A developmentally regulated gene cluster involved in conidial pigment biosynthesis in Aspergillus fumigatus

Aspergillus fumigatus, a filamentous fungus producing bluish-green conidia, is an important opportunistic pathogen that primarily affects immunocompro mised patients. Conidial pigmentation of A. fumigatus significantly influen ces its virulence in a murine model. In the present study, six genes, formi ng a gene cluster spanning 19 kb, were identified as involved in conidial p igment 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-pin k 1"), and arp2 have high similarity to polyketide synthases, scytalone deh ydratases, and hydroxynaphthalene reductases, respectively, found in the di hydroxynaphthalene (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 conidi al color phenotypes that resulted from disruption of each gene in A. fumiga tus. 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 flaviol in are well-known signature metabolites of the DHN-melanin pathway. Based o n DNA sequence similarity, gene disruption results, and biochemical analyse s, we conclude that the 19-kb DNA fragment contains a six-gene cluster whic h is required for conidial pigment biosynthesis in A. fumigatus. However, t he presence of abr1, abr2, and ayg1 in addition to alb1, arp1, and arp2 sug gests that conidial pigment biosynthesis in A. fumigatus is more complex th an the known DHN-melanin pathway.