title: Pentaketide melanin biosynthesis in Aspergillus fumigatus requires chain-length shortening of a heptaketide precursor.
abstract: Chain lengths and cyclization patterns of microbial polyketides are generally determined by polyketide synthases alone. Fungal polyketide melanins are often derived from a pentaketide 1,8-dihydroxynaphthalene, and pentaketide synthases are used for synthesis of the upstream pentaketide precursor, 1,3,6,8-tetrahydroxynaphthalene (1,3,6,8-THN). However, Aspergillus fumigatus, a human fungal pathogen, uses a heptaketide synthase (Alb1p) to synthesize its conidial pigment through a pentaketide pathway similar to that which produces 1,8-dihydroxynaphthalene-melanin. In this study we demonstrate that a novel protein, Ayg1p, is involved in the formation of 1,3,6,8-THN by chain-length shortening of a heptaketide precursor in A. fumigatus. Deletion of the ayg1 gene prevented the accumulation of 1,3,6,8-THN suggesting the involvement of ayg1 in 1,3,6,8-THN production. Genetic analyses of double-gene deletants suggested that Ayg1p catalyzes a novel biosynthetic step downstream of Alb1p and upstream of Arp2p (1,3,6,8-THN reductase). Further genetic and biochemical analyses of the reconstituted strains carrying alb1, ayg1, or alb1 + ayg1 indicated that Ayg1p is essential for synthesis of 1,3,6,8-THN in addition to Alb1p. Cell-free enzyme assays, using the crude Ayg1p protein extract, revealed that Ayg1p enzymatically shortened the heptaketide product of Alb1p to 1,3,6,8-THN. Thus, the protein Ayg1p facilitates the participation of a heptaketide synthase in a pentaketide pathway via a novel polyketide-shortening mechanism in A. fumigatus.
authors.affiliation: Laboratory of Clinical Investigation, NIAID, National Institutes of Health, Bethesda, Maryland 20892-1882, USA.
authors.forename: H F
authors.initials: HF
authors.lastname: Tsai
authors.forename: I
authors.initials: I
authors.lastname: Fujii
authors.forename: A
authors.initials: A
authors.lastname: Watanabe
authors.forename: M H
authors.initials: MH
authors.lastname: Wheeler
authors.forename: Y C
authors.initials: YC
authors.lastname: Chang
authors.forename: Y
authors.initials: Y
authors.lastname: Yasuoka
authors.forename: Y
authors.initials: Y
authors.lastname: Ebizuka
authors.forename: K J
authors.initials: KJ
authors.lastname: Kwon-Chung
mesh_terms: D000429:Alcohol Oxidoreductases / Q000235:genetics / Q000378:metabolism*; D001232:Aspergillus fumigatus / Q000235:genetics / Q000254:growth & development / Q000502:physiology*; D001426:Bacterial Proteins; D002474:Cell-Free System; D002851:Chromatography, High Pressure Liquid; D017353:Gene Deletion; D005800:Genes, Fungal; D005838:Genotype; D008543:Melanins / Q000096:biosynthesis*; D009097:Multienzyme Complexes / Q000235:genetics / Q000378:metabolism*; D005810:Multigene Family; D009284:Naphthols / Q000378:metabolism; D010641:Phenotype; D013172:Spores, Fungal
chemical_list: C121288:1,3,6,8-tetrahydroxynaphthalene; D001426:Bacterial Proteins; D008543:Melanins; D009097:Multienzyme Complexes; D009284:Naphthols; C041660:1,8-dihydroxynaphthalene; D000429:Alcohol Oxidoreductases; C087192:polyketide synthase ketoreductase