Characterization of siderophores from Ustilago maydis

Siderophore Biosynthesis and Transport Systems in Model and Pathogenic Fungi

Fungi employ diverse mechanisms for iron uptake to ensure proliferation and survival in iron-limited environments. Siderophores are secondary metabolite small molecules with a high affinity specifically for ferric iron; these molecules play an essential role in iron acquisition in fungi and significantly influence fungal physiology and virulence. Fungal siderophores, which are primarily hydroxamate types, are synthesized via non-ribosomal peptide synthetases (NRPS) or NRPS-independent pathways. Following synthesis, siderophores are excreted, chelate iron, and are transported into the cell by specific cell membrane transporters. In several human pathogenic fungi, siderophores are pivotal for virulence, as inhibition of their synthesis or transport significantly reduces disease in murine models of infection. This review briefly highlights siderophore biosynthesis and transport mechanisms in fungal pathogens as well the model fungi Saccharomyces cerevisiae and Schizosaccharomyces pombe. Understanding siderophore biosynthesis and transport in pathogenic fungi provides valuable insights into fungal biology and illuminates potential therapeutic targets for combating fungal infections.

Fungal siderophore metabolism with a focus on Aspergillus fumigatus: impact on biotic interactions and potential translational applications

Iron is an essential trace element that is limiting in most habitats including hosts for fungal pathogens. Siderophores are iron-chelators synthesized by most fungal species for high-affinity uptake and intracellular handling of iron. Moreover, virtually all fungal species including those lacking siderophore biosynthesis appear to be able to utilize siderophores produced by other species. Siderophore biosynthesis has been shown to be crucial for virulence of several fungal pathogens infecting animals and plants revealing induction of this iron acquisition system during virulence, which offers translational potential of this fungal-specific system. The present article summarizes the current knowledge on the fungal siderophore system with a focus on Aspergillus fumigatus and its potential translational application including noninvasive diagnosis of fungal infections via urine samples, imaging of fungal infections via labeling of siderophores with radionuclides such as Gallium-68 for detection with positron emission tomography, conjugation of siderophores with fluorescent probes, and development of novel antifungal strategies.

Differential Biosynthesis and Roles of Two Ferrichrome-Type Siderophores, ASP2397/AS2488053 and Ferricrocin, in Acremonium persicinum

Ferrichromes are a family of fungal siderophores with cyclic hexapeptide structures. Most fungi produce one or two ferrichrome-type siderophores. Acremonium persicinum MF-347833 produces ferrichrome-like potent Trojan horse antifungal antibiotics ASP2397 and AS2488053, the aluminum- and iron-chelating forms of AS2488059, respectively. Here, we show by gene sequencing followed by gene deletion experiments that A. persicinum MF-347833 possesses two nonribosomal peptide synthetase genes responsible for AS2488059 and ferricrocin assembly. AS2488059 was produced under iron starvation conditions and excreted into the media to serve as a defense metabolite and probably an iron courier. In contrast, ferricrocin was produced under iron-replete conditions and retained inside the cells, likely serving as an iron-sequestering molecule. Notably, the phylogenetic analyses suggest the different evolutionary origin of AS2488059 from that of conventional ferrichrome-type siderophores. Harnessing two ferrichrome-type siderophores with distinct biological properties may give A. persicinum a competitive advantage for surviving the natural environment.

Fungal iron homeostasis with a focus on Aspergillus fumigatus

To maintain iron homeostasis, fungi have to balance iron acquisition, storage, and utilization to ensure sufficient supply and to avoid toxic excess of this essential trace element. As pathogens usually encounter iron limitation in the host niche, this metal plays a particular role during virulence. Siderophores are iron-chelators synthesized by most, but not all fungal species to sequester iron extra- and intracellularly. In recent years, the facultative human pathogen Aspergillus fumigatus has become a model for fungal iron homeostasis of siderophore-producing fungal species. This article summarizes the knowledge on fungal iron homeostasis and its links to virulence with a focus on A. fumigatus. It covers mechanisms for iron acquisition, storage, and detoxification, as well as the modes of transcriptional iron regulation and iron sensing in A. fumigatus in comparison to other fungal species. Moreover, potential translational applications of the peculiarities of fungal iron metabolism for treatment and diagnosis of fungal infections is addressed.

Metals in fungal virulence

Metals are essential for life, and they play a central role in the struggle between infecting microbes and their hosts. In fact, an important aspect of microbial pathogenesis is the 'nutritional immunity', in which metals are actively restricted (or, in an extended definition of the term, locally enriched) by the host to hinder microbial growth and virulence. Consequently, fungi have evolved often complex regulatory networks, uptake and detoxification systems for essential metals such as iron, zinc, copper, nickel and manganese. These systems often differ fundamentally from their bacterial counterparts, but even within the fungal pathogens we can find common and unique solutions to maintain metal homeostasis. Thus, we here compare the common and species-specific mechanisms used for different metals among different fungal species-focusing on important human pathogens such as Candida albicans, Aspergillus fumigatus or Cryptococcus neoformans, but also looking at model fungi such as Saccharomyces cerevisiae or A. nidulans as well-studied examples for the underlying principles. These direct comparisons of our current knowledge reveal that we have a good understanding how model fungal pathogens take up iron or zinc, but that much is still to learn about other metals and specific adaptations of individual species-not the least to exploit this knowledge for new antifungal strategies.

HapX Mediates Iron Homeostasis in the Pathogenic Dermatophyte Arthroderma benhamiae but Is Dispensable for Virulence

For many pathogenic fungi, siderophore-mediated iron acquisition is essential for virulence. The process of siderophore production and further mechanisms to adapt to iron limitation are strictly controlled in fungi to maintain iron homeostasis. Here we demonstrate that the human pathogenic dermatophyte Arthroderma benhamiae produces the hydroxamate siderophores ferricrocin and ferrichrome C. Additionally, we show that the iron regulator HapX is crucial for the adaptation to iron starvation and iron excess, but is dispensable for virulence of A. benhamiae. Deletion of hapX caused downregulation of siderophore biosynthesis genes leading to a decreased production of siderophores during iron starvation. Furthermore, HapX was required for transcriptional repression of genes involved in iron-dependent pathways during iron-depleted conditions. Additionally, the ΔhapX mutant of A. benhamiae was sensitive to high-iron concentrations indicating that HapX also contributes to iron detoxification. In contrast to other pathogenic fungi, HapX of A. benhamiae was redundant for virulence and a ΔhapX mutant was still able to infect keratinized host tissues in vitro. Our findings underline the highly conserved role of the transcription factor HapX for maintaining iron homeostasis in ascomycetous fungi but, unlike in many other human and plant pathogenic fungi, HapX of A. benhamiae is not a virulence determinant.

Engineering rhizobial bioinoculants: a strategy to improve iron nutrition

Under field conditions, inoculated rhizobial strains are at a survival disadvantage as compared to indigenous strains. In order to out-compete native rhizobia it is not only important to develop strong nodulation efficiency but also increase their competence in the soil and rhizosphere. Competitive survival of the inoculated strain may be improved by employing strain selection and by genetic engineering of superior nitrogen fixing strains. Iron sufficiency is an important factor determining the survival and nodulation by rhizobia in soil. Siderophores, a class of ferric specific ligands that are involved in receptor specific iron transport into bacteria, constitute an important part of iron acquisition systems in rhizobia and have been shown to play a role in symbiosis as well as in saprophytic survival. Soils predominantly have iron bound to hydroxamate siderophores, a pool that is largely unavailable to catecholate-utilizing rhizobia. Outer membrane receptors for uptake of ferric hydroxamates include FhuA and FegA which are specific for ferrichrome siderophore. Increase in nodule occupancy and enhanced plant growth of the fegA and fhuA expressing engineered bioinoculants rhizobial strain have been reported. Engineering rhizobia for developing effective bioinoculants with improved ability to utilize heterologous siderophores could provide them with better iron acquisition ability and consequently, rhizospheric stability.

Regulation of petrobactin and bacillibactin biosynthesis in Bacillus anthracis under iron and oxygen variation

BACKGROUND

Bacillus anthracis produces two catecholate siderophores, petrobactin and bacillibactin, under iron-limited conditions. Here, we investigate how variable iron and oxygen concentrations influence the biosynthetic output of both siderophores in B. anthracis. In addition, we describe the differential levels of transcription of select genes within the B. anthracis siderophore biosynthetic operons that are responsible for synthesis of petrobactin and bacillibactin, during variable growth conditions.

METHODOLOGY/PRINCIPAL FINDINGS

Accumulation of bacillibactin in B. anthracis Sterne (34F(2)) and in a mutant lacking the major superoxide dismutase (ΔsodA1) was almost completely repressed by the addition of 20 µM of iron. In contrast, petrobactin synthesis in both strains continued up to 20 µM of iron. Accumulation of petrobactin and bacillibactin showed a slight increase with addition of low levels of paraquat-induced oxidative stress in wild type B. anthracis Sterne. Cultures grown with high aeration resulted in greater accumulation of petrobactin relative to low aeration cultures, and delayed the repressive effect of added iron. Conversely, iron-depleted cultures grown with low aeration resulted in increased levels of bacillibactin. No difference was found in overall superoxide dismutase (SOD) activity or transcriptional levels of the sodA1 and sodA2 genes between iron-depleted and iron-replete conditions at high or low aeration, suggesting that SOD regulation and iron metabolism are separate in B. anthracis. The highest transcription of the gene asbB, part of the petrobactin biosynthetic operon, occurred under iron-limitation with high aeration, but transcription was readily detectable even under iron-replete conditions and in low aeration. The gene dhbC, a member of the bacillibactin biosynthetic operon, was only transcribed under conditions of iron-depletion, regardless of growth aeration.

CONCLUSION

These data suggest that bacillibactin regulation is highly sensitive to iron-concentration. In contrast, although regulation of petrobactin is less dependent on iron, it is likely subject to additional levels of regulation that may contribute to virulence of B. anthracis.

Growth and siderophores production in Alcaligenes faecalis is regulated by metal ions

During stationary phase of growth under low stress of iron in succinic acid medium, Alcaligenes feacalis BCCM ID 2374 produced microbial iron chelators. Increase in iron concentration supported bacterial growth but suppressed siderophores production, 1 μM and 2 μM of iron was optimum for maximum siderophore yield, i.e. 354 and 360 μg/ml in untreated and deferrated medium, respectively. Threshold level of iron, which suppressed siderophores production in A. feacalis BCCM ID 2374, was 20 μM. Ten micromoles and above concentration of CuCl(2) and CoCl(2), and 20 μM of MgCl(2), MgSO(4), ZnCl(2) and ZnSO(4) severely affected siderophores production.

Ustilago maydis as a Pathogen

The Ustilago maydis-maize pathosystem has emerged as the current model for plant pathogenic basidiomycetes and as one of the few models for a true biotrophic interaction that persists throughout fungal development inside the host plant. This is based on the highly advanced genetic system for both the pathogen and its host, the ability to propagate U. maydis in axenic culture, and its unique capacity to induce prominent disease symptoms (tumors) on all aerial parts of maize within less than a week. The corn smut pathogen, though economically not threatening, will continue to serve as a model for related obligate biotrophic fungi such as the rusts, but also for closely related smut species that induce symptoms only in the flower organs of their hosts. In this review we describe the most prominent features of the U. maydis-maize pathosystem as well as genes and pathways most relevant to disease. We highlight recent developments that place this system at the forefront of understanding the function of secreted effectors in eukaryotic pathogens and describe the expected spin-offs for closely related species exploiting comparative genomics approaches.

A ferroxidation/permeation iron uptake system is required for virulence in Ustilago maydis

In the smut fungus Ustilago maydis, a tightly regulated cAMP signaling cascade is necessary for pathogenic development. Transcriptome analysis using whole genome microarrays set up to identify putative target genes of the protein kinase A catalytic subunit Adr1 revealed nine genes with putative functions in two high-affinity iron uptake systems. These genes locate to three gene clusters on different chromosomes and include the previously identified complementing siderophore auxotroph genes sid1 and sid2 involved in siderophore biosynthesis. Transcription of all nine genes plus three additional genes associated with the gene clusters was also coregulated by iron through the Urbs1 transcription factor. Two components of a high-affinity iron uptake system were characterized in more detail: fer2, encoding a high-affinity iron permease; and fer1, encoding an iron multicopper oxidase. Fer2 localized to the plasma membrane and complemented an ftr1 mutant of Saccharomyces cerevisiae lacking a high-affinity iron permease. During pathogenic development, fer2 expression was confined to the phase of hyphal proliferation inside the plant. fer2 as well as fer1 deletion mutants were strongly affected in virulence. These data highlight the importance of the high-affinity iron uptake system via an iron permease and a multicopper oxidase for biotrophic development in the U. maydis/maize (Zea mays) pathosystem.

Characterization of the Ustilago maydis sid2 gene, encoding a multidomain peptide synthetase in the ferrichrome biosynthetic gene cluster

Ustilago maydis, the causal agent of corn smut disease, acquires and transports ferric ion by producing the extracellular, cyclic peptide, hydroxamate siderophores ferrichrome and ferrichrome A. Ferrichrome biosynthesis likely proceeds by hydroxylation and acetylation of L-ornithine, and later steps likely involve covalently bound thioester intermediates on a multimodular, nonribosomal peptide synthetase. sid1 encodes L-ornithine N(5)-oxygenase, which catalyzes hydroxylation of L-ornithine, the first committed step of ferrichrome and ferrichrome A biosynthesis in U. maydis. In this report we characterize sid2, another biosynthetic gene in the pathway, by gene complementation, gene replacement, DNA sequence, and Northern hybridization analysis. Nucleotide sequencing has revealed that sid2 is located 3.7 kb upstream of sid1 and encodes an intronless polypeptide of 3,947 amino acids with three iterated modules of an approximate length of 1,000 amino acids each. Multiple motifs characteristic of the nonribosomal peptide synthetase protein family were identified in each module. A corresponding iron-regulated sid2 transcript of 11 kb was detected by Northern hybridization analysis. By contrast, constitutive accumulation of this large transcript was observed in a mutant carrying a disruption of urbs1, a zinc finger, GATA family transcription factor previously shown to regulate siderophore biosynthesis in Ustilago. Multiple GATA motifs are present in the intergenic region between sid1 and sid2, suggesting bidirectional transcription regulation by urbs1 of this pathway. Indeed, mutation of two of these motifs, known to be important to regulation of sid1, altered the differential regulation of sid2 by iron.

Human transferrin as a source of iron for Streptococcus intermedius

Streptococcus intermedius is well known to produce severe infections in various areas of the body. In this study, we evaluated the ability of S. intermedius to utilise human transferrin as a source of iron and investigated the mechanism by which iron can be obtained from this plasma protein. Adding either ferrous sulfate or holotransferrin to an iron-deficient culture medium allowed growth of S. intermedius. Cultivation of S. intermedius under an iron-poor condition was associated with the over expression of a 58 kDa cell surface protein. Neither siderophore activity nor reductase activity could be detected. Moreover, cells of S. intermedius did not show transferrin-binding activity or proteolytic activity toward transferrin. It was found that S. intermedius could rapidly decrease the pH of the medium during cell growth, resulting in a release of iron from holotransferrin. When the buffering capacity of the culture medium was significantly increased, the holotransferrin could not support growth of S. intermedius. It is suggested that under certain circumstances, S. intermedius may migrate from its normal niche (oral cavity), reach a particular site and create a localised environment where the pH can be lowered with the subsequent release of iron from transferrin. This would allow bacterial growth and initiation of the infectious process.

Iron uptake in Ustilago maydis: studies with fluorescent ferrichrome analogues

Iron uptake by the phytopathogenic fungus Ustilago maydis was studied using synthetic biomimetic ferrichrome analogues and their fluorescently labelled derivatives as structural and dynamic probes, respectively. The use of structurally distinct analogues enabled determination of the structural requirements for recognition by the fungal iron-uptake system. The application of fluorescently labelled derivatives which convert from a non-fluorescent to a fluorescent state upon iron (III) release enabled monitoring of iron uptake in real time both fluorimetrically and microscopically. Different rates of 55Fe uptake were found for two structurally distinct synthetic analogues, B9 and B5, which differ in their amino acid building blocks. B9 mediated uptake of 55Fe at a higher rate than B5. The behaviour of the fluorescent derivatives B9-Ant (anthracene-labelled B9) and B5-Ant (anthracene-labelled B5) paralleled that of their non-labelled precursors. Exposure of fungal cells to B9-Ant led to a higher increase of fluorescence in the medium than exposure to B5-Ant, indicating a more effective iron uptake from B9-Ant. By using fluorescence microscopy it was possible to trace the label of B9-Ant. Fluorescence was localized in regularly shaped vesicles in the treated cells. The rate of fluorescence appearance within the cells lagged behind the rate of iron uptake, suggesting use of the siderophores for iron storage.

The second finger of Urbs1 is required for iron-mediated repression of sid1 in Ustilago maydis

The urbs1 gene encodes a transcriptional regulator of siderophore biosynthesis in Ustilago maydis. Biological and DNA-binding activities of the two putative zinc-finger motifs of Urbs1 were studied by analyzing mutants containing altered finger domains. The mutated urbs1 alleles from three previously described N'-methyl-N'-nitro-N-nitrosoguanidine (NTG) mutants were mapped and cloned by a gap-repair procedure. Sequence analyses revealed single amino acid substitutions in two of the NTG mutants. Both mutations (G-507 to D in urbs1-1 and P-491 to L in urbs1-3), which are located in the Urbs1 C-terminal finger domain, reduced DNA-binding activity by 10-fold and were sufficient to confer a urbs1-minus phenotype. The third NTG urbs1 mutant (urbs1-2) also contained a mutation in one of the conserved amino acids (P-518 to S) in the C-terminal finger domain, but this mutation alone was not sufficient to confer a urbs1-minus phenotype. A second frame shift mutation was identified in urbs1-2 and is necessary for the urbs1-minus phenotype. In an analysis of the function of the N-terminal finger of Urbs1, the conserved amino acid Arg-350 was mutated to leucine. A Urbs1 protein with this mutation complemented a urbs1 null mutant strain. By contrast, a similar mutation in the C-terminal domain abolished the ability of Urbs1 to regulate siderophore biosynthesis and greatly reduced its ability to bind target DNA.

The distal GATA sequences of the sid1 promoter of Ustilago maydis mediate iron repression of siderophore production and interact directly with Urbs1, a GATA family transcription factor

The sid1 and urbs1 genes encode L-ornithine N5-oxygenase and a GATA family transcription regulator, respectively, for siderophore biosynthesis in Ustilago maydis. The basic promoter and iron-regulatory sequences of the U. maydis sid1 gene were defined by fusing restriction and Bal31 nuclease-generated deletion fragments of the promoter region with the Escherichia coli beta-glucuronidase (GUS) reporter gene. Sequences required for basal expression of sid1 mapped within 1043 bp upstream of the translation start site and include the first untranslated exon and first intron. Sequences needed for iron-regulated expression of sid1 were localized to a 306 bp region mapping 2.3 and 2.6 kb upstream of the ATG. The 306 bp region contains two G/TGATAA sequences, consensus DNA binding sites of GATA family transcription factors. Deletion or site-directed mutation of either or both GATA sequences resulted in deregulated expression of sid1. In vitro DNA binding studies showed that Urbs1 binds to the 3'-GATA site in the 306 bp iron-responsive region. However, deletion of 1.1 kb between the distal GATA sites and the basal promoter region led to deregulated expression of GUS, indicating that these GATA sequences are by themselves insufficient to regulate sid1. In vitro DNA binding and in vivo reporter gene analysis revealed that siderophores are not co-repressors of Urbs1.

Sequences and proteins required for iron-regulated expression ofsid1ofUstilago maydis

The molecular biology of the high affinity, siderophore-mediated iron uptake system of the basidiomycete fungus Ustilago maydis is under investigation. Ustilago maydis produces two cyclic peptide siderophores, ferrichrome and ferrichrome A. Biosynthesis of both siderophores is initiated by ornithine-N5-oxygenase, the product of sid1. sid1 mRNA accumulates only during growth under iron starvation conditions in wild-type cells or constitutively in urbs1 mutants, urbs1 encodes a 100-kDa protein with putative Zn finger domains that share sequence identity with those of the GATA family of transcription factors. The promoter region of sid1 was defined by deletion analysis of a 3.0-kb region 5′ to the translational start of sid1 using the Escherichia coli GUS gene as a reporter. Three regions were defined by this analysis to be critical to expression of sid1. These include (i) a 306-bp region containing two GATA sequences and mapping 2.4 kb from the start of translation; (ii) a 439-bp region immediately 5′ to the start of transcription; and (iii) a region encompassing the first intron of sid1. Deletion of the GATA sequences resulted in deregulated expression of sid1, while elimination of the latter two sequences ablated expression of the gene under all circumstances. Current efforts are focused on determining whether Urbsl interacts directly with the sid1 promoter via the GATA sequences and whether this interaction is dependent upon iron. Key words: GATA, transcription factor, siderophore, ferrichrome, iron, Urbs1.

urbs1, a gene regulating siderophore biosynthesis in Ustilago maydis, encodes a protein similar to the erythroid transcription factor GATA-1

Ustilago maydis secretes ferrichrome-type siderophores, ferric-ion-binding compounds, in response to iron starvation. TA2701, a non-enterobactin-producing, non-ferrichrome-utilizing mutant of Salmonella typhimurium LT-2, was employed as a biological indicator in a novel screening method to isolate three N-methyl-N'-nitro-N-nitrosoguanidine-induced U. maydis mutants defective in the regulation of ferrichrome-type siderophore biosynthesis. These mutants displayed a constitutive phenotype; they produced siderophores in the presence of iron concentrations that would typically repress siderophore synthesis in wild-type strains. A 4.8-kb fragment of U. maydis genomic DNA capable of restoring normal regulation of siderophore biosynthesis in the constitutive mutants was identified. This segment of DNA contains an intronless open reading frame that specifies a protein of 950 amino acids containing two finger motifs similar to those found in the erythroid transcription factor GATA-1. Disruption of this open reading frame in a wild-type strain gave rise to cells that produced siderophores constitutively. Genetic studies indicated that the disruption mutation was allelic to the chemically induced mutations, confirming that the structural gene for a regulator rather than a suppressor gene had been cloned. Northern (RNA) analysis of the gene revealed a 4.2-kb transcript that is expressed constitutively at low levels in wild-type cells. The data support the hypothesis that this gene, which we designate urbs1 (Ustilago regulator of biosynthesis of siderophores), acts directly or indirectly to repress biosynthesis of siderophores in U. maydis.

urbs1, a gene regulating siderophore biosynthesis in Ustilago maydis, encodes a protein similar to the erythroid transcription factor GATA-1

Ustilago maydis secretes ferrichrome-type siderophores, ferric-ion-binding compounds, in response to iron starvation. TA2701, a non-enterobactin-producing, non-ferrichrome-utilizing mutant of Salmonella typhimurium LT-2, was employed as a biological indicator in a novel screening method to isolate three N-methyl-N'-nitro-N-nitrosoguanidine-induced U. maydis mutants defective in the regulation of ferrichrome-type siderophore biosynthesis. These mutants displayed a constitutive phenotype; they produced siderophores in the presence of iron concentrations that would typically repress siderophore synthesis in wild-type strains. A 4.8-kb fragment of U. maydis genomic DNA capable of restoring normal regulation of siderophore biosynthesis in the constitutive mutants was identified. This segment of DNA contains an intronless open reading frame that specifies a protein of 950 amino acids containing two finger motifs similar to those found in the erythroid transcription factor GATA-1. Disruption of this open reading frame in a wild-type strain gave rise to cells that produced siderophores constitutively. Genetic studies indicated that the disruption mutation was allelic to the chemically induced mutations, confirming that the structural gene for a regulator rather than a suppressor gene had been cloned. Northern (RNA) analysis of the gene revealed a 4.2-kb transcript that is expressed constitutively at low levels in wild-type cells. The data support the hypothesis that this gene, which we designate urbs1 (Ustilago regulator of biosynthesis of siderophores), acts directly or indirectly to repress biosynthesis of siderophores in U. maydis.

sid1, a gene initiating siderophore biosynthesis in Ustilago maydis: molecular characterization, regulation by iron, and role in phytopathogenicity

Iron uptake in Ustilago maydis is mediated by production of extracellular hydroxamate siderophores. L-Or-nithine N5-oxygenase catalyzes hydroxylation of L-ornithine, which is the first committed step of ferrichrome and ferrichrome A biosynthesis in U. maydis. We have characterized sid1, a gene coding for this enzyme, by complementation in trans, gene disruption, and DNA sequence analysis. A comparison of genomic DNA and cDNA sequences has shown that the gene is interrupted by three introns. The putative amino acid sequence revealed similarity with Escherichia coli lysine N6-hydroxylase, which catalyzes the hydroxylation of lysine, the first step in biosynthesis of aerobactin. Two transcription initiation points have been determined, both by PCR amplification of the 5' end of the mRNA and by primer extension. A 2.3-kb transcript which accumulates in cells grown under low iron conditions was detected by Northern hybridization. A less abundant 2.7-kb transcript was observed in cells grown in iron-containing medium. By contrast, constitutive accumulation of the 2.3-kb transcript was observed in a mutant carrying a disruption of urbs1, a gene involved in regulation of siderophore biosynthesis. Analysis of the pathogenicity of mutants carrying a null allele of sid1 suggests that the biosynthetic pathway of siderophores does not play an essential role in the infection of maize by U. maydis.