The Zn2Cys6-type transcription factor LeuB cross-links regulation of leucine biosynthesis and iron acquisition in Aspergillus fumigatus

Both branched-chain amino acids (BCAA) and iron are essential nutrients for eukaryotic cells. Previously, the Zn₂Cys₆-type transcription factor Leu3/LeuB was shown to play a crucial role in regulation of BCAA biosynthesis and nitrogen metabolism in Saccharomyces cerevisiae and Aspergillus nidulans. In this study, we found that the A. fumigatus homolog LeuB is involved in regulation of not only BCAA biosynthesis and nitrogen metabolism but also iron acquisition including siderophore metabolism. Lack of LeuB caused a growth defect, which was cured by supplementation with leucine or iron. Moreover, simultaneous inactivation of LeuB and HapX, a bZIP transcription factor required for adaptation to iron starvation, significantly aggravated the growth defect caused by inactivation of one of these regulators during iron starvation. In agreement with a direct role in regulation of both BCAA and iron metabolism, LeuB was found to bind to phylogenetically conserved motifs in promoters of genes involved in BCAA biosynthesis, nitrogen metabolism, and iron acquisition in vitro and in vivo, and was required for full activation of their expression. Lack of LeuB also caused activation of protease activity and autophagy via leucine depletion. Moreover, LeuB inactivation resulted in virulence attenuation of A. fumigatus in Galleria mellonella. Taken together, this study identified a previously uncharacterized direct cross-regulation of BCCA biosynthesis, nitrogen metabolism and iron homeostasis as well as proteolysis.

Adaptation to the host niche is an essential attribute of pathogens. Here we found that the Zn₂Cys₆-type transcription factor LeuB cross-regulates branched-chain amino acid biosynthesis, nitrogen metabolism, iron acquisition via siderophores, and proteasome activity in the mold Aspergillus fumigatus. Lack of this regulatory circuit impaired virulence in an insect infection model. Mammals do neither express Zn₂Cys₆-type transcription factors nor have the capacity to produce branched-chain amino acids or siderophores. Consequently, this regulatory circuit is a paradigm for fungal pathogen-specific adaptation to the host niche.