Analysis of the role of the Aspergillus niger aminolevulinic acid synthase (hemA) gene illustrates the difference between regulation of yeast and fungal haem- and sirohaem-dependent pathways.

To increase knowledge on haem biosynthesis in filamentous fungi like Aspergillus niger, pathway-specific gene expression in response to haem and haem intermediates was analysed. This analysis showed that iron, 5'-aminolevulinic acid (ALA) and possibly haem control haem biosynthesis mostly via modulating expression of hemA [coding for 5'-aminolevulinic acid synthase (ALAS)]. A hemA deletion mutant (ΔhemA) was constructed, which showed conditional lethality. Growth of ΔhemA was supported on standard nitrate-containing media with ALA, but not by hemin. Growth of ΔhemA could be sustained in the presence of hemin in combination with ammonium instead of nitrate as N-source. Our results suggest that a branch-off within the haem biosynthesis pathway required for sirohaem synthesis is responsible for lack of growth of ΔhemA in media containing nitrate as sole N-source, because of the requirement of sirohaem for nitrate assimilation, as a cofactor of nitrite reductase. In contrast to the situation in Saccharomyces cerevisiae, cysteine, but not methionine, was found to further improve growth of ΔhemA. These results demonstrate that A. niger can use exogenous hemin for its cellular processes. They also illustrate important differences in regulation of haem biosynthesis and in the role of haem and sirohaem in A. niger compared to S. cerevisiae.