were used to isolate a fragment of a novel PKS gene from A. terreus. Targeted gene disruption identifies the novel gene fragment as that from the emodin anthrone PKS. Consequently, the gene disruption event eliminated the production of metabolites from the sulochrin biosynthetic pathway.

Physiology of antibiotic production in actinomycetes and some underlying control mechanisms

Some of the accumulated information on the physiology and nutritional control of antibiotic production in actinomycetes can now be related to recent discoveries in the field of actinomycete molecular biology. This review focuses on aspects of genetic and metabolic control of antibiotic biosynthesis. It surveys some well established principles in the relationship between primary and secondary metabolism, and summarizes briefly the areas where progress is being made in elucidating the molecular organization of regulatory systems underlying this relationship.

Physiological control of trophophase-idiophase separation in streptomycete cultures producing secondary metabolites

Cultures of Streptomyces coelicolor A3(2) produced actinorhodin in defined media with various carbon and nitrogen sources. Production occurred during biomass accumulation if assimilation of either the carbon or the nitrogen source limited the rate of growth. High growth rates tended to delay product synthesis until after biomass accumulation was complete, but fully biphasic fermentation profiles were achieved only with media supporting very rapid growth. The onset of actinorhodin production then coincided with a decline in the growth rate during transition of carbon-sufficient cultures to stationary phase. In cultures with maltose as a growth-limiting carbon source, depletion of phosphate increased the rate of actinorhodin biosynthesis, but did not alter the timing of its initiation. With defined media, the use of spores rather than vegetative mycelium as inocula reduced the overlap between trophophase and idiophase. The general guidelines for achieving biphasic production of actinorhodin in S. coelicolor A3(2) cultures could be used to obtain trophophase-idiophase separation in cultures of Streptomyces venezuelae producing chloramphenicol. However, the conditions needed to be modified to give optimized biphasic fermentations with individual strains. Under conditions favouring chloramphenicol production in a distinct idiophase, aromatic amine secondary metabolites in the same cultures of S. venezuelae were produced in a pattern that overlapped the trophophase, suggesting that conditions need to be tailored also to meet differences in the regulation of secondary metabolites.

Ammonium repression of antibiotic and intracellular proteinase production in Penicillium urticae

In this study the addition of ammonium ions (5-30 mM) to Penicillium urticae shake-flask cultures before, during and after the onset of polyketide biosynthesis was examined in a time-dependent manner for its repressive effect on metabolites and a marker enzyme of the patulin pathway and on the intracellular proteinases that also appear during the non-growth or idiophase. A study of the effect of ammonium ion addition, showed that both secondary enzyme and proteinase appearance were maximally delayed if the addition was made before the normal 7 h period of derepression/induction. If added during this period the effect of ammonium ions was progressively less. A reduction in the extracellular ammonium ion concentration from 30 to 4 mM appeared to be required to initiate the derepression/induction process. Adding ammonium ions during the appearance of secondary enzymes caused a rapid decrease in specific activity, about 67% for the patulin pathway enzyme and 12% for proteinase. Nitrogen repression exerts a much stronger effect on the expression of polyketide genes as opposed to proteinase genes. Both patulin pathway enzymes and proteinases are subjected to proteolysis, but the proteinases retain much of their activity, whereas the polyketide biosynthetic enzymes do not.

Kinetics of the repression of tylosin biosynthesis by ammonium ion in Streptomyces fradiae

Nitrogen regulation of tylosin synthesis in Streptomyces fradiae NRRL 2702 was studied in batch and chemostat cultures using a soluble synthetic medium. In batch cultures, valine dehydrogenase (VDH; EC 1.4.1.8), threonine dehydratase (TDT; EC 4.2.1.16) and aspartate aminotransferase (ASAT; EC 2.6.1.1) reached their highest specific activities at 120 h. The specific activities of the three enzymes showed close correlation with the value of specific tylosin formation rate (qTYL). In chemostat cultures, the maximum value of qTYL was 1.14 tylosin per mycelial mass per h (mg g-1 h-1) at the specific growth rate of 0.05 h-1, and after reaching a rate of 0.1 h-1, qTYL decreased with increasing levels of the specific growth rate. This value of qTYL was 3.5-times as large as that of maximum qTYL observed in the batch culture. The specific formation rates of VDH, TDT, ASAT and tylosin were repressed by high levels of specific ammonium ion uptake rate.

Influences of chemical fertilizers (in vitro) on aflatoxin and citrinin synthesis by two strains of aspergilli

The selective effect of various levels of phosphate and nitrate (as fertilizers) on biosynthesis of aflatoxin by Aspergillus parasiticus var. globosus, and citrinin by A. terreus var. aureus was studied in defined culture medium. Phosphate at 35-175 mmol per 50 mL decreased aflatoxin production, but increased citrinin synthesis. Nitrate at 73-365 mmol per 50 mL stimulated the synthesis of aflatoxin but depressed that of citrinin. A rise in the levels of nitrate and phosphate led to a decrease in aflatoxin production, an increase in citrinin production and an accumulation of mycelial phosphate and nitrate contents.

Purification and properties of 6-methylsalicylic acid synthase from Penicillium patulum

6-Methylsalicylic acid synthase has been isolated in homogeneous form from Penicillium patulum grown in liquid culture from a spore inoculum. The enzyme is highly susceptible to proteolytic degradation in vivo and in vitro, but may be stabilized during purification by incorporating proteinase inhibitors in the buffers. The enzyme exists as a homotetramer of M(r) 750,000, with a subunit M(r) of 180,000. 6-Methylsalicyclic acid synthase also accepts acetoacetyl-CoA as an alternative starter molecule to acetyl-CoA. The enzyme also catalyses the formation of small amounts of triacetic acid lactone as an oligatory by-product of the reaction. In the absence of NADPH, triacetic acid lactone is the exclusive enzymic product, being formed at 10% of the rate of 6-methylsalicylic acid. The enzyme is inactivated by 1,3-dibromopropan-2-one, leading to the formation of cross-linked dimers similar to that observed with type I fatty acid synthases. Acetyl-CoA protects the enzyme against the inactivation and inhibits dimer formation. An adaptation of the purification method for 6-methylsalicylic acid synthase may be used for the isolation of fatty acid sythase from Penicillium patulum.

Applications of high-performance liquid chromatography to quantitation of metabolites and enzymes of the patulin pathway from Penicillium patulum

Conditions for extraction and high-performance liquid chromatographic (HPLC) analysis for fourteen of the patulin pathway metabolites from Penicillium patulum are described which allow quantitation of the metabolite content of cultures at hourly intervals. The HPLC analysis is more sensitive than gas-liquid chromatographic analysis and is more quantitative than thin-layer chromatographic analysis. Separations on a preparative column allow for the collection and identification of new metabolites. The column elution program can be varied to optimize analysis time for individual metabolites, allowing individual enzymes of the pathway to be assayed by following the conversion of substrate to product. Analysis of product formation in crude enzyme mixtures can be used to assay an enzyme in the presence of subsequent enzymes of the pathway and to establish the pathway reaction sequence.

Roles of trace metals in transcriptional control of microbial secondary metabolism

Secondary metabolism in bacteria and fungi requires a much narrower range of environmental concentrations of key trace metals than that permitted for primary metabolism. The cells acquire appropriate quantities of the key metals at the initiation of the shift from primary to secondary metabolism. Evidence suggests that these essential micronutrients participate in regulating the expression of genes responsible for synthesis of secondary metabolites and/or morphological alterations associated with cellular differentiation.

Nitrogen inhibition of mycotoxin production by Alternaria alternata

Alternaria alternata produces the polyketides alternariol (AOH) and alternariol monomethyl ether (AME) during the stationary growth phase. Addition of 12 mM NaNO3 to the cultures before initiation of polyketide production reduced the AOH and AME content to 5 to 10% of that of controls. Glutamate and urea also reduced AOH and AME accumulation, whereas increasing the ionic strength did not affect the polyketide content. Adding NaNO3 after polyketide production had started did not inhibit further AOH accumulation, although over 90% of the added NO3- disappeared from the medium within 24 h. Activity of an AME-synthesizing enzyme, alternariol-O-methyltransferase (AOH-MT), appeared in control mycelia during the early stationary growth phase. No AOH-MT activity appeared in mycelia blocked in polyketide synthesis by addition of NaNO3. Later addition of NaNO3 reduced the AOH-MT specific activity to 50% of that of the control, whereas the total of activity per mycelium was the same. The AOH-MT activity in vitro was not affected by 100 mM NaNO3. The results suggest that nitrogen in some way inhibited the formation of active enzymes in the polyketide-synthesizing pathway in A. alternata when it was added before these enzymes were formed.

Thin-layer chromatography of mycotoxins

TLC has become an extremely powerful, rapid and in most instances inexpensive separation technique in mycotoxicology. This review presents achievements of its applications in this field. General technical aspects of the TLC of mycotoxins that are discussed include extraction and clean-up procedures, adsorbents and solvent systems, detection methods, two-dimensional TLC, high-performance TLC (HPTLC), quantitation and preparative TLC (PLC). Special applications of TLC deal with multi-mycotoxin analyses and with structurally related or individual mycotoxins (aflatoxins, sterigmatocystins, versicolorins, ochratoxins, rubratoxins, patulin, penicillic acid, mycophenolic acid, butenolide, citreoviridin, trichothecenes, cytochalasans, tremorgenic toxins, epipolythiopiperazine-3,6-diones, hydroxyanthraquinones, zearalenone, citrinin, secalonic acids, cyclopiazonic acid, PR toxin, roquefortine, xanthomegnin, viomellein and naphtho-gamma-pyrones).

Action of patulin on a yeast

The action of patulin on Saccharomyces cerevisiae was studied. At weak doses, the drug inhibited growth, but inhibition was transient. After 10 min, syntheses of rRNA, tRNA, and probably mRNA were blocked; this was shown by radioactive precursor incorporation assays and gel electrophoresis of RNAs. After recovery of growth, patulin disappeared from the medium. It seemed that this degradation resulted from the activity of an inducible enzymatic system. Induced cells resisted very high patulin concentrations.

6-Methylsalicylic Acid Production in Solid Cultures of Penicillium patulum Occurs Only When an Aerial Mycelium Is Present

When Penicillium patulum was grown on Czapek-Dox agar, 6-methylsalicylic acid was produced as an aerial mycelium was forming. Nutrients were often plentiful in the medium when biosynthesis began. If the formation of an aerial mycelium was prevented by growing the fungus between two sheets of dialysis membrane, no 6-methylsalicylic acid was produced even when nutrients were completely consumed. If the upper sheet of dialysis membrane was stripped off cultures of the latter type, an aerial mycelium formed; concomitantly, 6-methylsalicylic acid biosynthesis was observed. We conclude that 6-methylsalicylic acid was produced only by P. patulum colonies that possessed an aerial mycelium.