Biosynthesis and uptake of siderophores is controlled by the PacC-mediated ambient-pH Regulatory system in Aspergillus nidulans.

Biosynthesis and uptake of siderophores in Aspergillus nidulans are regulated not only by iron availability but also by ambient pH: expression of this high-affinity iron uptake system is elevated by an increase in the ambient pH. Mediation of this regulation by the transcriptional regulator PacC has been confirmed via acidity- and alkalinity-mimicking mutants.

4'-phosphopantetheinyl transferase-encoding npgA is essential for siderophore biosynthesis in Aspergillus nidulans.

Aspergillus nidulans produces two major siderophores: it excretes triacetylfusarinine C to capture iron and contains ferricrocin as an intracellular iron-storage compound. Siderophore biosynthesis involves the enzymatic activity of nonribosomal peptide synthetases (NRPS). NRPS contain 4'-phosphopantetheine as an essential prosthetic group, which is attached by 4'-phosphopantetheinyl transferases. A. nidulans appears to possess at least one gene, npgA, encoding such an enzyme. Using a strain carrying a temperature-sensitive allele, cfwA2, we showed that NpgA is essential for biosynthesis of both the peptide bond-containing ferricrocin and the ester bond-containing triacetylfusarinene C. The cfwA2 strain was found to be iron-starved at the restrictive temperature during iron-replete conditions, consistent with the siderophore system being the major iron-uptake system-as we recently demonstrated. Northern analysis indicated that, in contrast to other genes which are involved in siderophore biosynthesis and uptake, expression of npgA is not controlled by the GATA-transcription factor SreA. It was shown previously that NpgA is required for biosynthesis of penicillin, pigment, and potentially lysine via the alpha-aminoadipate pathway. Supplementation with lysine plus triacetylfusarinine C restored normal growth of the cfwA2 strain at the restrictive temperature, suggesting that the growth defect of the mutant is mainly due to impaired biosynthesis of siderophores and lysine.

The siderophore system is essential for viability of Aspergillus nidulans: functional analysis of two genes encoding l-ornithine N 5-monooxygenase (sidA) and a non-ribosomal peptide synthetase (sidC).

The filamentous ascomycete A. nidulans produces two major siderophores: it excretes triacetylfusarinine C to capture iron and contains ferricrocin intracellularly. In this study we report the characterization of two siderophore biosynthetic genes, sidA encoding l-ornithine N(5)-monooxygenase and sidC encoding a non-ribosomal peptide synthetase respectively. Disruption of sidC eliminated synthesis of ferricrocin and deletion of sidA completely blocked siderophore biosynthesis. Siderophore-deficient strains were unable to grow, unless the growth medium was supplemented with siderophores, suggesting that the siderophore system is the major iron assimilatory system of A. nidulans during both iron depleted and iron-replete conditions. Partial restoration of the growth of siderophore-deficient mutants by high concentrations of Fe(2+) (but not Fe(3+)) indicates the presence of an additional ferrous transport system and the absence of an efficient reductive iron assmilatory system. Uptake studies demonstrated that TAFC-bound iron is transferred to cellular ferricrocin whereas ferricrocin is stored after uptake. The siderophore-deficient mutant was able to synthesize ferricrocin from triacetylfusarinine C. Ferricrocin-deficiency caused an increased intracellular labile iron pool, upregulation of antioxidative enzymes and elevated sensitivity to the redox cycler paraquat. This indicates that the lack of this cellular iron storage compound causes oxidative stress. Moreover, ferricrocin biosynthesis was found to be crucial for efficient conidiation.

Characterization of the Aspergillus nidulans transporters for the siderophores enterobactin and triacetylfusarinine C.

The filamentous ascomycete Aspergillus nidulans produces three major siderophores: fusigen, triacetylfusarinine C, and ferricrocin. Biosynthesis and uptake of iron from these siderophores, as well as from various heterologous siderophores, is repressed by iron and this regulation is mediated in part by the transcriptional repressor SREA. Recently we have characterized a putative siderophore-transporter-encoding gene ( mirA ). Here we present the characterization of two further SREA- and iron-regulated paralogues (mirB and mirC ), including the chromosomal localization and the complete exon/intron structure. Expression of mirA and mirB in a Saccharomyces cerevisiae strain, which lacks high affinity iron transport systems, showed that MIRA transports specifically the heterologous siderophore enterobactin and that MIRB transports exclusively the native siderophore triacetylfusarinine C. Construction and analysis of an A. nidulans mirA deletion mutant confirmed the substrate specificity of MIRA. Phylogenetic analysis of the available sequences suggests that the split of the species A. nidulans and S. cerevisiae predates the divergence of the paralogous Aspergillus siderophore transporters.

Regulation of freA, acoA, lysF, and cycA expression by iron availability in Aspergillus nidulans.

In the filamentous fungus Aspergillus nidulans, iron homeostasis is regulated at the transcriptional level by the negative-acting GATA factor SREA. In this study the expression of a putative heme-containing metalloreductase-encoding gene, freA, was found to be upregulated by iron limitation independently of SREA, demonstrating the existence of an iron-regulatory mechanism which does not involve SREA. In contrast to freA, various other genes encoding proteins in need of iron-containing cofactors-acoA, lysF, and cycA-were downregulated in response to iron depletion. Remarkably, SREA deficiency led to increased expression of acoA, lysF, and cycA under iron-replete growth conditions.