Pentaketide Melanin Biosynthesis in Aspergillus fumigatus Requires Chain-length Shortening of a Heptaketide Precursor

Huei-Fung Tsai,Isao Fujii,Akira Watanabe,Michael H Wheeler,Yun C Chang,Yoshinori Yasuoka,Yutaka Ebizuka,K.J Kwon-Chung

doi:10.1074/jbc.m101998200

Huei-Fung Tsai, Isao Fujii + Show 6 more

Open Access

https://doi.org/10.1074/jbc.m101998200

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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.

Highlights

Chain lengths and cyclization patterns of microbial polyketides are generally determined by polyketide synthases alone
To understand how A. fumigatus uses a heptaketide synthase to initiate the biosynthesis of a pentaketide melanin, we explored the possible involvement of accessory protein(s) in the biosynthetic pathway
Genetic and biochemical analyses indicated that A. fumigatus synthesizes its conidial pigment through a pentaketide pathway similar to the DHN-melanin pathway found in many brown-to-black fungi

Summary

Introduction

Chain lengths and cyclization patterns of microbial polyketides are generally determined by polyketide synthases alone. 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. Genetic and biochemical investigations have shown that biosynthesis of the conidial pentaketide melanin in A. fumigatus requires a six-gene cluster that includes the genes alb1, arp2, and arp1 coding for PKS, 1,3,6,8THN reductase, and scytalone dehydratase, respectively [24].

Results

Conclusion