Objectives: Scedosporium apiospermum, a ubiquitous environmental mold, is now recognized worldwide as an important cause of infection in both immunocompetent and immunocompromised individuals. It also ranks second among the filamentous fungi colonizing the airways of patients with cystic fibrosis (CF), after Aspergillus fumigatus. However, there are still few data about the virulence genes required for Scedosporium growth and persistence in vivo. Iron acquisition is crucial for survival of microorganisms and is implicated in the virulence of numerous pathogens colonizing the CF lungs. To fulfill their iron requirements, fungi have evolved fine-tuned systems such as the production of siderophores (low-molecular weight iron-specific chelators) or the reduction of insoluble ferric iron into the more soluble ferrous iron, termed reductive iron assimilation (RIA). The aim of this study was to investigate the presence of genes involved in iron homeostasis in the recently sequenced genome of S. apiospermum.

Methods: Firstly, we performed a search for orthologs of Aspergillus fumigatus iron-related proteins through tBLASTn analysis against S. apiospermum genome. Organization into clusters of the genes identified by genome mining in S. apiospermum was then compared with those of A. fumigatus, A. nidulans, A. niger, Colletotrichum higginsianum and Trichoderma reesei. All relevant loci were further confirmed by studying their expression level when the fungus was grown in liquid medium under Fe-depleted (< 0.5 μM Fe) and Fe-overloaded (20 μM) conditions.

Results: The tBLASTn analysis demonstrated that the S. apiospermum genome harbors almost all genes of iron metabolism described in A. fumigatus. Interestingly, siderophore biosynthesis and transporter genes were organized in two clusters, each containing a gene encoding a non-ribosomal peptide synthase (NRPS) that was demonstrated to drive the production of the hydroxamate-type siderophore fusarinin C or ferricrocin in A. fumigatus. Comparison of the siderophore-related clusters, however, showed that S. apiospermum shares more similarities with phytopathogenic molds than with human pathogenic Aspergillus species. As described in other fungi, the genes putatively involved in RIA were randomly distributed along the S. apiospermum genome. Moreover, several orthologs involved in siderophore-mediated iron assimilation or in RIA were significantly overexpressed during iron starvation, and conversely downregulated during iron overload.

Conclusion: Our results revealed that the S. apiospermum genome contains all of the information needed to efficiently compete for iron. Transcriptomic analyses also suggest the siderophore production system as the major system for iron uptake. Studies are now undertaken to identify the siderophores produced by the fungus according to the iron status, and to determine the relative contribution of both siderophore production and reductive assimilation systems in its virulence.

1 / Genome mining and transcriptomic analysis of iron metabolism genes in Scedosporium apiospermum.