Functional expression of a foreign gene in Aspergillus oryzae producing new pyrone compounds.

Fungi from the genus Xylaria produce a wide range of polyketides with diverse structures, which provide important sources for pharmaceutical agents. At least seven polyketide synthase (PKS) genes, including pksmt, were found in Xylaria sp. BCC 1067. The multifunctional enzyme pksmt contains the following catalytic motifs: β-ketosynthase (KS), acyltransferase (AT), dehydratase (DH), methyltransferase (MT), enoylreductase (ER), ketoreductase (KR), and acyl carrier region (ACP). The presence of multiple domains indicated that pksmt was an iterative type I highly-reduced-type PKS gene. To identify the gene function, pksmt was fused with a gene encoding green fluorescent protein (GFP) and introduced into a surrogate host, Aspergillus oryzae, and expressed under the control of a constitutive gpdA promoter. In the transformant, the pksmt gene was functionally expressed and translated as detected by a green fluorescence signal. This transformant produced two new 2-pyrone compounds, 4-(hydroxymethyl)-5,6-dihydro-pyran-2-one and 5-hydroxy-4-methyl-5,6-dihydro-pyran-2-one, as well as a previously identified 4-methyl-5,6-dihydro-pyran-2-one. Our results suggested that pksmt from Xylaria sp. BCC 1067 represents a family of fungal PKSs that can synthesize 2-pyrone-containing compounds.