Heterologous expression and product identification of Colletotrichum lagenarium polyketide synthase encoded by the PKS1 gene involved in melanin biosynthesis

The Colletotrichum lagenarium PKS1 gene was expressed in the heterologous f ungal host, Aspergillus oryzae, under the starch-inducible alpha-amylase pr omoter to identify the direct product of polyketide synthase (PKS) encoded by the PKS1 gene. The main compound produced by an A. oryzae transformant w as isolated and characterized to be 1,3,6,8-tetrahydroxynaphthalene (T4HN) as its tetraacetate. Since the PKS1 gene was cloned from C, lagenarium to c omplement the nonmelanizing albino mutant, T4HN was assumed to be an initia l biosynthetic intermediate, and thus the product of the PKS reaction, but had not been isolated from the fungus. The production of T4HN by the PKS1 t ransformant unambiguously identified the gene to encode a PKS of pentaketid e T4HN. In addition, tetraketide orsellinic acid and pentaketide isocoumari n were isolated, the latter being derived from a pentaketide monocyclic car boxylic acid, as by-products of the PKS1 PKS reaction. Production of the pe ntaketide carboxylic acid provided insights into the mechanism for the PKS1 polyketide synthase reaction to form T4HN.