Pyomelanin Formation in Aspergillus fumigatus Requires HmgX and the Transcriptional Activator HmgR but Is Dispensable for Virulence

Abstract

The opportunistic human pathogenic fungus Aspergillus fumigatus is able to produce the dark brown pigment pyomelanin by degradation of L-tyrosine. Pyomelanin was shown to protect the fungus against reactive oxygen intermediates as well as cell wall disturbing compounds and is therefore assumed to protect against immune effector cells during the infection process. Several genes for tyrosine degradation and pyomelanin formation are organized in a cluster in the genome of A. fumigatus. Here, we aimed at further analyzing tyrosine degradation and a possible role of pyomelanin in virulence. For this purpose, the function of two not yet characterized genes of the cluster, i.e., hmgX and hmgR, was analyzed. Generation of corresponding gene deletion mutants and reconstituted strains revealed that hmgX and hmgR are essential for tyrosine degradation. Both mutants, ΔhmgX and ΔhmgR, were not able to use tyrosine as sole carbon or nitrogen source and revealed impaired pyomelanin production. HmgR harbors a Zn(II)2Cys6-DNA binding domain. Analyses of the steady state mRNA levels revealed that HmgR acts as a transcriptional activator for the genes of the tyrosine degradation cluster. Consistently, an HmgR-eGFP fusion protein was localized in the nucleus of A. fumigatus cells. By contrast, HmgX was found to be localized in the cytoplasm and does not contribute to regulation of gene transcription. HPLC analyses showed that HmgX is crucial for the conversion of p-hydroxyphenylpyruvate to homogentisic acid, the main intermediate in pyomelanin formation. Thus, HmgX is supposed to function as an accessory factor to mediate specific activity of HppD. Remarkably, the ability to degrade tyrosine and to form pyomelanin is dispensable for virulence of A. fumigatus in a murine infection model.

Biosynthesis and Functions of a Melanoid Pigment Produced by Species of the Sporothrix Complex in the Presence of l -Tyrosine

“…The inhibition of tyrosine degradation in Paracoccidioides brasiliensis impairs fungal growth and differentiation for the parasitic yeast phase of the fungus (16). A. fumigatus is able to produce pyomelanin (18), but despite the protection against reactive oxygen intermediates conferred by this pigment, it is not necessary for fungal virulence (8). Our explorations suggest several pathways for protective responses in Sporothrix in the presence of tyrosine, and our findings open up interesting avenues for further exploration into the survival of Sporothrix spp.…”

Unraveling the Molecular Basis of Temperature-Dependent Genetic Regulation in Penicillium marneffei

“…Melanin may play a role in inhibiting cytokine production in the host cells (58) and apoptosis in macrophages (59). In A. fumigatus, the two kinds of melanin, dihydroxynaphthalene-melanin (DHN-melanin) and pyomelanin (60)(61)(62)(63), are generated. Six genes related to the biosynthesis of DHN-melanin were clustered together (60,61) and genetically regulated by the expression of brlA (64).…”

“…Pyomelanin biosynthesis pathway is tightly linked with the pathway of tyrosine degradation: two genes, hppD and hmgX, known to be specifically involved in the pyomelanin biosynthesis pathway, are colocated in the same cluster with the other four genes in the tyrosine degradation pathway (62,63) (Fig. 3).…”

Surface Structure Characterization of Aspergillus fumigatus Conidia Mutated in the Melanin Synthesis Pathway and Their Human Cellular Immune Response

“…DHN-melanin is also essential for the proper assembly of cell wall layers in A. fumigatus. Pyomelanin was shown to protect the fungus from host defense mechanism, i.e., reactive oxygen intermediates and hence considered to be protecting the fungus against immune effector cells during infection (11). DOPA-melanin contributes to host death, fungal burden, and dissemination (8).…”