Comparisons of fungal melanin biosynthesis in ascomycetous, imperfect and basidiomycetous fungi

Melanin biosynthesis was studied in 26 species of fungi belonging to 16 genera using the melanin inhibitor tricyclazole (5-methyl-1,2,4-triazolo(3,4 b )-benzothiazole) and enzyme reactions in cell-free homogenates. All but one of the 20 dark brown and black ascomycetous and imperfect fungi apparently made melanin from 1,8-dihydroxynaphthalene. Cell-free homogenates of these fungi metabolized two or more melanin substrates (1,3,6,8-tetrahydroxynaphthalene, (+)-scytalone, 1,3,8-trihydroxynaphthalene or vermelone) to intermediates in the melanin pathway, including 1,8-dihydroxynaphthalene, and/or the melanin biosynthetic pathway in these fungi was blocked in vivo by tricyclazole, causing a change in fungal colour and an accumulation of 2-hydroxyjuglone, flaviolin, and other melanin shunt products. Aspergillus niger and six basidiomycetes did not metabolize these melanin substrates, and melanin biosynthesis in these fungi was not affected by tricyclazole. Thus, many ascomycetous and related imperfect fungi appear to make melanin from 1,8-dihydroxynaphthalene, whereas basidiomycetes and at least some imperfect fungi probably use alternative pathways.