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A developmentally regulated gene cluster involved in conidial pigment biosynthesis in Aspergillus fumigatus
H F Tsai1, M H Wheeler, Y C Chang
1Laboratory of Clinical Investigation, National Institute of Allergy and Infectious Diseases, Bethesda, Maryland 20892-1882, USA.
Abstract:
Aspergillus fumigatus, a filamentous fungus producing bluish-green conidia, is an important opportunistic pathogen that primarily affects immunocompromised patients. Conidial pigmentation of A. fumigatus significantly influences its virulence in a murine model. In the present study, six genes, forming a gene cluster spanning 19 kb, were identified as involved in conidial pigment biosynthesis in A. fumigatus. Northern blot analyses showed the six genes to be developmentally regulated and expressed during conidiation. The gene products of alb1 (for "albino 1"), arp1 (for "aspergillus reddish-pink 1"), and arp2 have high similarity to polyketide synthases, scytalone dehydratases, and hydroxynaphthalene reductases, respectively, found in the dihydroxynaphthalene (DHN)-melanin pathway of brown and black fungi. The abr1 gene (for "aspergillus brown 1") encodes a putative protein possessing two signatures of multicopper oxidases. The abr2 gene product has homology to the laccase encoded by the yA gene of Aspergillus nidulans. The function of ayg1 (for "aspergillus yellowish-green 1") remains unknown. Involvement of the six genes in conidial pigmentation was confirmed by the altered conidial color phenotypes that resulted from disruption of each gene in A. fumigatus. The presence of a DHN-melanin pathway in A. fumigatus was supported by the accumulation of scytalone and flaviolin in the arp1 deletant, whereas only flaviolin was accumulated in the arp2 deletants. Scytalone and flaviolin are well-known signature metabolites of the DHN-melanin pathway. Based on DNA sequence similarity, gene disruption results, and biochemical analyses, we conclude that the 19-kb DNA fragment contains a six-gene cluster which is required for conidial pigment biosynthesis in A. fumigatus. However, the presence of abr1, abr2, and ayg1 in addition to alb1, arp1, and arp2 suggests that conidial pigment biosynthesis in A. fumigatus is more complex than the known DHN-melanin pathway.
Insights
Researchers identified a six-gene cluster in Aspergillus fumigatus responsible for conidial pigment biosynthesis. This discovery reveals a more complex melanin pathway than previously understood, impacting fungal virulence.
Area of Science:
- Medical Mycology
- Molecular Biology
- Biochemistry
Background:
- Aspergillus fumigatus is an opportunistic fungal pathogen affecting immunocompromised individuals.
- Conidial pigmentation in A. fumigatus is linked to its virulence.
- Understanding pigment biosynthesis is crucial for targeting fungal pathogens.
Purpose of the Study:
- To identify and characterize genes involved in conidial pigment biosynthesis in A. fumigatus.
- To elucidate the pathway of melanin production in this fungus.
- To compare the identified pathway with known melanin synthesis pathways in other fungi.
Main Methods:
- Gene identification and cluster analysis (19 kb region).
- Northern blot analyses for gene expression during conidiation.
- Gene disruption and phenotypic analysis of conidial color.
- Biochemical analysis of metabolite accumulation (scytalone, flaviolin).
Main Results:
- Six genes (alb1, arp1, arp2, abr1, abr2, ayg1) identified in a pigment biosynthesis gene cluster.
- Genes are developmentally regulated and expressed during conidiation.
- Disruption of each gene resulted in altered conidial pigmentation.
- Evidence supports a dihydroxynaphthalene (DHN)-melanin pathway, with scytalone and flaviolin accumulation.
- A. fumigatus pathway appears more complex than the canonical DHN-melanin pathway due to additional genes (abr1, abr2, ayg1).
Conclusions:
- A six-gene cluster is essential for conidial pigment biosynthesis in A. fumigatus.
- The fungus utilizes a DHN-melanin pathway, but with unique complexities.
- Further research is needed to fully understand the novel aspects of this pathway.