Multifaceted transcriptional regulation of siderophore biosynthesis by the bZIP transcription factor HapX in the entomopathogenic fungus Beauveria bassiana under the iron-deficiency conditions

Iron acquisition is important for Fungal metabolism, growth, development, and pathogenicity. In response to iron limitation/starvation, filamentous fungi excrete siderophores to chelate iron ions from environment. As an iron responsive transcription factor, HapX was found to be required for the siderophore production in the entomopathogenic fungus Beauveria bassiana. Comparative transcriptomic analysis indicated that the HapX-mediated transcriptome was significantly associated with metabolism, energy generation, replication and repair, and membrane transport. Significantly, HapX was required for transcriptional activation of siderophore synthesis genes. Chromatin immunoprecipitation Sequencing (ChIP-seq) indicated HapX displayed the binding activity to the promoters of the comprehensive genes in B. bassiana, and FunCat analysis indicated the genes of the highly enriched promoters were over-presented in metabolism, energy, biogenesis of cellular components, and so on. The genes associated with peroxisome included a number of peroxin genes. Further experiments demonstrated that two peroxin genes (pex2 and pex12) were required for the siderophore production in B. bassiana and functioned as the direct targets of HapX. These data suggest that HapX regulates the siderophore production in B. bassiana through transcriptional control of peroxisomal functionality and the siderophore synthesis pathway compartmentalized in this organelle. This study highlights the new insights into the transcriptional regulatory mechanisms involved in the siderophore production of filamentous fungi.