Cooperative cluster-binding regulates the functional transitions of the Aspergillus fumigatus iron regulator HapX for adaptation to iron starvation, sufficiency and excess

Abstract

Accurate sensing of cellular iron levels is vital, as this metal is essential but toxic in excess. The iron-sensing transcription factor HapX is crucial for virulence of Aspergillus fumigatus, the predominant human mold pathogen. Its absence impairs growth under iron limitation and excess, but not under moderate iron availability, suggesting that HapX switches between three states to adapt to varying iron availability. This study suggests that the HapX state transitions are regulated by the different propensities of four phylogenetically conserved cysteine-rich regions (CRRs) to coordinate [2Fe-2S] clusters resulting in cumulative occupancies that depend on iron availability. In the iron starvation state, CRR-B and -C lack [2Fe-2S] clusters, the iron sufficiency/"neutral" state features clusters in CRR-B and/or -C and the iron excess state has clusters in all CRR-A, B, and -C, while CRR-D plays a minor role. Combinatorial mutation of CRR-B and -C blocked growth by locking HapX in the iron starvation state, leading to uncontrolled iron uptake, iron accumulation, repression of iron-consuming pathways and impaired iron detoxification. Loss of the C-terminal 27 amino acid region of HapX, which is crucial for the iron starvation state and was found to contain a degron, rescued the severe growth defect. Noteworthy, the - Fe state of HapX induced several gene clusters encoding secondary metabolites.