Biosynthetic Pathways and Functions of Indole-3-Acetic Acid in Microorganisms

Indole-3-acetic acid (IAA) belongs to the family of auxin indole derivatives. IAA regulates almost all aspects of plant growth and development, and is one of the most important plant hormones. In microorganisms too, IAA plays an important role in growth, development, and even plant interaction. Therefore, mechanism studies on the biosynthesis and functions of IAA in microorganisms can promote the production and utilization of IAA in agriculture. This mini-review mainly summarizes the biosynthesis pathways that have been reported in microorganisms, including the indole-3-acetamide pathway, indole-3-pyruvate pathway, tryptamine pathway, indole-3-acetonitrile pathway, tryptophan side chain oxidase pathway, and non-tryptophan dependent pathway. Some pathways interact with each other through common key genes to constitute a network of IAA biosynthesis. In addition, functional studies of IAA in microorganisms, divided into three categories, have also been summarized: the effects on microorganisms, the virulence on plants, and the beneficial impacts on plants.

Huitlacoche (Ustilago maydis), an Iconic Mexican Fungal Resource: Biocultural Importance, Nutritional Content, Bioactive Compounds, and Potential Biotechnological Applications

Worldwide, the fungus known as huitlacoche (Ustilago maydis (DC.) Corda) is a phytopathogen of maize plants that causes important economic losses in different countries. Conversely, it is an iconic edible fungus of Mexican culture and cuisine, and it has high commercial value in the domestic market, though recently there has been a growing interest in the international market. Huitlacoche is an excellent source of nutritional compounds such as protein, dietary fiber, fatty acids, minerals, and vitamins. It is also an important source of bioactive compounds with health-enhancing properties. Furthermore, scientific evidence shows that extracts or compounds isolated from huitlacoche have antioxidant, antimicrobial, anti-inflammatory, antimutagenic, antiplatelet, and dopaminergic properties. Additionally, the technological uses of huitlacoche include stabilizing and capping agents for inorganic nanoparticle synthesis, removing heavy metals from aqueous media, having biocontrol properties for wine production, and containing biosurfactant compounds and enzymes with potential industrial applications. Furthermore, huitlacoche has been used as a functional ingredient to develop foods with potential health-promoting benefits. The present review focuses on the biocultural importance, nutritional content, and phytochemical profile of huitlacoche and its related biological properties as a strategy to contribute to global food security through food diversification; moreover, the biotechnological uses of huitlacoche are also discussed with the aim of contributing to the use, propagation, and conservation of this valuable but overlooked fungal resource.

Identification and Characterization of Auxin/IAA Biosynthesis Pathway in the Rice Blast Fungus Magnaporthe oryzae

The rice blast fungus Magnaporthe oryzae has been known to produce the phytohormone auxin/IAA from its hyphae and conidia, but the detailed biological function and biosynthesis pathway is largely unknown. By sequence homology, we identified a complete indole-3-pyruvic acid (IPA)-based IAA biosynthesis pathway in M. oryzae, consisting of the tryptophan aminotransferase (MoTam1) and the indole-3-pyruvate decarboxylase (MoIpd1). In comparison to the wild type, IAA production was significantly reduced in the motam1Δ mutant, and further reduced in the moipd1Δ mutant. Correspondingly, mycelial growth, conidiation, and pathogenicity were defective in the motam1Δ and the moipd1Δ mutants to various degrees. Targeted metabolomics analysis further confirmed the presence of a functional IPA pathway, catalyzed by MoIpd1, which contributes to IAA/auxin production in M. oryzae. Furthermore, the well-established IAA biosynthesis inhibitor, yucasin, suppressed mycelial growth, conidiation, and pathogenicity in M. oryzae. Overall, this study identified an IPA-dependent IAA synthesis pathway crucial for M. oryzae mycelial growth and pathogenic development.

A sustainable C-H functionalization of indoles, pyrroles and furans under a blue LED with iodonium ylides

Pyrrole and indole derivatives are functionalized via a green initiative with the dimethyl malonate derived phenyl iodonium ylide 4a in the presence of a blue LED via C-H functionalization of the respective heterocycles in methanol to generate the desired compounds 5-7 in moderate to good yields. Control experiments provide insight into the probable reaction mechanism. Finally, the strategy is successfully applied in the generation of azepino[4,5-b]indole 12a/b.

Marine Alkaloid Pityriacitrin and Its Analogues: Discovery, Structures, Synthetic Methods and Biological Properties

Pityriacitrin is a natural marine alkaloid with a typical β-carboline scaffold, and which has been demonstrated to exhibit diverse biological functions. The special structural features for pityriacitrin lead to the increasing research interest and the emergence of versatile derivatives, and many pityriacitrin analogues have been isolated or synthesized over the past decades. The structural diversity and evolved biological activity of these natural alkaloids can offer opportunities for the development of highly potential novel drugs with a new mechanism of action, and therefore, the aim of this brief review is to describe the discovery, synthesis, and biological properties of natural pityriacitrin and its derivatives, as well as the isolation source.

Burkholderia as a Source of Natural Products

Burkholderia bacteria are multifaceted organisms that are ecologically and metabolically diverse. The Burkholderia genus has gained prominence because it includes human pathogens; however, many strains are nonpathogenic and have desirable characteristics such as beneficial plant associations and degradation of pollutants. The diversity of the Burkholderia genus is reflected within the large genomes that feature multiple replicons. Burkholderia genomes encode a plethora of natural products with potential therapeutic relevance and biotechnological applications. This review highlights Burkholderia as an emerging source of natural products. An overview of the taxonomy of the Burkholderia genus, which is currently being revised, is provided. We then present a curated compilation of natural products isolated from Burkholderia sensu lato and analyze their characteristics in terms of biosynthetic class, discovery method, and bioactivity. Finally, we describe and discuss genome characteristics and highlight the biosynthesis of a select number of natural products that are encoded in unusual biosynthetic gene clusters. The availability of >1000 Burkholderia genomes in public databases provides an opportunity to realize the genetic potential of this underexplored taxon for natural product discovery.

Synthesis of a leopolic acid-inspired tetramic acid with antimicrobial activity against multidrug-resistant bacteria

The increasing emergence of multidrug-resistant pathogens is one of the biggest threats to human health and food security. The discovery of new antibacterials, and in particular the finding of new scaffolds, is an imperative goal to stay ahead of the evolution of antibiotic resistance. Herein we report the synthesis of a 3-decyltetramic acid analogue of the ureido dipeptide natural antibiotic leopolic acid A. The key step in the synthetic strategy is an intramolecular Lacey–Dieckmann cyclization reaction of a linear precursor to obtain the desired 3-alkyl-substituted tetramic acid core. The synthesized analogue is more effective than the parent leopolic acid A against Gram-positive (Staphylococcus pseudintermedius) and Gram-negative (E. coli) bacteria (MIC 8 µg/mL and 64 µg/mL, respectively). Interestingly, the compound shows a significant activity against Staphylococcus pseudintermedius strains expressing a multidrug-resistant phenotype (average MIC 32 µg/mL on 30 strains tested). These results suggest that this molecule can be considered a promising starting point for the development of a novel class of antibacterial agents active also against resistant strains.

Skin Fungi from Colonization to Infection

Humans are exceptional among vertebrates in that their living tissue is directly exposed to the outside world. In the absence of protective scales, feathers, or fur, the skin has to be highly effective in defending the organism against the gamut of opportunistic fungi surrounding us. Most (sub)cutaneous infections enter the body by implantation through the skin barrier. On intact skin, two types of fungal expansion are noted: (A) colonization by commensals, i.e., growth enabled by conditions prevailing on the skin surface without degradation of tissue, and (B) infection by superficial pathogens that assimilate epidermal keratin and interact with the cellular immune system. In a response-damage framework, all fungi are potentially able to cause disease, as a balance between their natural predilection and the immune status of the host. For this reason, we will not attribute a fixed ecological term to each species, but rather describe them as growing in a commensal state (A) or in a pathogenic state (B).

Characterization of indole-3-pyruvic acid pathway-mediated biosynthesis of auxin in Neurospora crassa

Plants, bacteria and some fungi are known to produce indole-3-acetic acid (IAA) by employing various pathways. Among these pathways, the indole-3-pyruvic acid (IPA) pathway is the best studied in green plants and plant-associated beneficial microbes. While IAA production circuitry in plants has been studied for decades, little is known regarding the IAA biosynthesis pathway in fungal species. Here, we present the first data for IAA-producing genes and the associated biosynthesis pathway in a non-pathogenic fungus, Neurospora crassa. For this purpose, we used a computational approach to determine the genes and outlined the IAA production circuitry in N. crassa. We then validated these data with experimental evidence. Here, we describe the homologous genes that are present in the IPA pathway of IAA production in N. crassa. High-performance liquid chromatography and thin-layer chromatography unambiguously identified IAA, indole-3-lactic acid (ILA) and tryptophol (TOL) from cultures supplemented with tryptophan. Deletion of the gene (cfp) that encodes the enzyme indole-3-pyruvate decarboxylase, which converts IPA to indole-3-acetaldehyde (IAAld), results in an accumulation of higher levels of ILA in the N. crassa culture medium. A double knock-out strain (Δcbs-3;Δahd-2) for the enzyme IAAld dehydrogenase, which converts IAAld to IAA, shows a many fold decrease in IAA production compared with the wild type strain. The Δcbs-3;Δahd-2 strain also displays slower conidiation and produces many fewer conidiospores than the wild type strain.

RNA-seq analysis provides insight into reprogramming of culm development in Zizania latifolia induced by Ustilago esculenta

We report a transcriptome assembly and expression profiles from RNA-Seq data and identify genes responsible for culm gall formation in Zizania latifolia induced by Ustilago esculenta. The smut fungus Ustilago esculenta can induce culm gall in Zizania latifolia, which is used as a vegetable in Asian countries. However, the underlying molecular mechanism of culm gall formation is still unclear. To characterize the processes underlying this host-fungus association, we performed transcriptomic and expression profiling analyses of culms from Z. latifolia infected by the fungus U. esculenta. Transcriptomic analysis detected U. esculenta induced differential expression of 19,033 and 17,669 genes in Jiaobai (JB) and Huijiao (HJ) type of gall, respectively. Additionally, to detect the potential gall inducing genes, expression profiles of infected culms collected at -7, 1 and 10 DAS of culm gall development were  analyzed. Compared to control, we detected 8089 genes (4389 up-regulated, 3700 down-regulated) and 5251 genes (3121 up-regulated, 2130 down-regulated) were differentially expressed in JB and HJ, respectively. And we identified 376 host and 187 fungal candidate genes that showed stage-specific expression pattern, which are  possibly responsible for gall formation at the initial and later phases, respectively. Our results indicated that cytokinins play more prominent roles in regulating gall formation than do auxins. Together, our work provides general implications for the understanding of gene regulatory networks for culm gall development in Z. latifolia, and potential targets for genetic manipulation to improve the future yield   of  this crop.

Iridium-Catalyzed Asymmetric Allylic Dearomatization by a Desymmetrization Strategy

A desymmetrization strategy was developed involving iridium-catalyzed allylic dearomatization of indoles. The six-membered-ring spiroindolenines contain three contiguous stereogenic centers, including an all-carbon quaternary center, and were obtained in up to 99 % yield with 99 % ee and >95:5 d.r. When treated with a catalytic amount of tosylic acid, six-membered spiroindolenine undergoes an unprecedented six-to-seven-membered ring expansion, affording the corresponding hexahydroazepino[4,5-b]indole.

A simple and efficient method for constructing azepino[4,5-b]indole derivatives via acid catalysis

A new and efficient synthetic methodology has been developed to prepare azepino[4,5-b]indole derivatives under Brønsted acid catalysis.

Total synthesis of leopolic acid A, a natural 2,3-pyrrolidinedione with antimicrobial activity

The first total synthesis of leopolic acid A, a fungal metabolite with a rare 2,3-pyrrolidinedione nucleus linked to an ureido dipeptide, was designed and carried out. Crucial steps for the strategy include a Dieckmann cyclization to obtain the 2,3-pyrrolidinedione ring and a Wittig olefination to install the polymethylene chain. An oxazolidinone-containing leopolic acid A analogue was also synthesized. The antibacterial activity showed by both compounds suggests that they could be considered as promising candidates for future developments.

BEM46 shows eisosomal localization and association with tryptophan-derived auxin pathway in Neurospora crassa

BEM46 proteins are evolutionarily conserved, but their functions remain elusive. We reported previously that the BEM46 protein in Neurospora crassa is targeted to the endoplasmic reticulum (ER) and is essential for ascospore germination. In the present study, we established a bem46 knockout strain of N. crassa. This Δbem46 mutant exhibited a level of ascospore germination lower than that of the wild type but much higher than those of the previously characterized bem46-overexpressing and RNA interference (RNAi) lines. Reinvestigation of the RNAi transformants revealed two types of alternatively spliced bem46 mRNA; expression of either type led to a loss of ascospore germination. Our results indicated that the phenotype was not due to bem46 mRNA downregulation or loss but was caused by the alternatively spliced mRNAs and the peptides they encoded. Using the N. crassa ortholog of the eisosomal protein PILA from Aspergillus nidulans, we further demonstrated the colocalization of BEM46 with eisosomes. Employing the yeast two-hybrid system, we identified a single interaction partner: anthranilate synthase component II (encoded by trp-1). This interaction was confirmed in vivo by a split-YFP (yellow fluorescent protein) approach. The Δtrp-1 mutant showed reduced ascospore germination and increased indole production, and we used bioinformatic tools to identify a putative auxin biosynthetic pathway. The genes involved exhibited various levels of transcriptional regulation in the different bem46 transformant and mutant strains. We also investigated the indole production of the strains in different developmental stages. Our findings suggested that the regulation of indole biosynthesis genes was influenced by bem46 overexpression. Furthermore, we uncovered evidence of colocalization of BEM46 with the neutral amino acid transporter MTR.

Fungi on the skin: dermatophytes and Malassezia

Several human skin diseases and disorders are associated with two groups of fungi, the dermatophytes and Malassezia. Although these skin-related problems are not generally life threatening, they are among the most common diseases and disorders of mankind. These fungi are phylogenetically divergent, with the dermatophytes within the Ascomycota and Malassezia within Basidiomycota. Genome analysis indicates that the adaptations to the skin environment are different in these two groups of fungi. Malassezia are dependent on host lipids and secrete lipases and phospholipases that likely release host fatty acids. The dermatophytes encode multiple enzymes with potential roles in modulating host interactions: polyketide synthases, nonribosomal peptide synthetases, LysM, proteases, kinases, and pseudokinases. These two fungal groups have maximized their interactions with the host using two very different mechanisms.

[Pityriasis versicolor : new aspects of an old disease]

Pityriasis versicolor (PV) is one of the most common infectious skin diseases, as well as the most common dermatosis associated with pigmentation alterations of the skin. PV is prevalent in 1% of the population living in temperate climate zones and more common during the summer. In tropical areas, PV is found in up to 50% of all patients consulting a dermatologist. Of the known Malassezia species, M. globosa is currently felt to play a key role in the pathogenesis of PV, as it is most commonly found in PV lesions. In addition, its round-shaped cells may contribute to the characteristic histology of the disease ("spaghetti and meatballs"). However, the clinical appearance of PV including hyper- and hypopigmentation, fluorescence of the lesions, as well as a lack of inflammation despite high fungal load cannot fully be explained by the presence of M. globosa, which is also found on healthy skin. In M. furfur a tryptophan-dependent metabolic pathway generates a number of indole pigments, which may be associated with the clinical appearance of PV. In the model organism Ustilago maydis it was shown that the formation of the indole compounds occurs spontaneously after initial conversion of tryptophan into indole pyruvate controlled by the key enzyme aminotransferase Tam 1. We review the present knowledge of PV and highlight the potential role of Tam1 in explaining the poorly understood aspects of the disease. Promising therapeutic results using the application of Tam1 inhibitors to treat PV support the enzyme's important role in the disease pathogenesis.

Interactions between Fusarium verticillioides, Ustilago maydis, and Zea mays: an endophyte, a pathogen, and their shared plant host

Highly diverse communities of microbial symbionts occupy eukaryotic organisms, including plants. While many well-studied symbionts may be characterized as either parasites or as mutualists, the prevalent but cryptic endophytic fungi are less easily qualified because they do not cause observable symptoms of their presence within their host. Here, we investigate the interactions of an endophytic fungus, Fusarium verticillioides with a pathogen, Ustilago maydis, as they occur within maize (Zea mays). We used experimental inoculations to evaluate metabolic mechanisms by which these three organisms might interact. We assessed the impacts of fungal-fungal interactions on endophyte and pathogen growth within the plant, and on plant growth. We find that F. verticillioides modulates the growth of U. maydis and thus decreases the pathogen's aggressiveness toward the plant. With co-inoculation of the endophyte with the pathogen, plant growth is similar to that which would be gained without the pathogen present. However, the endophyte may also break down plant compounds that limit U. maydis growth, and obtains a growth benefit from the presence of the pathogen. Thus, an endophyte such as F. verticillioides may function as both a defensive mutualist and a parasite, and express nutritional modes that depend on ecological context.

The Malassezia genus in skin and systemic diseases

In the last 15 years, the genus Malassezia has been a topic of intense basic research on taxonomy, physiology, biochemistry, ecology, immunology, and metabolomics. Currently, the genus encompasses 14 species. The 1996 revision of the genus resulted in seven accepted taxa: M. furfur, M. pachydermatis, M. sympodialis, M. globosa, M. obtusa, M. restricta, and M. slooffiae. In the last decade, seven new taxa isolated from healthy and lesional human and animal skin have been accepted: M. dermatis, M. japonica, M. yamatoensis, M. nana, M. caprae, M. equina, and M. cuniculi. However, forthcoming multidisciplinary research is expected to show the etiopathological relationships between these new species and skin diseases. Hitherto, basic and clinical research has established etiological links between Malassezia yeasts, pityriasis versicolor, and sepsis of neonates and immunocompromised individuals. Their role in aggravating seborrheic dermatitis, dandruff, folliculitis, and onychomycosis, though often supported by histopathological evidence and favorable antifungal therapeutic outcomes, remains under investigation. A close association between skin and Malassezia IgE binding allergens in atopic eczema has been shown, while laboratory data support a role in psoriasis exacerbations. Finally, metabolomic research resulted in the proposal of a hypothesis on the contribution of Malassezia-synthesized aryl hydrocarbon receptor (AhR) ligands to basal cell carcinoma through UV radiation-induced carcinogenesis.

Malassezia virulence determinants

PURPOSE OF REVIEW

Malassezia yeasts are associated with a number of dermatologic and systemic diseases in humans and animals. Pityriasis versicolor is amongst these diseases and represents one of the most common human skin diseases. Beyond that, the role of Malassezia yeasts in the pathogenesis of other skin diseases such as psoriasis, seborrheic dermatitis and confluent and reticulate papillomatosis is discussed but remains less clear. Clear pathogenetic mechanisms of the above-mentioned diseases are not known so far. The review presents new findings on virulence factors of Malassezia yeasts, shedding light on the pathogenesis of Malassezia-associated diseases.

RECENT FINDINGS

Several virulence factors in Malassezia yeasts are known, based on their enzymatic lipolytic activity resulting in the production of distinct metabolites and special cell wall features. Recently, a secondary metabolic pathway possibly implicated in the pathogenesis of pityriasis versicolor was described.

SUMMARY

The article presents virulence factors of Malassezia yeasts ranging from irritant metabolic byproducts to highly bioactive indole derivatives and attempts to clarify their pathogenic implications in the different diseases. Special emphasis is given to the pathogenesis of pityriasis versicolor, as it represents the disease wherein the causative relationship with Malassezia yeasts appears the most obvious.

Glycine as a regulator of tryptophan-dependent pigment synthesis in Malassezia furfur

The effects of the addition of different amino nitrogens on growth, morphology and secondary metabolism of Malassezia furfur were investigated. After primary culture on Dixon agar, M. furfur CBS 1878 was transferred into a fluid medium together with the nitrogen sources, glycine (Gly) or tryptophan (Trp), or a combination of both. Growth was measured by means of a direct cell counting method and pigment synthesis was photometrically assessed. Addition of glycine resulted in an exponential increase in biomass, but not in pigment production. Tryptophan as the sole nitrogen source caused distinct brown staining of the medium, without increasing biomass. Simultaneous equimolar addition of both amino acids resulted in an initial increase in biomass as a sign of preferential metabolism of glycine, followed by a growth plateau and pigment production which, caused by higher biomass, occurred more rapidly than after addition of tryptophan alone. The yeast-cell morphology changed from round to oval. Addition of glycine to the tryptophan-containing liquid culture stopped pigment formation with simultaneous growth induction. These in vitro on-off phenomena depending on the nitrogen source might be significant in the pathogenesis of pityriasis versicolor: hyperhidrosis followed by preferential consumption of individual nitrogen sources such as glycine with exponential growth and thereafter transamination of tryptophan and TRP-dependent pigment synthesis.

Huitlacoche (corn smut), caused by the phytopathogenic fungus Ustilago maydis, as a functional food

BACKGROUND

In recent years the need has arisen to study and develop (or re-discover) foods that have nutritional characteristics as well as specific functions, such as improving health and/or reducing the risk of disease. For this reason knowledge of the nutritional value of food is important to promote greater consumer acceptance. In Mexico huitlacoche (also, cuitlacoche) has traditionally been prized as a delicacy since the time of the Aztecs and is currently being studied as a potential functional food and as a producer of natural bioactive substances that are used in fortifying foods.

AIMS

To present an updated review about the properties of the huitlacoche (corn smut) as functional food.

METHODS

A bibliographic search was performed and data were discussed.

RESULTS

The data of the works reviewed here show that huitlacoche contains many compounds that confer to it unique organoleptic and nutraceutical characteristics.

CONCLUSIONS

The content of bioactive substances in huitlacoche supports the proposal that this is a good functional food as well as producer of compounds to enrich other foods.

L-Tryptophan catabolism by Rubrivivax benzoatilyticus JA2 occurs through indole 3-pyruvic acid pathway

Rubrivivax benzoatilyticus JA2 utilizes L: -tryptophan as the sole source of nitrogen for growth, and it has a doubling time of approximately 11 h (compared to 8 h with ammonium chloride). With cell free extracts in the presence of 2-oxoglutarate, indole-3-pyruvic acid, indole-3-acetaldehyde, indole-3-acetic acid, isatin, benzaldehyde, gallic acid and pyrogallol were identified using high performance liquid chromatography (HPLC) and liquid chromatography-mass spectroscopy (LC-MS) analysis. The conversion of L: -tryptophan into indole 3-pyruvic acid and glutamate by an enzyme aminotransferase was confirmed and the catabolism of indole-3-pyruvic acid via side chain oxidation followed by ring oxidation, gallic acid and pyrogallol were confirmed as metabolites. In addition, the proposed pathway sequential conversion of indole-3-pyruvic acid to the end product of pyrogallol was identified, including an enzymatic step that would convert isatin to benzaldehyde by an enzyme yet to be identified. At this stage of the study, the enzyme tryptophan aminotransferase in R. benzoatilyticus JA2 was demonstrated.

Candida glabrata tryptophan-based pigment production via the Ehrlich pathway

Pigments contribute to the pathogenicity of many fungi, mainly by protecting fungal cells from host defence activities. Here, we have dissected the biosynthetic pathway of a tryptophan-derived pigment of the human pathogen Candida glabrata, identified key genes involved in pigment production and have begun to elucidate the possible biological function of the pigment. Using transcriptional analyses and a transposon insertion library, we have identified genes associated with pigment production. Targeted deletion mutants revealed that the pigment is a by-product of the Ehrlich pathway of tryptophan degradation: a mutant lacking a tryptophan-upregulated aromatic aminotransferase (Aro8) displayed significantly reduced pigmentation and a recombinantly expressed version of this protein was sufficient for pigment production in vitro. Pigment production is tightly regulated as the synthesis is affected by the presence of alternative nitrogen sources, carbon sources, cyclic AMP and oxygen. Growth of C. glabrata on pigment inducing medium leads to an increased resistance to hydrogen peroxide, an effect which was not observed with a mutant defective in pigmentation. Furthermore, pigmented yeast cells had a higher survival rate when exposed to human neutrophils and caused increased damage in a monolayer model of human epithelia, indicating a possible role of pigmentation during interactions with host cells.

Divergent pathways in the biosynthesis of bisindole natural products

Two molecules of the amino acid L-tryptophan are the biosynthetic precursors to a class of natural products named the "bisindoles." Hundreds of these bisindole molecules have been isolated from natural sources, and many of these molecules have potent medicinal properties. Recent studies have clarified the biosynthetic construction of six bisindole molecules, revealing novel enzymatic mechanisms and leading to combinatorial synthesis of new bisindole compounds. Collectively, these results provide a vantage point for understanding how much of the diversity of the bisindole class is generated from a small number of diverging pathways from L-tryptophan, as well as enabling identification of bisindoles that are likely derived via completely distinct biosynthetic pathways.

Elucidation of the complete ferrichrome A biosynthetic pathway in Ustilago maydis

Iron is an important element for many essential processes in living organisms. To acquire iron, the basidiomycete Ustilago maydis synthesizes the iron-chelating siderophores ferrichrome and ferrichrome A. The chemical structures of these siderophores have been elucidated long time ago but so far only two enzymes involved in their biosynthesis have been described. Sid1, an ornithine monoxygenase, is needed for the biosynthesis of both siderophores, and Sid2, a non-ribosomal peptide synthetase (NRPS), is involved in ferrichrome generation. In this work we identified four novel enzymes, Fer3, Fer4, Fer5 and Hcs1, involved in ferrichrome A biosynthesis in U. maydis. By HPLC-MS analysis of siderophore accumulation in culture supernatants of deletion strains, we show that Fer3, an NRPS, Fer4, an enoyl-coenzyme A (CoA)-hydratase, and Fer5, an acylase, are required for ferrichrome A production. We demonstrate by conditional expression of the hydroxymethyl glutaryl (HMG)-CoA synthase Hcs1 in U. maydis that HMG-CoA is an essential precursor for ferrichrome A. In addition, we heterologously expressed and purified Hcs1, Fer4 and Fer5, and demonstrated the enzymatic activities by in vitro experiments. Thus, we describe the first complete fungal siderophore biosynthetic pathway by functionally characterizing four novel genes responsible for ferrichrome A biosynthesis in U. maydis.