dffA Gene fromAspergillus oryzae encodesl-ornithineN5-oxygenase and is indispensable for deferriferrichrysin biosynthesis

Functional analysis of a fungal P450 enzyme

Fungal cytochrome P450s participate in various physiological reactions, including the synthesis of internal cellular components, metabolic detoxification of xenobiotic compounds, and oxidative modification of natural products. Although functional analysis reports of fungal P450s continue to grow, there are still some difficulties as compared to prokaryotic P450s, because most of these fungal enzymes are transmembrane proteins. In this chapter, we will describe the methods for heterologous expression, in vivo analysis, enzyme preparation, and in vitro enzyme assays of the fungal P450 enzyme Trt6 and isomerase Trt14, which play important roles in the divergence of the biosynthetic pathway of terretonins, as a model for the functional analysis of fungal P450 enzymes.

Talaropeptins A and B, Tripeptides with an N-trans-Cinnamoyl Moiety from the Marine-Derived Fungus Talaromyces purpureogenus CX11

We report the discovery of talaropeptins A (1) and B (2), tripeptides with an unusual 5/6/5 heterocyclic scaffold and an N-trans-cinnamoyl moiety, which were identified from the marine-derived fungus Talaromyces purpureogenus CX11. A bioinformatic analysis of the genome of T. purpureogenus CX11 and gene inactivation revealed that the biosynthesis of talaropeptins involves a nonribosomal peptide synthase gene cluster. Their chemical structures were elucidated using a combination of 1D and 2D NMR spectroscopy and mass spectrometry. The absolute configurations of 1 and 2 were established by electronic circular dichroism calculations and Marfey's method. The plausible biosynthesis of 1 and 2 is also proposed on the basis of gene deletion, substrate feeding, and heterologous expression. Compounds 1 and 2 showed moderate antifungal activity against phytopathogenic fungus Fusarium oxysporum with MIC values of 12.5 and 25 μg/mL, respectively.

Antibacterial diphenyl ether production induced by co-culture of Aspergillus nidulans and Aspergillus fumigatus

Fungi are a rich source of secondary metabolites with potent biological activities. Co-culturing a fungus with another microorganism has drawn much attention as a practical method for stimulating fungal secondary metabolism. However, in most cases, the molecular mechanisms underlying the activation of secondary metabolite production in co-culture are poorly understood. To elucidate such a mechanism, in this study, we established a model fungal-fungal co-culture system, composed of Aspergillus nidulans and Aspergillus fumigatus. In the co-culture of A. nidulans and A. fumigatus, production of antibacterial diphenyl ethers was enhanced. Transcriptome analysis by RNA-sequencing showed that the co-culture activated expression of siderophore biosynthesis genes in A. fumigatus and two polyketide biosynthetic gene clusters (the ors and cic clusters) in A. nidulans. Gene disruption experiments revealed that the ors cluster is responsible for diphenyl ether production in the co-culture. Interestingly, the ors cluster was previously reported to be upregulated by co-culture of A. nidulans with the bacterium Streptomyces rapamycinicus; orsellinic acid was the main product of the cluster in that co-culture. In other words, the main product of the ors cluster was different in fungal-fungal and bacterial-fungal co-culture. The genes responsible for biosynthesis of the bacterial- and fungal-induced polyketides were deduced using a heterologous expression system in Aspergillus oryzae. The molecular genetic mechanisms that trigger biosynthesis of two different types of compounds in A. nidulans in response to the fungus and the bacterium were demonstrated, which provides an insight into complex secondary metabolic response of fungi to microorganisms. KEY POINTS: • Co-culture of two fungal species triggered antibiotic diphenyl ether production. • The co-culture affected expression levels of several genes for secondary metabolism. • Gene cluster essential for induction of the antibiotics production was determined.

Reconstitution of Polyketide-Derived Meroterpenoid Biosynthetic Pathway in Aspergillus oryzae

The heterologous gene expression system with Aspergillus oryzae as the host is an effective method to investigate fungal secondary metabolite biosynthetic pathways for reconstruction to produce un-natural molecules due to its high productivity and genetic tractability. In this review, we focus on biosynthetic studies of fungal polyketide-derived meroterpenoids, a group of bioactive natural products, by means of the A. oryzae heterologous expression system. The heterologous expression methods and the biosynthetic reactions are described in detail for future prospects to create un-natural molecules via biosynthetic re-design.

Flavin-dependent N-hydroxylating enzymes: distribution and application

Amino groups derived from naturally abundant amino acids or (di)amines can be used as "shuttles" in nature for oxygen transfer to provide intermediates or products comprising N-O functional groups such as N-hydroxy, oxazine, isoxazolidine, nitro, nitrone, oxime, C-, S-, or N-nitroso, and azoxy units. To this end, molecular oxygen is activated by flavin, heme, or metal cofactor-containing enzymes and transferred to initially obtain N-hydroxy compounds, which can be further functionalized. In this review, we focus on flavin-dependent N-hydroxylating enzymes, which play a major role in the production of secondary metabolites, such as siderophores or antimicrobial agents. Flavoprotein monooxygenases of higher organisms (among others, in humans) can interact with nitrogen-bearing secondary metabolites or are relevant with respect to detoxification metabolism and are thus of importance to understand potential medical applications. Many enzymes that catalyze N-hydroxylation reactions have specific substrate scopes and others are rather relaxed. The subsequent conversion towards various N-O or N-N comprising molecules is also described. Overall, flavin-dependent N-hydroxylating enzymes can accept amines, diamines, amino acids, amino sugars, and amino aromatic compounds and thus provide access to versatile families of compounds containing the N-O motif. Natural roles as well as synthetic applications are highlighted. Key points • N-O and N-N comprising natural and (semi)synthetic products are highlighted. • Flavin-based NMOs with respect to mechanism, structure, and phylogeny are reviewed. • Applications in natural product formation and synthetic approaches are provided. Graphical abstract .

A unique Zn(II)2-Cys6-type protein, KpeA, is involved in secondary metabolism and conidiation in Aspergillus oryzae

Aspergillus oryzae is an important microorganism in the bio- and food industries; therefore, understanding the mechanism underlying its secondary metabolism regulation is important for ensuring its safe use. Here, we screened a novel Zn(II)₂-Cys₆-type protein-encoding gene, AO090003001186, designated as kpeA (kojic acid production enhancement A), from an A. oryzae disruption mutant library of transcriptional regulators. kpeA is highly conserved among filamentous fungi and encodes a protein with Zn(II)₂-Cys₆ motif located in the middle of the sequence. Phylogenetic analysis revealed that KpeA was classified into a distal group compared to other fungal Zn(II)₂-Cys₆-type transcriptional regulators. A Cys to Ala substitution mutant of KpeA showed identical phenotype to the kpeA disruption strain, confirming that KpeA is novel type Zn(II)₂-Cys₆ binding protein. Colonies of the kpeA disruption strain (ΔkpeA) had longer aerial hyphae and showed decreased conidia production. Microscopic analysis suggested that the reduced vesicle size and conidial head formation in ΔkpeA strain account for the decreased conidia production. Transcriptional levels of brlA and downstream abaA and wetA were decreased in ΔkpeA strain. Moreover, ΔkpeA strain produced 6-fold more kojic acid than the control strains, and the expression of kojR and kojA was increased in ΔkpeA strain. Therefore, KpeA is a novel Zn(II)₂-Cys₆-type protein likely involved in conidiation and kojic acid production at the transcriptional level.

Genome mining for fungal polyketide-diterpenoid hybrids: discovery of key terpene cyclases and multifunctional P450s for structural diversification

The biosynthesis of the fungal meroterpenoid chevalone E and its derivatives has been successfully elucidated and reconstituted.

O-Methylation steps during strobilurin and bolineol biosynthesis

Strobilurins are potent antifungal polyketides produced by basidiomycete fungi. Two genes encoding O-methyltransferases (O-MeT) are present in the biosynthetic gene cluster of strobilurin A 1. In previous studies, the two O-MeT enzymes Str2 and Str3 were found to catalyse the final steps of the biosynthesis of 1. Here, we show by in vivo expression experiments, that O-methylation during strobilurin biosynthesis is regiospecific. O-MeT Str2 acts first and selectively catalyses the methylation of the carboxyl group of strobilurin and bolineol precursors. Str3 catalyses the subsequent methyl transfer to the enol group to form strobilurin A 1, but cannot methylate bolineol 4. Toxicity tests showed increasing antifungal activity of intermediates through the pathway and that bolineol 4 shows antifungal activity against A. oryzae NSAR1 with an MIC of 0.1 mg ml⁻¹.

Two methyltransferase genes involving in the biosynthesis of the antifungal agent strobilurin A were investigated. The antifungal activity of pathway intermediates increses with methylation.

Strobilurin biosynthesis in Basidiomycete fungi

Strobilurins from fungi are the inspiration for the creation of the β-methoxyacrylate class of agricultural fungicides. However, molecular details of the biosynthesis of strobilurins have remained cryptic. Here we report the sequence of genomes of two fungi that produce strobilurins and show that each contains a biosynthetic gene cluster, which encodes a highly reducing polyketide synthase with very unusual C-terminal hydrolase and methyltransferase domains. Expression of stpks1 in Aspergillus oryzae leads to the production of prestrobilurin A when the fermentation is supplemented with a benzoyl coenzyme A (CoA) analogue. This enables the discovery of a previously unobserved route to benzoyl CoA. Reconstruction of the gene cluster in A. oryzae leads to the formation of prestrobilurin A, and addition of the gene str9 encoding an FAD-dependent oxygenase leads to the key oxidative rearrangement responsible for the creation of the β-methoxyacrylate toxophore. Finally, two methyltransferases are required to complete the synthesis.

Strobilurins are fungal metabolites that inspired the creation of β-methoxyacrylate agricultural fungicides. Here, Nofiani et al. identify the strobilurin biosynthesis gene cluster, encoding a polyketide synthase as well as an FAD-dependent oxygenase for an oxidative rearrangement leading to β-methoxyacrylate formation.

Novofumigatonin biosynthesis involves a non-heme iron-dependent endoperoxide isomerase for orthoester formation

Novofumigatonin (1), isolated from the fungus Aspergillus novofumigatus, is a heavily oxygenated meroterpenoid containing a unique orthoester moiety. Despite the wide distribution of orthoesters in nature and their biological importance, little is known about the biogenesis of orthoesters. Here we show the elucidation of the biosynthetic pathway of 1 and the identification of key enzymes for the orthoester formation by a series of CRISPR-Cas9-based gene-deletion experiments and in vivo and in vitro reconstitutions of the biosynthesis. The novofumigatonin pathway involves endoperoxy compounds as key precursors for the orthoester synthesis, in which the Fe(II)/α-ketoglutarate-dependent enzyme NvfI performs the endoperoxidation. NvfE, the enzyme catalyzing the orthoester synthesis, is an Fe(II)-dependent, but cosubstrate-free, endoperoxide isomerase, despite the fact that NvfE shares sequence homology with the known Fe(II)/α-ketoglutarate-dependent dioxygenases. NvfE thus belongs to a class of enzymes that gained an isomerase activity by losing the α-ketoglutarate-binding ability.

Development of a promoter shutoff system in Aspergillus oryzae using a sorbitol-sensitive promoter

Promoter shutoff is a general method for analyzing essential genes, but in the fungus Aspergillus oryzae, no tightly repressed promoters have been reported. To overcome the current limitations of conditional promoters, we examined sorbitol- and galactose-responsive genes using microarrays to identify regulatable genes with only minor physiological and genetic effects. We identified two sorbitol-induced genes (designated as sorA and sorB), cloned their promoters, and built a regulated egfp and brlA expression system. Growth medium-dependent enhanced green fluorescence protein (EGFP) fluorescence and conidiation were confirmed for egfp and brlA under the control of their respective promoters. We also used this shutoff system to regulate the essential rhoA, which demonstrated the expected growth inhibition under repressed growth conditions. Our new sorbitol promoter shutoff system developed can serve as a valuable new tool for essential gene analyses of filamentous fungi.

Biosynthesis of oxygen and nitrogen-containing heterocycles in polyketides

This review highlights the biosynthesis of heterocycles in polyketide natural products with a focus on oxygen and nitrogen-containing heterocycles with ring sizes between 3 and 6 atoms. Heterocycles are abundant structural elements of natural products from all classes and they often contribute significantly to their biological activity. Progress in recent years has led to a much better understanding of their biosynthesis. In this context, plenty of novel enzymology has been discovered, suggesting that these pathways are an attractive target for future studies.

Characterization of two acetyltransferase genes in the pyripyropene biosynthetic gene cluster from Penicillium coprobium

Pyripyropenes potently and selectively inhibit acyl-CoA:cholesterol acyltransferase 2 (ACAT-2). Among multiple isomers of pyripyropene (A to R), pyripyropene A (PyA) has insecticidal properties in addition to its growth inhibition properties against human umbilical vein endothelial cells. Based on the predicted biosynthetic gene cluster of pyripyropene A, two genes (ppb8 and ppb9) encoding two acetyltransferases (ATs) were separately isolated and introduced into the model fungus Aspergillus oryzae, using the protoplast–polyethylene glycol method. The bioconversion of certain predicted intermediates in the transformants revealed the manner by which acetylation occurred in the biosynthetic pathway by the products expressed by these two genes (AT-1 and AT-2). The acetylated products detected by high-performance liquid chromatography (HPLC) in the extracts from AT-1 and AT-2 transformant clones were not present in the extract from the transformant clone with an empty vector. The HLPC charts of each bioconversion study exhibited high peaks at 12, 10.5 and 9 min, respectively. Further ultraviolet absorption and mass spectrometry analyses identified the products as PyE, PyO and PyA, respectively. AT-1 acetylated the C-1 of deacetyl-pyripyropene E (deAc-PyE), while AT-2 played an active role in acetylating the C-11 of 11-deAc-PyO and C-7 of deAc-PyA at two different steps of the biosynthetic pathway.

Fungal siderophore metabolism with a focus on Aspergillus fumigatus

Siderophores are chelators synthesized by microbes to sequester iron. This article summarizes the knowledge on the fungal siderophore metabolism with a focus on Aspergillus fumigatus. In recent years, A. fumigatus became a role model for fungal biosynthesis, uptake and degradation of siderophores as well as regulation of siderophore-mediated iron handling and the elucidation of siderophore functions. Siderophore functions comprise uptake, intracellular transport and storage of iron. This proved to be crucial not only for adaptation to iron starvation conditions but also for germination, asexual and sexual propagation, antioxidative defense, mutual interaction, microbial competition as well as virulence in plant and animal hosts. Recent studies also indicate the high potential of siderophores and its biosynthetic pathway to improve diagnosis and therapy of fungal infections.

Fungus-specific sirtuin HstD coordinates secondary metabolism and development through control of LaeA

The sirtuins are members of the NAD(+)-dependent histone deacetylase family that contribute to various cellular functions that affect aging, disease, and cancer development in metazoans. However, the physiological roles of the fungus-specific sirtuin family are still poorly understood. Here, we determined a novel function of the fungus-specific sirtuin HstD/Aspergillus oryzae Hst4 (AoHst4), which is a homolog of Hst4 in A. oryzae yeast. The deletion of all histone deacetylases in A. oryzae demonstrated that the fungus-specific sirtuin HstD/AoHst4 is required for the coordination of fungal development and secondary metabolite production. We also show that the expression of the laeA gene, which is the most studied fungus-specific coordinator for the regulation of secondary metabolism and fungal development, was induced in a ΔhstD strain. Genetic interaction analysis of hstD/Aohst4 and laeA clearly indicated that HstD/AoHst4 works upstream of LaeA to coordinate secondary metabolism and fungal development. The hstD/Aohst4 and laeA genes are fungus specific but conserved in the vast family of filamentous fungi. Thus, we conclude that the fungus-specific sirtuin HstD/AoHst4 coordinates fungal development and secondary metabolism via the regulation of LaeA in filamentous fungi.

Isolation and characterisation of a ferrirhodin synthetase gene from the sugarcane pathogen Fusarium sacchari

FSN1, a gene isolated from the sugar-cane pathogen Fusarium sacchari, encodes a 4707-residue nonribosomal peptide synthetase consisting of three complete adenylation, thiolation and condensation modules followed by two additional thiolation and condensation domain repeats. This structure is similar to that of ferricrocin synthetase, which makes a siderophore that is involved in intracellular iron storage in other filamentous fungi. Heterologous expression of FSN1 in Aspergillus oryzae resulted in the accumulation of a secreted metabolite that was identified as ferrirhodin. This siderophore was found to be present in both mycelium and culture filtrates of F. sacchari, whereas ferricrocin is found only in the mycelium, thus suggesting that ferricrocin is an intracellular storage siderophore in F. sacchari, whereas ferrirhodin is used for iron acquisition. To our knowledge, this is the first report to characterise a ferrirhodin synthetase gene functionally.

Isotope-assisted screening for iron-containing metabolites reveals a high degree of diversity among known and unknown siderophores produced by Trichoderma spp

Due to low iron availability under environmental conditions, many microorganisms excrete iron-chelating agents (siderophores) to cover their iron demands. A novel screening approach for the detection of siderophores using liquid chromatography coupled to high-resolution tandem mass spectrometry was developed to study the production of extracellular siderophores of 10 wild-type Trichoderma strains. For annotation of siderophores, an in-house library comprising 422 known microbial siderophores was established. After 96 h of cultivation, 18 different iron chelators were detected. Four of those (dimerum acid, fusigen, coprogen, and ferricrocin) were identified by measuring authentic standards. cis-Fusarinine, fusarinine A and B, and des-diserylglycylferrirhodin were annotated based on high-accuracy mass spectral analysis. In total, at least 10 novel iron-containing metabolites of the hydroxamate type were found. On average Trichoderma spp. produced 12 to 14 siderophores, with 6 common to all species tested. The highest number (15) of siderophores was detected for the most common environmental opportunistic and strongly fungicidic species, Trichoderma harzianum, which, however, did not have any unique compounds. The tropical species T. reesei had the most distinctive pattern, producing one unique siderophore (cis-fusarinine) and three others that were present only in T. harzianum and not in other species. The diversity of siderophores did not directly correlate with the antifungal potential of the species tested. Our data suggest that the high diversity of siderophores produced by Trichoderma spp. might be the result of further modifications of the nonribosomal peptide synthetase (NRPS) products and not due to diverse NRPS-encoding genes.

Isolation of industrial strains of Aspergillus oryzae lacking ferrichrysin by disruption of the dffA gene

Based on studies using laboratory strains, the efficiency of gene disruption in Aspergillus oryzae, commonly known as koji mold, is low; thus, gene disruption has rarely been applied to the breeding of koji mold. To evaluate the efficiency of gene disruption in industrial strains of A. oryzae, we produced ferrichrysin biosynthesis gene (dffA) disruptants using three different industrial strains as hosts. PCR analysis of 438 pyrithiamine-resistant transformants showed dffA gene disruption efficiency of 42.9%-64.1%, which is much higher than previously reported. Analysis of the physiological characteristics of the disruptants indicated that dffA gene disruption results in hypersensitivity to hydrogen peroxide. To investigate the industrial characteristics of dffA gene disruptants, two strains were used to make rice koji and their properties were compared to those of the host strains. No differences were found between the dffA gene disruptants and the host strains, except that the disruptants did not produce ferrichrysin. Thus, this gene disruption technique is much more effective than conventional mutagenesis for A. oryzae breeding.

Secretory expression of the non-secretory-type Lentinula edodes laccase by Aspergillus oryzae

The shiitake mushroom, Lentinula edodes, has an extracelluar secretory-type laccase, Lcc1, and a fruiting-body-accumulation-type laccase, Lcc4. We previously reported the production of Lcc1 by plant cells, but had difficulty producing Lcc4. Here, we report the production of Lcc1 and Lcc4 by Aspergillus oryzae and the extracellular secretory production of Lcc4 using a modified secretion signal peptide (SP) from Lcc1. Sp-Lcc4 produced by A. oryzae had biochemical activities similar to Lcc4 produced by L. edodes. Lcc1 did not react with beta-(3,4-dihydroxyphenol) alanine (DOPA), but Lcc4 from L. edodes and A. oryzae could oxidize DOPA. K(M) values for the substrates 2,2'-azino-di-(3-ethylbenzthiazolinsulfonate), 2,6-dimethoxyphenol, guaiacol, pyrogallol, and catechol were similar for Lcc4 and Sp-Lcc4. In conclusion, a non-secretory-type fungal laccase is secreted into the culture media with its original enzymatic properties by exploiting modified secretory signal peptide.

Iron and siderophores in fungal-host interactions

Most fungi and bacteria express specific mechanisms for the acquisition of iron from the hosts they infect for their own survival. This is primarily because iron plays a key catalytic role in various vital cellular reactions in conjunction with the fact that iron is not freely available in these environments due to host sequestration. High-affinity iron uptake systems, such as siderophore-mediated iron uptake and reductive iron assimilation, enable fungi to acquire limited iron from animal or plant hosts. Regulating iron uptake is crucial to maintain iron homeostasis, a state necessary to avoid iron-induced toxicity from iron abundance, while simultaneously supplying iron required for biochemical demand. Siderophores play diverse roles in fungal-host interactions, many of which have been principally delineated from gene deletions in non-ribosomal peptide synthetases, enzymes required for siderophore biosynthesis. These analyses have demonstrated that siderophores are required for virulence, resistance to oxidative stress, asexual/sexual development, iron storage, and protection against iron-induced toxicity in some fungal organisms. In this review, the strategies fungi employ to obtain iron, siderophore biosynthesis, and the regulatory mechanisms governing iron homeostasis will be discussed with an emphasis on siderophore function and relevance for fungal organisms in their interactions with their hosts.

Identification, isolation, and analysis of a gene cluster involved in iron acquisition by Pseudomonas mendocina ymp

Microbial acquisition of iron from natural sources in aerobic environments is a little-studied process that may lead to mineral instability and trace metal mobilization. Pseudomonas mendocina ymp was isolated from the Yucca Mountain Site for long-term nuclear waste storage. Its ability to solubilize a variety of Fe-containing minerals under aerobic conditions has been previously investigated but its molecular and genetic potential remained uncharacterized. Here, we have shown that the organism produces a hydroxamate and not a catecholate-based siderophore that is synthesized via non-ribosomal peptide synthetases. Gene clustering patterns observed in other Pseudomonads suggested that hybridizing multiple probes to the same library could allow for the identification of one or more clusters of syntenic siderophore-associated genes. Using this approach, two independent clusters were identified. An unfinished draft genome sequence of P. mendocina ymp indicated that these mapped to two independent contigs. The sequenced clusters were investigated informatically and shown to contain respectively a potentially complete set of genes responsible for siderophore biosynthesis, uptake, and regulation, and an incomplete set of genes with low individual homology to siderophore-associated genes. A mutation in the cluster's pvdA homolog (pmhA) resulted in a siderophore-null phenotype, which could be reversed by complementation. The organism likely produces one siderophore with possibly different isoforms and a peptide backbone structure containing seven residues (predicted sequence: Acyl-Asp-Dab-Ser-fOHOrn-Ser-fOHorn). A similar approach could be applied for discovery of Fe- and siderophore-associated genes in unsequenced or poorly annotated organisms.

The siderophore biosynthetic gene SID1, but not the ferroxidase gene FET3, is required for full Fusarium graminearum virulence

To acquire iron from plant hosts, fungal pathogens have evolved at least two pathways for iron uptake. One system is hinged on the secretion and subsequent uptake of low-molecular-weight iron chelators termed siderophores, while the other uses cell-surface reductases to solubilize ferric iron by reducing it to ferrous iron for uptake. We identified five iron uptake-related genes from the head blight pathogen Fusarium graminearum and showed that they were transcribed in response to iron limitation. To examine the relative contribution of the reductive and siderophore pathways of iron uptake, we created mutants disrupted at the ferroxidase gene FET3 (Deltafet3) or the siderophore biosynthetic gene SID1 (Deltasid1). The Deltafet3 mutants produced wild-type amounts of siderophores and grew at the same rate as the wild-type under iron limitation, but accumulated high levels of free intracellular iron. The Deltasid1 mutants did not produce siderophores and grew slowly under low iron conditions. Transcription of the iron uptake-related genes was induced in the Deltasid1 mutant regardless of the growth medium iron content, whereas these genes were transcribed normally in the Deltafet3 mutant. Finally, the Deltasid1 mutants could infect single, inoculated spikelets, but were unable to spread from spikelet-to-spikelet through the rachises of wheat spikes, while the Deltafet3 mutants behaved as wild-type throughout infection. Together, our data suggest that siderophore-mediated iron uptake is the major pathway of cellular iron uptake and is required for full virulence in F. graminearum.

Analysis of expressed sequence tags from the fungus Aspergillus oryzae cultured under different conditions

We performed random sequencing of cDNAs from nine biologically or industrially important cultures of the industrially valuable fungus Aspergillus oryzae to obtain expressed sequence tags (ESTs). Consequently, 21 446 raw ESTs were accumulated and subsequently assembled to 7589 non-redundant consensus sequences (contigs). Among all contigs, 5491 (72.4%) were derived from only a particular culture. These included 4735 (62.4%) singletons, i.e. lone ESTs overlapping with no others. These data showed that consideration of culture grown under various conditions as cDNA sources enabled efficient collection of ESTs. BLAST searches against the public databases showed that 2953 (38.9%) of the EST contigs showed significant similarities to deposited sequences with known functions, 793 (10.5%) were similar to hypothetical proteins, and the remaining 3843 (50.6%) showed no significant similarity to sequences in the databases. Culture-specific contigs were extracted on the basis of the EST frequency normalized by the total number for each culture condition. In addition, contig sequences were compared with sequence sets in eukaryotic orthologous groups (KOGs), and classified into the KOG functional categories.

NPS6, encoding a nonribosomal peptide synthetase involved in siderophore-mediated iron metabolism, is a conserved virulence determinant of plant pathogenic ascomycetes

NPS6, encoding a nonribosomal peptide synthetase, is a virulence determinant in the maize (Zea mays) pathogen Cochliobolus heterostrophus and is involved in tolerance to H(2)O(2). Deletion of NPS6 orthologs in the rice (Oryza sativa) pathogen, Cochliobolus miyabeanus, the wheat (Triticum aestivum) pathogen, Fusarium graminearum, and the Arabidopsis thaliana pathogen, Alternaria brassicicola, resulted in reduced virulence and hypersensitivity to H(2)O(2). Introduction of the NPS6 ortholog from the saprobe Neurospora crassa to the Deltanps6 strain of C. heterostrophus restored wild-type virulence to maize and tolerance to H(2)O(2), demonstrating functional conservation in filamentous ascomycete phytopathogens and saprobes. Increased sensitivity to iron depletion was identified as a conserved phenotype of Deltanps6 strains. Exogenous application of iron enhanced the virulence of Deltanps6 strains of C. heterostrophus, C. miyabeanus, F. graminearum, and A. brassicicola to each host. NPS6 is responsible for the biosynthesis of extracellular siderophores by C. heterostrophus, F. graminearum, and A. brassicicola. Application of the extracellular siderophore of A. brassicicola restored wild-type virulence of the DeltaAbnps6 strain to Arabidopsis. It is proposed that the role of extracellular siderophores in fungal virulence to plants is to supply an essential nutrient, iron, to their producers in planta and not to act as phytotoxins, depriving their hosts of iron.

Iron uptake in fungi: a system for every source

Fungi have a remarkable capacity to take up iron when present in any of a wide variety of forms, which include free iron ions, low-affinity iron chelates, siderophore-iron chelates, transferrin, heme, and hemoglobin. Appropriately, these unicellular eukaryotes express a variety of iron uptake systems, some of which are unique to fungi and some of which are present in plants and animals, as well. The reductive system of uptake relies upon the external reduction of ferric salts, chelates, and proteins prior to uptake by a high-affinity, ferrous-specific, oxidase/permease complex. This system recognizes a broad range of substrates. The non-reductive system exhibits specificity for siderophore-iron chelates, and transporters of this system exhibit multiple substrate-dependent intracellular trafficking events.

The Aspergillus fumigatus siderophore biosynthetic gene sidA, encoding L-ornithine N5-oxygenase, is required for virulence

Aspergillus fumigatus is the leading cause of invasive mold infection and is a serious problem in immunocompromised populations worldwide. We have previously shown that survival of A. fumigatus in serum may be related to secretion of siderophores. In this study, we identified and characterized the sidA gene of A. fumigatus, which encodes l-ornithine N(5)-oxygenase, the first committed step in hydroxamate siderophore biosynthesis. A. fumigatus sidA codes for a protein of 501 amino acids with significant homology to other fungal l-ornithine N(5)-oxygenases. A stable DeltasidA strain was created by deletion of A. fumigatus sidA. This strain was unable to synthesize the siderophores N',N",N'''-triacetylfusarinine C (TAF) and ferricrocin. Growth of the DeltasidA strain was the same as that of the wild type in rich media; however, the DeltasidA strain was unable to grow in low-iron defined media or media containing 10% human serum unless supplemented with TAF or ferricrocin. No significant differences in ferric reduction activities were observed between the parental strain and the DeltasidA strain, indicating that blocking siderophore secretion did not result in upregulation of this pathway. Unlike the parental strain, the DeltasidA strain was unable to remove iron from human transferrin. A rescued strain (DeltasidA + sidA) was constructed; it produced siderophores and had the same growth as the wild type on iron-limited media. Unlike the wild-type and rescued strains, the DeltasidA strain was avirulent in a mouse model of invasive aspergillosis, indicating that sidA is necessary for A. fumigatus virulence.

Intracellular and extracellular growth ofAspergillus fumigatus

Aspergillus fumigatus is a filamentous fungus that can cause a life-threatening systemic mycosis in immunocompromised patients. We have studied the growth of A. fumigatus inside cultured cells, and the extracellular growth requirements (in serum). We measured the uptake of bound conidia by the cultured human type II pneumocyte cell line (A549) and a mouse macrophage cell line (J774). The extent of internalization was determined using a nystatin protection assay and by confocal microscopy. Both assays showed that A549 cells internalized 30% of bound conidia after three hours. In contrast, the value for J774 cells was 90%. In both J774 and A549 cells, conidia entered the endosomal pathway and ultimately co-localized with lysosomal markers. Lysosomes containing conidia were acidified. Internalized conidia germinated, and after 24-36 h of incubation with A549 cells, the hyphal tips of some intracellular germlings became exposed to the extracellular space. The importance of iron acquisition to extracellular growth was assessed by creating a strain of A. fumigatus in which the gene encoding the first step of hydroxamate siderophore biosynthesis, ornithine N5-oxygenase (AfusidA), was inactivated by gene replacement. Mutant strains were avirulent in a mouse model of invasive aspergillosis indicating that siderophore biosynthesis is a virulence factor in A. fumigatus.